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Carburetors

Contents:
Solex
Weber
  table: jet settings
Dellorto
Stromberg
  table: needles & springs
SU
Nikki
Reece Fish
Zenith
Choke cable attachment
Imp manifolds
 

Carburation is an area where opinions differ.

A carburetor conversion would in the UK probably affect insurance. On the other hand - it could make the car more economical, if you don't use the extra performance... ;-)

The carburettor dilemma
To run the engine smoothly at low speeds, air must pass through the carb fast enough to pick up fuel and atomise it properly. Therefore a small choke diameter is preferred.
However, to get the absolute maximum power at top speed, a large diameter is needed to pass planty of air. Perhaps the carburettor cannot satisfy both requirements, in which case for a road car the top end power will be sacrificed.

Carburators used on Imps

From 1963 to 1966 (when the first series-production 'sporting' derivatives were announced) all cars had carburation by a single semi-downdraught Solex B30 PIHT unit. At first Imps used an automatic choke and a pneumatic throttle control. These were included because of the lengthy run from the driver controls to the carburetor. The pneumatic throttle was dropped within two years as it was unreliable, and so was the automatic choke. (There was a Chrysler kit #432: the auto-choke override.) And, by the way, Imps were not the first car to feature an automatic choke. 1932 sees Oldsmobile become the first manufacturer to offer automatic choke on an automobile.
Single carburetors were used in Hillman Imp, Singer Chamois, Hillman Husky and Commer Van models. Twin carbs were standard in the Sunbeam Imp Sport, Singer Chamois Sport and Sunbeam Stiletto, and in the Rallye Imp.


A twin choke carb can overcome the carburettor dilemma by running a small choke tube in the primary barrel, to give good low speed atomisation. At high speeds, when the secpnd choke is brought into use, the additional air flow allows the motor to develop full power. A progressive twin choke, complete with a suitable manifold, may give you a slight power boost.

The chances of properly setting up a 4-cylinder 4-stroke engine are greater if two carburetors are used. To get the engine running as good as possible, the four cylinders should have equal compression ratios; equal valve spring pressures; equal valve opening angles; and equal induction lengths from the venturis to the inlet valves. Same goes for the exhaust system.

Twin carburetors will not make an engine run less economical. It may even consume marginally less fuel.

All things being equal, twin carbs (in good condition and properly tuned) will prove superior to a single one. Getting the final mixture right and the airflow balanced makes all the difference from the point of a smooth and reliable idle.
The potential advantage with twin carburettors is in the manifold design. Compared to the manifold of a single carb or a twin choke, the flow pattern of a two carbs manifold hold curves that are much smoother and the lengths of the inlet tracts are more equal. Up to a point, the carburettors themselves are just incidential to an increased engine power.
The most efficient lay-out is the one choke per cylinder: each carburettor barrel feeds only one inlet tract, leading without substantial bends to one inlet valve. Such a set-up is justified on a highly tuned motor. But with one choke per cylinder, low speed running is impaired again, compared to a twin choke.

[Malcolm Smith (over 12 years of carb tuning) in Hot Car, July 1976]


Imp tuning for the impecunious

In my opinion, many owners make the mistake of fitting twin Strombergs or a Weber 28/36 to a standard head. The Imp's long, narrow inlet ports are the problem. Even with the Sport cam and exhaust fitted, the carbs are working on a much reduced gas flow. Consequently, low speed performance is often worse than with a Solex. With standard ports, the single Stromberg conversion (which uses the Solex manifold via an adaptor) is the best all-rounder.

With enlarged ports it's a different story. Twin Strombergs or a Weber will give results equivalent or better than an Imp Sport, depending on head modifications. This is about as far as the standard bottom end should go, without being balanced. There may be cooling problems, so fitment of of the Sport's oil cooler is a good idea. At this stage the insurance man will expect you to fit a brake servo, and a Sport distributor will also be required.

If you have a standard unit and wish to play with carburation (which upsets insurance men), you must improve the port's gas flow, as well as cam and exhaust mods, otherwise you will be paying a lot of money for fancy carbs, and may be disappointed with the end results.

Imp tuning : tuning on a budget / Richard Freeman. - Impressions 1983, Dec.

One of the precautions you could take (and a possible cure against cutting out), is to put a gauze filter in the petrol tank, covering the outlet.
The tank has been designed to supply every last drop, but this also predicts where any foreign matter will go.

Carburetor jets and needles are made to very fine tolerances (about 0.0001" accuracy) and wear is sometimes impossible for the amateur to measure. Try comparing the fit between an new and an old one.
Worn jets or needles can cause rough running or flat spots.

Always arrange the throttle mechanism so that the carbs will shut down if anything breaks. Fail safe. Ensure that the mechanism can not go over centre and thus jam in the fully open position.


Solex logo

Carburetors; Solex

Performance Solex Carburetors are identical to the standard Solex, except for a larger diameter and a larger main sprayer.
The Solex carburetor that was used on the L4 series has a smaller choke (19mm) and jets than the previous Solexes (22mm choke). The addition of a Sport exhaust and an earlier carb (or re-jet / choke of the later sort) should give an L4 better performance.
There is a modification kit for the Solex carb which involves a 24mm choke tube.

If you use the 22mm on a more recent Imp and are not satisfied (too temperamental), it may be worth it to resort to the 19mm. On the other hand, if you're not satisfied with your 19mm it may be worth it giving the 22mm choke conversion a try. All you need from an early carb is the choke (which has a 22 stamped on it), the 115 mainjet, 125 econostat jet, 45 slow running jets and the 190 air correction and emulsion tube. After this conversion your Imp may go a lot better. However, you may need to use the choke more, even after short stops...
Beware new Carburetors / Nigel Turner. - Impressions 1990 Spring, p. 34. (A warning to make sure the new unit is the correct size with the right jets for your engine).

The standard Solex is not that bad and it can be rejetted to work on a 998 to give excellent economy.

If your Solex is flooding and trying to cut out (or succeeding), it may be that the inlet manifold is too cold. If it's chilly, the petrol won't vaporize no matter what carb you use to pour it in... (The Sport twin-Stromberg setup is water-heated.)
In the case of flooding (and the choke isn't on when it shouldn't be), the usual culprit is the float needle and seat leaking.

When you change a standard camshaft for a Sport on, while you keep the standard Solex (possibly increase the main jet by one size), you can still notice a large power gain.

The settings for a standard Solex on a 875 are found in the top bit of the Weber table.

Literature

Tuning Solex carburetters for performance / by R.C. Pack and Charles Wheeler. - Brentford (51 York Rd, Brentford, Middx) : SpeedSport Motobooks, 1974. - [1],127p. : ill. ; 22cm.
ISBN: 0851130690 (Pbk)

British Solex carburetters. - [Brentford] : [Speedsport Motobooks], [1976]. - 218p. : ill., port. ; 21cm.
ISBN: 0903192705 (Pbk)

News in Autocar of 8 May 1964:
following the succesful integration of the engineering departments of Zenith Carburettor Co. Ltd. and Solex Ltd., J.D. Richards (chairman) said in his annual report that it is proposed to merge the trading activities of both companies in the future.

Weber logo

Carburetors; Weber

Web site
Weber range of performance Carburetors

Books:

Weber Carburettors / John Passini. - Speed & Sports, 1968. - [Speed sport books ; 'Cars & Car Conversions' publications]. - 75p., ill., 22cm.
Errata as insert.

Weber carburettors : Vol 1: Theory / John Passini. - Brentford : Speedsport Motobooks, 1968 (1973)
ISBN: 0903192403

Weber Carburettors, Part 2: Tuning and maintenance / John Passini. - Brentford : Speedsport, 1973. - 116p : ill ; 22cm.
ISBN: 0851130607

Webers have much scope for adjustment and therefore have a good potential for maladjustment. To have them correctly set up, consider a trip to a rolling road establishment. The same goes for Dellortos.

A 998 with Webers may suffer badly from flat spots, but this fault is more likely due to the manifold than the carburetor.

The 28/36 works on the first choke until about 3/4 then the 2nd choke opens.
A single 40DCOE Weber has 2 chokes working together as a pair.

Weber 40 DCOE

The famous Weber DCOE, horizontal twin, can be used with barrel size 40 for the Imp. These are sometimes used as single applications, but it is said (Malcolm Smith, Hot Car, July 1976) that if the accompanying manifold is a compromise for space reasons, you may find that the advantage of the carb is lost. Either prepare to modify everything in order to fit the carb, or don't convert to the DCOE.
Dual applications are remarkable (their price, too). Maybe if your motor warrants it...

Things to watch out for:

The settings of the main jets depend on a multiplication factor
twin 40s: 3.8 to 4.5 x choke diameter
single 40: 4.5 to 5.4 x choke diameter
This gives a selection of jets from which to choose. Final selection will still be a matter of trial and error. For the DCOE a variety of chokes and jets are available.
The choke diameter depends on the revolutions:

  5000 Sport
6000
R17
7000
8000 R23
9000
rpm
single 40  
twin 40  
21
-
28
25
31
28
33
31
-
35
choke diameter in mm.

Selection of the other jets is not as straightforward. Some suggestions are available, but all settings on this page must be considered as starting points. And they may be well of the mark in certain configurations.
Auxiliary venture
Air corrector
Emulsion tube
Idle jet
Pump jet
Intake and discharge valve 
  4.5
Main jet plus .3 to .5
F15 to F16 for chokes smaller than 36mm. Possibly F2 to F9 for a single 40
45F8, 45F9, 50F9. Idle jet screw ¼ to 1¼ turns unscrewed for correct mixture strength
35, 40
40, 45, 50, 55
  single Weber 40 DCOE
  a single 40 DCOE Weber on this 80 bhp Hartwell 998cc Imp. It helped propel the little screamer from zero to 60mph in 10.6 seconds. This no-compromise set-up only gave a modest 18-27 mpg usage!
Hotcar 1976, July. - p.72

A single 40 Weber on a combined Hartwell manifold goes very well and gives between 25 and 35mpg. on a 998. Twin 40's work superbly, but require a careful right foot to watch the consumption.

The cheapest way to get new twin 40's is to look through Cars and Car Conversions they are about 400 a pair including VAT. (Jun-99)


 

V.W. Derrengton text on Advanced Tuning for the Imp range     twin 40 dcoe webers
     
    DCOE kit of Derrington in Imp
 
 
Jet settings 28/36
 875 998

Primary
Main
Air
Choke
Idle
Pump
105
220
21
50
50
  115
230
22
40
50
Secondary
Main
Air
Choke
Idle
110
220
22
55
  130
200
24
50
Neil Lenton
  Weber 28/36 DCD

Weber 28/36 DCD

In 1976 the 28/36 DCD (a progressive twin choke with removable choke tubes) was still the backbone of Weber conversion business, although they admitted to it getting a bit old. DCD kits generally work quite nicely.

Neil Lenton on Weber 28/36 vs. twin Strombergs:

Weber 28/36 DCD
Weber part no.
71701 22mm
71702 22mm
22mm secondary venturi/choke
22mm primary choke

If you don't want to buy new jets every time you want to change the mixture on your DCD Weber carb:
Drill and tap the jet holder at M5x .8mm all the way through. A Weber idle mixture screw then alters the mixture.
To stop leaks, drill jet holder head 7mm, so a small o-ring can slide in this hole on a groove filed round mix screw. - Cam Johnson

In their article on "Tuning for the Imp range" V.W. Derrington say:
The first improvement should be to the breathing by the fitting of a twin choke Weber carburetter, proved by road tests to be the superior in performance and economy to any other constant depression type of carburetter, single or twin.
A two stage instrument is used, which gives good low speed torque for acceleration, with econimical fuel consumption up to half throttle opening, when the larger secondary barrel comes into action, providing the increased volume of mixture necessary for high performance.
The 28/36 DCD Weber carburetter is mounted on a specially designed water heated inlet manifold, which has a built in chamber for even distribution and a platform on to which the cable operation for the throttle is mounted.
Will fit with the standard cast manifold and exhaust system or the tubular manifold.
Due to the larger carburetter fouling, the mechanical fuel pump - an S.U. fuel pump - should be fitted, giving a quicker starting and a full head of fuel for higher revolutions.
Comparative tests have proved that the 28/36 Weber carburetter unit gives better accelerations, performance and fuel consumption than twin C.D. type carburetters. The design has been approved by the Weber Works at Bologna, after having been tested by them.
A separate pancake type air cleaner can be used.
  £ P/P
28/36 Weber, water heated inlet, fuel pipe line, water hoses, adaptors, connections (state if servo connection required). Complete kit plus air cleaner 36.50 50 p.
Separate choke control (if required) 1.75  
S.U. fuel pump for Weber; better head of fuel 6.30 30 p.
Flexible fuel pipes and conncetions 1.00 20 p.
Extractor exhaust manifold in tubular steel, metal sprayed 13.50 50 p.
Inlet manifold, cast L/A for two 1½" H4 or 150 CD Stromberg 12.00 40 p.
Inlet manifold, water heated for 150 Stromberg 12.50 40 p.
Inlet manifold, water heated for 28/36 Weber, with hoses, adaptors 17.50 45 p.
Lucas high voltage coil, essential for high revolutions 2.73 25 p.

 

Comparative performance figures showing superior results by only a 28/36 Weber, over alternative carburation
 
standard 875 Imp
   
tuned 875 Imp
with a: Solex 28/36 Weber   twin CD Stromberg 28/36 Weber
0 - 30 mph
6.5 secs
5.1 secs
4.0 secs
3.4 secs;
0 - 60 mph
23.0 secs
19.0 secs
15.0 secs
11.5 secs
0 - 70 mph
37.5 secs
29.0 secs
     
Max. top
80 mph
90 mph
 
89 mph
94 mph
Fuel cons.
34-36 mpg
38-40 mpg
 
34-36 mpg
38-40 mpg

 

    Weber twin choke , two stage carburetter, 28/36 DCD
Weber twin choke, two stage carburetter, 28/36 DCD
     

Table
This sort of table is only a guideline. If you use anything slightly out of the ordinary, like bigger valves, a special cylinder head or so, you're on your own. The best way to determine the exact settings is to have the engine set up on a dynometer.
Rule of thumb: high rpm as generated by competition camshafts (fitted to suitably tuned engines) requires the carburetor to be fitted with larger chokes and jets.

standard Solex875standardstandard110standard 21   
standard Solex875standardstandard115170 22
carburetor c.c.  cyl. head  camshaft main air
correction
emulsion
tube
choke idle    accel.
pump
pump
return
single 40 DCOE Weber 875   a single 40 DCOE can be jetted for 875
twin 40 DCOE Weber
with air cleaner
998   r17/ r20 115160F153045F93555
twin 40 DCOE Weber
without air cleaner 
998   r23 130 180 F16 32 45F9 40 55
two 40 DCOE Weber 850 racingracing110180/190F162850F11 slow run
two 40 DCOE Weber998racing/
stage III
racing/
36 long dwell
120190F153150F935
single 28/36 DCD
on heated manifold
875   Sport Prim.: 105 
Sec.: 130
220
200
 21
24
40
50
50 
28/36 875     Prim.: 105 
Sec.: 110
220
220
 21
22
50
55
50 
28/36 998     Prim.: 115 
Sec.: 130
230
200
  22
23
40
50
50 
single 28/36 DCD
on heated manifold
998   r17 Prim.: 115
Sec.: 130
230
200
 22
24
40
50
50

Janspeed of Salisbury gave these specifications for a standard 875 Imp with a single 28/36 DCD:
starter accel.
pump
accel. pump
back bleed
needle
valve
main
jet
air
correction
emulsion
tube
choke  idle  air idle   auxiliary
venturi
105 FI 50 50 175 Primary: 105
Second: 105
220
220
F30
F30
21mm
22mm
50
55
150
155
450
450

Weber U.K. suggested in 1981 these figures
for a twin choke carb, 36 DCD 7, on a Neal Davis manifold (a racing manifold) for an 875cc Imp:
intake
valve
float
level
float max.
travel
pump
jet 
needle
valve
main
jet
air
corrector
emulsion
tube
choke
tube
idling
jet
venturi 
.50 5mm 8.5mm .60 175 Prim.: 115
Sec.: 135
230
200
F30
F30
Prim.: 21
Sec.: 23
.50 Primary: 4.50
Second: 4.50

Others suggest that with a standard 875cc Imp Webers would not be worth all the effort.
Give it a Sport exhaust, Stromberg 125 carburettors on a sport inlet manifold and a Sport camshaft as a start.

JanĀ­speed Engineering, of Park Street, Salisbury, based their tuning on the introduction of twin carburettors, using either HS2 or H4 S.U.s., 150 CD Strombergs or the 40DCOE Weber, and supply specially flowed manifolding for all or any of these set-ups. As well as these, Jan-speed supplied complete conversions, from one giving pepped-up road performance to full Group 2 performance.

There exists a combined inlet and outlet Janspeed manifold for single and twin 40 DCOE Webers (or Dellorto's equivalent, the 40DHLA). Janspeed still produce these Imp manifolds or at least they did in the early nineties. The manifold for the single 40 is only avaliable as a combined inlet/ exhaust unit. It has the problem of cross-over inlet pipes, which can result in poor gas flow and low-down flat spots.
There are small bore and big bore systems. Small bore is for rallying. It produces less power until you reach very high revs. Large bore is for racing. A large bore on a 998cc can produce a very respectable torque from 3300rev/min on a R17. (75 lb/ft at around 6200).

Twin 40s have the theoretical ideal of one choke per cylinder. They are not necessarily more expensive than a single 40, due to the simpler, cheaper manifold outweighing the cost of the extra carburettor and filter.

If you are planning to use twin 40's. The you need a big valve head, R17 or higher 360th cam and the bigbore Janspeed. Fitting twin 40's with a small bore is problematic as the inlet/exhaust are separate. The small bore would throttle the above spec. If your engine is a 998 sport with twin stroms then the small bore is about right and makes a lovely road engine, you need the big one if you are serious about competing. A sport cam gives power between 3-6,000. The R17 etc give power give power from 5-8,000 or more depending partly on bravery and depth of pocket. Big valve heads and high lift cams with high revs get through valve seats rapidly on leaded petrol

Tecnocarbur: an Italian (0471) source for Weber and Solex carburetors. (Inquiries)

 

Technical INFO
A letter by W. Lane, Merriott, to Hot Car
answered in Hot Car 1971, February. - p. 20

Q.: I have a Mk II Imp with an engine modded as follows - the head has been skimmed 65 thou; modified combustion chambers; enlarged inlet and exhaust ports 1 5/16 inlet valves; double springs; a high lift camshaft (30,60,60,30 timing); the flywheel has been lightened; and everything has been balanced.
Would you please advise me on a couple of points. I have a 28/36 DCD Weber and would like to know which jets to use. Would it be worth having the engine bored out to 900cc or go straight out to 1000cc?

A.:You can take your Imp out to around 900cc by using normal OS bore pistons. This will be worthwhile. It is expensive to go any further. A 998cc block and pistons costed about £65 (March 1975)
Try these Weber settings:  

  Primary Secondary
Choke 22 24
Em Tube F23 F23
Main 115 120
Air corr 200 180
Pump 70  
Idle 40 40

V.W. Derrington were Weber distributors (for sales, servicing, rejetting and overhauling) since 1954. They advertised, saying: "Weber make the carburettors, Derrington make the kits." Weber advised to first make enquiries as to the ideal carb and setting from Weber and then to buy a complete conversion package from a Weber dealer.

Literature

Service Manual 28-36 DCD    Manuale di Servizio = Service Manual 28-36 DCD / Edoardo Weber. - Italia; Bologna, 1977. - 28p.
Introduction

The purpose of this booklet is to supply to both private users and to the maintenance Staff of Service Stations a service handbook which should meet the practical needs of periodical service and cleaning of the carburetor.

In order to simplify these instructions, the various operations necessary for the proper maintenance of the carburetor have been described, and the various devices which compose it have been individually dealt with.

This type of work must be done with the greatest care in order to enjoy all the benefits which the WEBER carburetors Model 28/36 DCD can give.

The instructions given in this booklet are purely of a general nature, as it is not always possible to follow the order we have suggested for disassembling and assembling the carburetors, this depending on the existing conditions.

Bolt-on conversions - do they work? / Roger Bell. - Motor 1967 May 20
Motor compared the Hartwell Stromberg bolt-on kit with the Neal Davis twin choke Weber kit for the Imp. 2 pages

Directions for levelling the float of Weber 45 DCOE. - Weber Concessionaires LTD. - 9555000800/ MARCH 95

Carburetors; Dellorto

Dellorto single choke carbs are semi down-draught, fixed choke types, where the float-chamber straddles the barrel. Twin boom floats. The flooding angle is said to be very good.

Imported by Contact Developments. They claim benefits both in power increase and greater economy.

Literature

Small bore power / David Vizard. - Cars & Car Conversions
Reproduced in Impressions vol. 3 (1983), no. 8/9 (Sept./Oct.)
David Vizard tested both a single and twin Dellorto 40 DHLA Carburetors on a 875 Sport engine with a special head and cam.

Diagram of an exploded Dell' Orto DHLA

dellorto.co.uk DHLA parts

  Strombergs on Sport manifold
  Twin Strombergs 125CD on an Imp Sport manifold (no carburettor linkage fitted)
  twin Stromberg on C/T manifold
  Twin Strombergs 150CD on a C/T manifold (no carburettor linkage fitted)

Carburetors; Stromberg

There are two models of 125 CD Stromberg carburetors. The main difference is the size of the flange onto the air filter. The type used on the Imp has the two filter fixing holes 68.3mm apart. The other version has them 60.3mm apart.
The filter used on the Imp must also be of the offset type: the fixing holes are not in the centre of the filter, but offset on one side.
Alexander part no. 724 will fit. Perhaps others, too.

If you decide to switch from Solex to Stromberg, you might consider fitting twin Strombergs as the Imp Sport has them. Fuel economy will be about the same. 125 CDs are readily available.
The Imp Sport throttle cable (inner and outer) is longer. If you fit twins to a standard car, buy a new Sport length cable.

An adapter is needed to fit a 150 CD Stromberg to the Imp. You can still get them new (after a search).

The Sport twin-Stromberg setup is water-heated, so the petrol will vapourize even when it's chilly.

Some Strombergs have a filter-gauze concealed above the float-needle valve assembly. This will eventually block up.

It is possible to use twin 150 CD Stromberg Carburetors (CDS and CDE are newer, slightly modified versions) on a 998 engine. They are still available new, but may have to be modified. Triumph used them on their Dolomite and Vitesse models (as an emission control alternative to the more common SU setup).

In one of his articles David Vizard shows Strombergs to produce more power than the Weber.

Things to watch out for:

If they leak badly around the the bottom mixture adjusting nuts, a cheap solution is to apply a coat of silicone grease around the sealing o-rings.

Removing the jet holder can cause problems (bimetallic corrosion causes stripped threads), so it is best avoided if posssible as the float chamber should come off with it in place.

A properly jetted Weber can be more economical over twin Strombergs. The difference may be something like 42mpg vs. 38mpg.
Twin Strombergs and 28/36 Weber both give good result. The Strombergs give a bit more power and the Weber a bit more economy.

Twin Strombergs and a 28/36 Weber are about equal in reliability and ease of tuning

The needle in a Stromberg carb is measured in 13 places along its length to provide the correct air/ petrol ratio. The standard 6K needle eg. tapers from 0.880" to 0.0540" at the tip.

 

Measured by Motor, journal article 20 May 1967 on Bolt-on conversions

 standardHartwell's 'Pack A'
Stromberg CD125 set
Max. speedm.p.h.m.p.h.
mean79.683.0
best81.185.8
   
Accelerationsec.sec.
0-30 m.p.h.  
0-408.37.5
0-5012.912.2
0-6020.318.5
0-7035.228.4
   
In topsec.sec.
20-40 m.p.h.13.313.7
30-5013.914.5
40-6016.016.5
50-7023.520.7
   
Fuel consumption atm.p.g.m.p.g.
30 m.p.h.55.255.7
5047.243.9
7034.632.2

 

    Stromberg 125 CD disc valve

Hot Car Tech Info Carbs April 1975
Cure for cold
Q. I own a '67 Mk2 Imp and have fitted a 125CDS Stromberg on an adaptor. I have just overhauled it and fitted a 6K needle and standard spring.
The problem is Starting from cold. When the choke is pulled out, it seems to be sucking too much air and the engine stalls.

A. A 125CD carb with a 6K needle and red spring is reckoned to be fine for the Imp, but you mention that you have a 125CDS carb. Only one type of CDS has been made for the Imp and this was designed to be used in pairs. These units have disc type chokes and, obviously, with twin carbs, each choke would only be sufficient to supply half the engine rich mixture requirement.
In addition, only one of the pair would have a fast idle mechanism to boost revs when cold - check to see if yours has this. If not, this could well account for the stalling problem.
We suggest that you device some system for giving a fast idle when the choke is pulled on.
As far as the disc valve is concerned (picture), write to Zenith Carbs at Honeypot Lane, Stansmore, Middx. and ask for an engineering drawing of the starter disc valve - this will show the diameters of the metering holes. You may be able to open these holes out slightly with a fine metric drill to give more enrichment.

For a 1964 850cc Mini, Hot Car advised (Febr. 1975) to keep the standard 0.090 jet and use a 7A needle, along with the blue spring.


Carburetion  c.c.  cyl. head  camshaft  needle  spring 
twin Stromberg 150CD 998   r17 6F red
twin 150CD 998   sport 6F blue
twin 150CD 998 stage III .310 6F red
twin 150CD 998 stage III .310 7B blue
twin 125CD 998   r17 6S red
twin 125CD 998   sport 6S blue
twin 125CD 998 stage III standard 6H blue
twin 125CD 998 stage III standard 6S standard
twin 125CD 875   standard 6L blue
twin 125CD 875 stage II standard 6L blue
twin 125CD 875 stage II .310 6K standard; blue for improved acceleration
twin 125CD 875   sport 6K blue
single 125CD 875   sport 6K blue
single 125CD 875 stage I standard 6K blue
single 125CD 875 standard standard 6K standard/ natural

Stromberg 150 CD on Sport manifold     
Stromberg 150 CD on Sport manifold     The 150 Strombergs are siamese twins due to the nylon clamps to make them behave the same. They can be set up individually and then be connected.
Stromberg CD150
Needle: 6F
mixture set: 3 turns (both)
piston springs: B18276
Stromberg 150 CD on Sport manifold     The Sport manifold (7010115E; SC7277) has been adjusted to the size of the 150 Strombergs.

Literature

  150 CD British Stromberg by the Zenith Carburetter Co. Ltd.
150 CD British Stromberg by the Zenith Carburetter Co. Ltd

A handbook for Strombergs is available from Zenith.

Bolt-on conversions - do they work? / Roger Bell. - Motor 1967 May 20
Motor compared the Hartwell Stromberg bolt-on kit with the Neal Davis twin choke Weber kit for the Imp. 2 pages

There is a book entitled 'Tuning Stromberg Carbs', but in 1976 it was already many years out of date and declared to be of little use.

Watkins, M.B.
Tuning Stromberg carburettors / M.B. Watkins. - Speedsport, 1971
ISBN 1-85113-006-2)

Stromberg sort-out. - Hot Car, 1971, March
(I haven't seen this article)

Stromberg carburetters. - Brentford (Bercourt House, York Rd, Brentford, Middx TW8 0QP) : Interauto Book Co. Ltd, 1973. - [Interauto automobile engineering reference series 11]. - 152p : ill ; 20cm.
ISBN: 0903192527 (pbk)

Stromberg CD carburettors : owners workshop manual / by Don Peers. - Sparkford, Eng. : Haynes Pub. Group ; Newbury Park, Calif. : distributed in the USA by Haynes Publications, 1976. - [Haynes owners workshop manuals, ISSN: 0305-4446]. - 56,[19]p. : ill. ; 27 cm.
ISBN: 0856963003

Chrysler UK Car Clinic / by Ken Dexter. - Popular Motoring, October 1973. - p.18

Q.
Cutting out
I have a Singer Chamois Mark 1 to which I have fitted a CD 12 Stromberg and manifold.
I find that at high speeds the engine cuts out, and only returns when I lift the throttle pedal. The fuel looks clean; the electrics also have been checked; the carburettor doesn't leak. Can you make any suggestions?
A.
The engine might be suffering with fuel starvation at hight revs. Pull the choke out a gnat's and then try the cazr again. If that helps, then the mixture is probably weak: make an adjustment or fit a slightly richer needle.
Maybe check that the petrol pipe from the tank is clear.
Maybe disconnect the pump outlet pipe and check that the pump delivers plenty of fuel.
Maybe check in the carburettor that the diaphragm isn't split or that the dashpot piston isn't falling freely from top to bottom position. Top up the oil in the dashpot - use 20 SAE preferably, otherwise 20W50. Clean out the float chamber to clear out any blockages.

 

Carburetors; S.U.

  1 1/4 SU carbs on a Lynx manifold
twin 1 ¼ SU on a Lynx manifold
   
   

One or two 1¼" SU carburetors can be used with an ordinary Imp engine. The Nerus Hillman Imp used twin S.U.s.

Some people prefer constant depression carbs like the SUs or Strombergs to Solex carbs. They say it smooths out the engine pulses and they claim an increase in performance, economy and reliability over the Solex.

To fit an SU a conversion 'elbow' is needed. A number of articles in Impressions showed how to make one. Cosmic used to make them as did C/T tuning plus a few others - search the established Motor Spares shops perhaps you'll find one. Secondhand ones come up from time to time, along with complete conversions ready to bolt on.

SUs from a Triumph Dolomite may fit on the Imp manifold. Although the mounting bolts are possibly in the wrong place. This means redrilling either the manifold or the carbs.
The Dolomite 1850 HL was also sold with twin 150 CDS Strombergs (see above).

1¼ SU. with red spring and H6 needle on a 875cc.


Cam Johnson's SUs, 17K

From: Cam Johnson
Subject: su carb
Date: Sat, 4 Dec 1999

re onelist, the SU carb I used came off a mini I think, the manifold adaptor a cast aluminium one. I needed to play around with grinding bits off the needle, but they are excellent carbs, especially set lean for economy. I went to the trouble of putting one on my Ford Fiesta, and it's been well worthwhile, because pulling the choke out for full power mixture doesn't involve a restrictive flap.
Years ago, I put a pair of 1.5" SUs on a sport manifold. If I remember correctly, I had to find another carb body to match the stud angle. They were again excellent, miles better than Strombergs in terms of reliability, throttle response, ease of adjustment. I don't have a clue about the needle specification, because I had soldered bits on and filed bits off. The 1.25" on my Fiesta has no mods to the needle at all, and the only mixture problem is a slightly too lean flat spot on a light throttle.

Western Services of Bristol were regarded by S.U. as most competent. Mr. Briginshaw senior and Mr. Briginshaw junior made S.U. kits for many vehicles. These kits are complete with all required bits, including throttle connections.

Malcolm Smith, Hot Car journalist, was critical about the angled adaptors which were used, as he found them to have an unfortunated effect on fuel distribution.

Maintenance: All that needs changing is the needle and sometimes the dashpot spring (when adapting).

Carburetion
c.c. 
cyl. head 
camshaft 
needle 
spring 
two 1¼" S.U. 875standardstandardV3blue
two 1¼" S.U.875stage IIstandardE3blue
two 1¼" S.U.875stage II.310No. 1red
two 1½" S.U.998standardstandardNo. 7red

Literature

Tuning S.U. carbs. - published by Zenith
very comprehensive

Solve SU snags. - Car Mechanics 1971, March. - pp.80-81
with Joss Joselyn's service guide

SU carburetters. - Brentford (Bercourt House, York Rd, Brentford, Middx TW8 0QP) : Interauto Book Co. Ltd, 1972. - [Interauto automobile engineering reference series 8]. - 171p : ill ; 20cm.
ISBN: 090319225x (pbk)

Tuning S.U. carburetters, including full needle charts. - Speed & Sports Publications, 1968. - ['Cars & Car Conversions' publications]. - 54p., ill., 22cm.

Tuning S.U. carburetters. (3rd edition.). - Brentford: Speedsport, 1975. - 128 p. : ill. ; 22 cm.
ISBN: 851130720p


 
  Nikki 28/32
  glass-sided float chamber
  the famous Nikki float chamber window
One of the most important settings in any carburetter is the fuel level. Nikki decided to develop a glass-sided float chamber. To check the fuel level 'at a glance', check the fuel delivery, fuel pressure and possible fuel contamination, instantly
- unfortunately with an Imp the window is on the blind side...
  Nikki counterbalance weight
  The counterbalance weight keeps the 2nd barrel closed until vacuum forces the weight butterfly to open.
  Nikki carburettor on a Nikki manifold

Carburetors; Nikki

Made by the Nippon Carburetor Company, imported by Brown & Geeson. Established in 1932, they were the first carburetor company in Japan. In 1976 only one type was available in the U.K. - the 28/32. It has a cast iron throttle body.

As carbs.freeuk.com says: "No diaphragms... No solenoids... No autochoke... No problems !!!"

It has fixed diameter chokes.
Primary choke: 21 mm tube diameter
Secondary choke: 27 mm.
So only smaller engines (smaller than 1500cc?) might derive extra power from this carburettor. Brown & Geeson did not promote the Nikki on the basis of performance, but rather their economy and flexibility.

Correct fuel/air ratio during all stages of operation is maintained by an auxiliary butterfly in the secondary barrel, operated by the engine demand. A 3rd throttle, which is offset and it is tightly shut by a weight.

   
on a conventional progressive carb
 
on a Nikki
fully depressed pedal
at lower speeds
  
both barrels will open 
  
the 3rd throttle will blank the 2nd choke
until air speed has risen sufficiently to displace it
         

If you convert your standard carburetor to Nikki twin choke carburetors, then use a Sport throttle cable, and not the sloppy extension which comes with the kit. A Sport cable is longer and it will give much more positive throttle control.

On the Brown and Geeson website, the word Nikki does not appear.
From: Carburetters Unlimited
Sent: Tue, Jan 11, 2011
Subject: RE: Nikki carbs for the Imp

[...] I have fitted Nikki carbs to many Imps, with great success. This entailed fitting a Nikki inlet manifold.
Unfortunately, I do not have any manifolds. I did once fit a Nikki to an Imp, using a Weber manifold with a 'Weber to Nikki' adaptor plate.
I bought the entire Nikki operation from Brown & Geeson over 35 years ago.
Best regards, Graham H. Hughes

Nikkis were not expensive.

You could not buy a bare carburettor. A conversion kit included a manifold, cleaner, fittings and instructions. Brown & Geeson would jet the Nikki according to the customer's requirements.

If you want to use a 2ndhand Nikki, it won't be easy to find a fitting manifold, as the flange dimensions are unusual.

Literature

The company brochures were said to be good.


 
Reece Fish carburettors

Carburetors; Reece Fish

The Fish carb is not calibrated by means of needles or jets. Instead, fuel is drawn up a swinging arm in the float chamber from whence it passes into the throttle spindle itself. Now just where it goes into the throttle spindle there is a screw which acts as an adjustable main jet. By screwing it in, you cut down the amount of fuel that can pass into that throttle spindle, and by screwing it out, you increase it.

  Reece Fish carburettor
Reece Fish carburetter - swinging arm
  The Reece Fish arm swinging through a graduated groove
at the side of the float chamber.
  Reece-Fish carburettor
 
  Imp with Reece-Fish carburettor
A Reece-Fish carburettor on an Imp

No needles, nor jets
Operation relies on the swinging arm, which is attached to the hollow throttle spindle. The arm runs up and down a tapered groove and this is basically how the mixture is varied. As the fuel is drawn up the arm and fed into the choke area to spray out of crossholes in the spindle. Altering the main running mixture is controlled by a taper screw, just where the fuel goes into the throttle spindle. Moving the throttle on the spindle controlles the part throttle mixture.
There is no choke tube and there are no jets. The restriction caused by the throttle and spindle sucks fuel into the air stream. Fuel trapped in front of the swinging arm gives an accelerator pump effect.

Therefore you tune a Reece Fish carb on your own motor. You need to be skilled at making gradual adjustments. Maybe take your car to a rolling road - tuning these does not take long.

Tuning

Setting the full power mixture:
Undo the grub screw in the float chamber case, so as to allow access to the main jet adjusting screw (Vizard says he has no better word for it). This screw can only be got at and adjusted when the throttle is fully open - which suits the need. Run the engine flat out on the dyno. The Reece-Fish comes with a special adjusting tool. Adjust the mixture screw, until it shows the maximum reading on the dyno.

Setting the part throttle mixture:
Situation: The fuel comes out of the throttle spindle through holes which start off live when the throttle is in the closed position (pointing forwards and backwards - this is in line with the airflow through the carb). When the butterfly is fully open, these holes are at 90 ° to the airflow.
Alter the position of the butterfly on the throttle spindle arm: either let more air or less air in. This will change the mixture as it changes the amount of fuel which happens to be sucked in with the throttle spindle at that particular position.
The pump action is achieved by the swinging arm in the float chamber.

In 1976 it was said:

The Reece Fish is a very novel carb with a flow potential much greater than a conventional carb of the same size. They do not come cheap.

Mr. Leonard Reece did not favour horizontal fittings, because in the event of spillage, fuel would run out of the intake. He would prefer it to run the other way: into the manifold. Therefore these carbs are available in downdraught pattern of various degrees of semi-downdraughting. Mounting the carb horizontally would not be a good idea.

From the brochure:

The level of the fuel in the float chamber, which is so critical in most carburetters, is relatively unimportant.

It is perhaps not too economical, as perhaps you need to run it rich on part throttle, to mask any flat spots?

Martin Bristow has a Reece Fish carburetor for an Imp, but 15 Dec 99 he hadn't fitted it yet.
Appartently when it's well set up, it should give performance similar to the 28/36 Weber.

It's a very strange thing to look at and its simplicity is legendary! It has no jets as such and is completely adjustable by screws
Whether this means that you can get the settings more accurate, or whether it means that you fiddle with it every week to try to get that little bit more has not been decided yet.

It has no cold-start device. Pump the pedal to inject fuel.

A heater device is needed on some cars, to keep it from icing up.

Special Reece Fish manifolds also exist. They're not cheap either. How much benifit is derived from these, is not clear. Vizard says that compared to the SU manifold it is a lot better and does away with the hot spot.

Literature

The Reece-Fish carburetter. - Motor 1969, week ending March 1. - p.23

Little fishes! - Hot Car 1975, April. - p. 66-67
"How a smaller carb can improve power and performance": the 1½in. SU on a 10,000-mile standard Triumph Toledo was changed for a 1¼in. Fish. It was rated a good buy.

Go Fishing on a Mini ! / David Vizard - Hot Car 1976, May. - p. 70-71
The unique design of the Reece Fish was thoroughly described by David Vizard
One single carburettor that would perform as well as a double carb set-up, need none of the constant attention and also please the insurance man. The Reece Fish nearly can do all this. David Vizard does extensive research on a dynamometer, using a standard (carefully set up) 1293cc Mini and then testing again with the Reece-Fish carburettor plus manifold. And again with a modified engine.

testing a Reece Fish carb against SU or Weber on 1293 Mini    one single Reece Fish carburettor measured against double carb set-ups - no other engine modifications.

The test engine for this experiment was a 1293cc Mini enine in absolutely standard form, right down to having the standard silencer etc.

With the engine carefully set up, some power figures were taken on the standard 1½" SU, making sure to optimize mixture and timing.

To test the Reece-Fish, we decided to use the manifold which Leonard Reece supplies with the carburettor. This was done for two reasons:

  1. the manifold is designed for the job
  2. this manifold is a lot better than the original one, as it does away with the hot spot which certainly can't help the power output any.

Tuning the Reece-Fish:
They ran the engine flat out at 5.000 rpm on the dyno. Then with the special adjusting tool supplied with the carburettor, the mixture screw was adjusted until it showed the maximum reading on the dyno. This, then, was our full power mixture set.
Adjusting the part throttle mixture:
alter the position of the butterfly on the throttle spindle arm to either let more air or less air in,


Then on to a tuned Mini engine for comparing the Reece-Fish against a Weber.

one barrel of the 40 DCOE is slightly bigger than one barrel of the Reece-Fish


For engines up to 70 hp-plus, the Fish will get the job done as well, if not better, than most other carbs.

From 75 hp up, David Vizard reckons the engine really needs to go to more venturi area. Twin Reece-Fish carbs haven't been tested yet.

The company brochures were good.


 
Zenith logo

Carburetors; Zenith

In 1976 they advertised:

Every Zenith Conversion is manufactured to original equipment standards and is guaranteed for twelve months or twelve thousand miles, by the largest independent carburetter manufacturer in Europe.
All jets are removable for cleaning, and size variations are readily available for individual tuning.

Zenith have a range of Stromberg (known as Zenith CD) kits for small british cars, often as a straight replacement for an S.U. No performance gain was claimed. The advantage was one of price and the design of the Stromberg was rather more modern. One handy outfit was the twin 125 CD set, which includes couplings, cleaners, adaptors and fuel tube.
The CD are widely used by independent firms as conversions. Zenith do not have any hand in the preparation of these. Kits bought from Zenith Carburetter Co. will have been expertly tuned.


  The Zenith Carburetter Company Ltd.
Honeypot Lane
Stanmore
Middlesex 

(Zenith Fuel Systems LLC, Bristol, Virginia, U.S.A., founded in 1911 is not the american branch of Zenith Carburetter Co.)

Literature

Dawson's Zenith Imp

Tuning Zenith Carburettors / David Kingston Harris. - [S.l.] : Speedsport, 1974 (1st ed.). - 100 p. : ill. ; 22 cm.
ISBN: 0851130739

Zenith had a motor book sideline, publishing books about more brands carberettors.

Periodical
Zenith news / Zenith Carburetter Co. - Stanmore, 1975-
British Library Holdings: no. 1, etc. Jan. 1975, etc.


Choke cable attachment

The choke design is not all we could wish for, so some Impers have adapted it.
The choke cable has a lever by the gearstick and it goes underneath the car to the carburettor. At either end the inner cable is held in little metal posts - going through a hole in the side of them and clamped by a screw in the top. Now, each of these posts are attached to their lever by a peg which is then squashed into shape on the other side.
The one at the carburettor end is loose and revolves so that the cable will not get bent back and forth as it moves the lever up and down.
The one at the choke lever isn't free to turn. It is firmly riveted to the lever and bends the cable as you move the lever.

The similar fitting is what breaks the throttle cables, especially at the carb end where traces of petrol wash away the grease so the inner can corrode as well...
Try running the cable through this pivoting fixture without clamping it, and then use a separate free screw-clamp nipple behind it. This means that the main flexing is not occurring at the position where the cable is crunched by the clamp-screw.

The correct design for the choke lever end is for the inner solid core to bend through 90 degrees for around the last 8mm or so and pass into the hole in the bottom of the choke lever. A plastic bush/clip then holds the end of the cable in place and allows the joint to pivot.
The little plastic clip can be difficult to locate correctly so some people alter the original design.

A replacement cable for the little catch that holds the choke outer cable to the bracket on the front carb can be gotten at eg. Volvo. Get the one for a 144 or early 244 which used a Stromberg CD175.

If you can pull the plastic peg out of the choke lever, you could try fitting it again with a kind of metal peg with a screw as used on the heater valve.


Manifolds

the effects of manifold pulsation can give a period of uneven running at certain speeds / frequencies.

Brown & Geeson

Brown & Geeson made a choice of manifolds for Nikki carburetor
Brown & Geeson adapters /manifold converters

Carburetters Unlimited bought the entire Nikki operation from Brown & Geeson over 35 years ago, according to Graham H. Hughes. They do not have any manifolds now.
C & T logo

C / T London

e.g. with the Weber 28/36 DCD.

   C T manifolds for Strombergs
   
  C/T manifold with 150 CD Strombergs
150 CD Strombergs on a C/T manifold

There were at least two C /T manifolds that fit the Stomberg 125CD or the 150CD.


V.W. Derrington manifolds

  The Derrinton name in the cast
   
  V.W. Derrington manifold for Webers or Delortoes

Lynx

  Lynx manifold for an Imp
   
  Lynx manifold for Webers

Lynx manifold adaptions for the standard imp. Lynx for twin Webers/ Dellortoes (for use with the standard exhaust).
Lynx did produce an inlet manifold for twin 40DCOE. They appear well made, but do require seperate extractors. Lynx is an Australian company. They may not produce manifolds anymore.
with a pair of 40 DHLAs (100 the lot), It is cast alloy and I think it is by Lynx, but I don't know for sure. something about originally enquiring about a manifold for twin 40's and ending up buying a job lot of these SU manifolds and getting them shipped over from Australia and I think he said they came from Lynx.


Rootes manifolds

  Manifold for two Solex carbs
  A Super Imp manifold, adapted to take two Solex carburettors
  Super Imp manifold
Super Imp K86 55 3


Nerus manifolds

twin SU manifolds

SU 1,25 +  Nerus inlet manifold

 

Mangoletsi did one of their well-known modified inlet manifolds to suit the Imp, while Motortune could supply a twin-Stromberg CD conversion, fitted for under £30.
Addresses:
Mangoletsi - Knutsford, Cheshire
Motortune - Kensington


Suppliers:
mention does not imply recommendation

Fuel Systems Carburetors ltd.
Ben Shallcross of FSC says: "we are solely a carb specialist, principally involved in classic and current carb reconditioning. We have a good specialist knowledge and parts supply and a philosophy of treating people in the manner we all miss so often when dealing with service providers !"
"We are also working on a range of comprehensive repair kits for many carbs, which are designed to allow owners to rebuild original carbs themselves, in a similar manner to a professional recon. To do this, we will be selling a kit with all the bits plus a reamer and spindle bushes etc. and an a-z instruction sheet."   (13 Dec 1999)


The Imp Site
   Imp Anatomy
      Carburetors (this file)
         Twin Carburetors - how to tune
         Carb trouble

Vizard, D.
Look-in on the Imp / by David Vizard. - Cars & Car Conversions 1973, February?, p.52-53,55
This month David Vizard concentrates on the induction and exhaust systems to use at various stages of tune

© Franka
File version: Nov. 15, 2013