Carburetors

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... ;-)

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.
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.

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.

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.

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

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, Pbk
ISBN/ISSN: 0851130690

Hillman Imp Carb For Sale
Mick, April 08 - Carb £5, Contact on 07932 032 341

Carburetors; Weber

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.

Things to watch out for:

• throttle spindle wear
• needle valver wear
• accelerator pump diaphragm operation
• idle jets occasionally block, but are easy to clean without stripping carb. In-line fuel filter cures this one.

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:

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

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

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

• A Weber 28/36 never needs mixture adjustment
• A Weber never needs re-balancing (as Strombergs regularly do). Re-balancing is an awkward job on an Imp
• A Weber does not have diaphragms that may split or o-rings that might leak
• A Weber takes up half the space, so it's much easier to get at and work on
• Its throttle and choke linkage is less complicated
• Throttle response
  It can be very economical when driven on primary choke. When floored, the second choke also operates and gives extremely good acceleration. Twin Strombergs operate best on light throttle and tend to feel flat when floored.
• Air filters are cheaper. Paper elements are vital on an Imp. An early Mini 850 air box can be adapted to fit the Weber. (T.J. filter B.94)

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.

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

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

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)

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

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.

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.

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)

Carburetors; Dellorto

David Vizard tested both a single and twin Dellorto 40 DHLA Carburetors on a 875 Sport engine with a special head and cam: Small bore power

Dell'Orto's UK importer

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 swith from Solex to Stromberg, you might consider fitting twin Strombergs as the Imp Sport has them. Fuel economy will be about the same. CD125s are readily available.
The 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).
eg. Autobitz in Dorchester, Dorset - £10

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:

• throttle spindle wear
  worn spindles are not cheap to replace
• needle valve wear

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.

Carburetors; S.U.

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.

From: Cam Johnson
Subject: su carb
Date: Sat, 4 Dec 1999 18:40:43 -0000

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.

Carburetors; Nikki

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.

Carburetors; Reece Fish

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 it's 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.

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

Lynx
manifold adaptions for the standard imp. Lynx for Twin webers/dellortoes (for use with the standard exhaust!). ( on the car was twin del 40's with a l o n g exhaust that went behind the transmission! Yes Lynx did produce an inlet manifold for twin 40DCOE. I recently sent one to Adelaide. They appear well made but do require seperate extractors. Lynx is an Australian company. They're well known in the Rootes community here for their Hunter manifolds. I didn't know they ever did anything for Imps. I don't think they 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.

SU manifolds
twin SU manifolds

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)