6 Cylinder Carburation Improvements



Worn butterfly spindles are sometimes the cause of erratic and unstable tick-overs. Replacement spindles and re-bushed bodies seem to be the only cure. Even then it becomes a good policy to blank off the non linkage end of the housing with a brass turned cup tapped home. To provide an air tight seal the linkage face may be sealed with a thin small diameter ‘0’ ring.

When the butterfly is in the closed condition, the mixture signal will be improved via the mixture screw. Also any fuel dribbling down the choke tube will not leak out via the spindles resulting in a much cleaner carburettor.

When a hot engine is switched off there is a sudden heat rise into the carburettors. This causes the fuel to boil which expands and is now forced up the emulsion tube then down to the butter fly hence leaking fuel out of worn spindles. Heat rise has always been a problem with all Bristol engines, 6 cylinder and V8s. ‘Tufnel’ or ‘Packsalin’ insulation plates provide an excellent barrier to this heat rise. The main body, on later V8s, was even made from ‘phenolic resin’ in an attempt to prevent this heat rise. These insulation plates are 0.062” or 1.5mm thick with the required gaskets. One is fitted direct to the head with a gasket top and bottom and then the fulcrum bracket is put on, a further green gasket, then the second insulation plate and then the grey gasket, finally the carburettor. When fitting the carburettor assembly to the head, do not forget the mounting flanges must be flat. Do not over tighten the thin retaining nuts, they can warp the flanges.

Note, the information in the handbook or workshop manual is only correct at the time when it was written. It does not apply now ie the jet sizes quoted are for standard engines on 82-3 octane fuel. Most engines are not standard by any means so, with heads that have been skimmed and distributors that are no longer accurate, engines will not be giving their best power. For an incremental rise in compression so must a change in the size in jets be considered.

Consider one 320cc cylinder using 83 octane fuel, the compression could be 8.5: l. For the same cylinder with a refurbished head that may have been skimmed and now running on 93-5 octane fuel, the compression could be as much as 10:1. What you really need to know is the clearance volume in the cylinder head to determine the compression ratio. This is when you need to look at your jet sizes. I know this is not easy, in fact it took BCL some time to finalise these settings. A word of warning; if receiving any rebuilt carburettors, strip and carefully reassemble them yourself. This was always done at BCL. It has been known for carburettors to arrive with different needle valves in one to another, different thickness washers one to another, throttle plates not centralised, etc. The choke plate will require lapping if it is to function at its best.

Also remember that fuel density is dependant on its temperature so the hotter the fuel the weaker the mixture. Remember, on fuel injected cars, fuel is in constant circulation in an effort to maintain this density.

Fuel pump pressure. Ex works, fuel pumps were always stripped and adjusted before fitting. Make sure yours is right, 2psi is about right. How? Drill and tap the front banjo bolt, fit screwed tube, connect to a sensitive low pressure gauge and observe the pressure reading, adjust the large fuel pump spring until correct. Remove the screwed tube and replace with a blanking bolt.

Starting from cold, the load on the starter motor and battery can be eased considerably by fitting an electric fuel pump. An ordinary SU fuel pump fitted in the fuel line with a manual electric switch with a warning light should ease this problem. How? From the delivery side of the fuel tap take a flexible pipe to the inlet side of the SU pump, from delivery side of the pump connect to the inlet side of the AC mechanical pump. Now just switch on then wait for the SU pump to stop ticking, switch off – instant engine start. Why? Carburettor float bowls are full and ready to go. Before with float bowls nearly empty it took a little while for them to fill. This is why a priming leaver is fitted to the AC pump.

Whatever fuel you use or condition your engine is in, the ignition must be adjusted accordingly. Marks on flywheels, or bits of metal and marked front pulleys are of very little use in this changed situation; usually a slight retarding is required. Remember your distributor was recommended for replacement at 40,000 miles or so. So how is yours?

How shock absorbers work


(Extracted by K. Lutz)

Shock absorbers don’t really absorb shock, they dampen it, hence they are often referred to as dampers. Why is this important for you to know? To improve ride quality. Basically put, a shock absorbers sole purpose is to dampen the compression and rebound of any suspension system by controlling the speed at which a suspension cycles. Without them, your car would continue to bounce up and down until the kinetic energy is finally dissipated from the suspension’s springs (e.g. leaf springs, coil springs, torsion bar, etc.) and sea sickness may follow.

Now let’s think about the law of conservation of energy. With this law in mind, shocks will perform two functions. The first function is to slow the suspension’s cycling of compressing or rebounding. Secondly, since energy can’t be destroyed, the shock transforms the kinetic energy into heat as it dampens the “bouncing” of the springs. That’s it. That’s what a shock does.

Or otherwise put – in a vehicle, it reduces the effect of travelling over rough ground, leading to improved ride quality, and increase in comfort due to substantially reduced amplitude of disturbances. Without shock absorbers, the vehicle would have a bouncing ride, as energy is stored in the spring and then released to the vehicle, possibly exceeding the allowed range of suspension movement. Control of excessive suspension movement without shock absorption requires stiffer (higher rate) springs, which would in turn give a harsh ride.

Shock absorbers allow the use of soft (lower rate) springs while controlling the rate of suspension movement in response to bumps. They also, along with “bounce” in your tyres, dampen the motion of the unsprung weight up and down on the springiness of the tyre. Since the tyre is not as soft as the springs, effective wheel bounce damping may require stiffer shocks than would be ideal for the vehicle motion alone.

Spring-based shock absorbers commonly use coil springs or leaf springs, though torsion bars can be used in torsional shocks as well. Ideal springs alone, however, are not shock absorbers as springs only store and do not dissipate or absorb energy. Vehicles typically employ both springs or torsion bars as well as hydraulic shock absorbers. In this combination, “shock absorber” is reserved specifically for the hydraulic piston that absorbs and dissipates vibration. Below are some images which illustrate these principles, and the technical features of dampers.

2 Litre shock absorbers

 Shock Absorbers

Investigation work is currently being carried out on shock absorbers as they relate to Bristol’s, and the confusing world of different and changing part numbers that have been used over the passing years, especially as a result of consolidation in the number of producing companies.

 The Bristol Workshop manual states “there are three types of Newton & Bennett shock absorbers fitted to Bristol Cars and they are not interchangeable with each other. One type was fitted to 400 cars; all 401 cars up to chassis 1155 have a second type and from chassis 1156 onwards a larger type is fitted”. Sadly they do not quote the three numbers, but see Bulletin 2 quoted below.

Paraphrased extract from the Bristol 400 Workshop Manual – Bulletin 2 August 1955

The Newton & Bennet shock absorbers originally fitted are S.12490 and are no longer available, but reconditioned units could be fitted OR you could fit S.14490 as the new recommendation. However, they were not interchangeable and if S.14490 was being used it necessitated the replacement of the bottom lugs. It was also important that both rear shock absorbers should be renewed.

One set of possible replacements, produced by Geoff Dowdle in Australia, is currently being checked out for possible local supply in the UK. Note that whilst the Koni numbers are valid in their type, they do not appear in the current Koni on-line catalogue. The SPAX numbers are also being checked out, and some makes possibly not relevant to the UK have been left out.


Action Setting Front Rear
Compression 6.5″/Sec.
5 – 25 Lbs
65 – 85 Lbs
70 – 70 Lbs
115 – 135 Lbs
Rebound 6.5″/Sec.
35 – 55 Lbs
105 – 125 Lbs
115 – 135 Lbs
190 – 215 Lbs


Model Setting Compression Rebound Possible Replacement
400 Rear Newton & Bennett
Max 21.75″ – Min 13″
100 – 140 Lbs 150 – 190 Lbs SPAX 170/561G
401 Front N&B Setting WE4C4
Max 425mm – Min 305mm
100 – 140 Lbs 150 – 190 Lbs KONI 80.1149
WYLIE 25 MR 36-2
401 Rear N&B Setting NE7C3
Max 15.5″ – Min 11.375″
120 – 190 Lbs 290 – 360 Lbs KONI 80.1080
403 Front ——– ——– ——– Spax 170/120G
Koni 80.1149
403 Rear Newton & Bennett ——– ——– Spax G4.5SL
Koni 82.1087
404 Front See Workshop
manual data below
Max 430mm – Min 290mm
——– ——– Spax G4.5SL
Koni 82.1087
See Girling below
404 Rear See Workshop
manual data below
——– ——– Koni 80.1149
See Girling below
Arnolt Front ——– ——– ——– Girling DAS6
Armstrong AT7 1185
Arnolt Rear ——– ——– ——– Girling DAS9
Armstrong AT7 1186A
405 Front See Workshop
manual data below
——– ——– Koni 80J.1107
See Girling below
405 Rear See Workshop
manual data below
——– ——– Koni 80.1215
See Girling below
406 Front ——– ——– ——– SPAX SG712
406 Rear ——– ——– ——– SPAX SG713

404-405 shock absorbers

Original data from the workshop manual



Salisbury Rear Axle


The first link does not apply to the earlier 2 Litre models, but is more relevant to 405/6 onwards through Chrysler V8′s etc. This and other information has kindly been donated by Per Blomquist and comes from a Motor Trader article from May 1958. It is reproduced here so that the technically minded can get a better understanding of how things work. The second link is an article by Phillip Herbert.

Radio Fitting

Radio Fitting

Time and technology has no doubt overtaken the types of radio originally available, but the fitting instructions may be of interest to those who do not yet have a radio fitted. Ongoing research is also being undertaken in regard to digital radios etc., and when this information is available will be included here.


2 litre Wheel oil seals and bearings

Oil seals


NA300 Oil seal

The following is a sample of cars that, according to the Payen Catalogue, also used NA300. Unfortunately, similar information has not yet been found for Oil Seal NA362.

Make Description Dates Models
Jaguar Transmission Rear Overdrive 1957 – 67 2.4, 3.4, “&” 3.8 (except “S” Type) (Jaguar No. 7980)
Jaguar Transmission Rear Overdrive 1963 – 68 3.4, “&” 3.8 “S” Type (Jaguar No. 7980)
Jaguar Transmission Rear Overdrive 1967 – 69 240 “&” 340 (Jaguar No. 7980)
Aston Martin Rear Extension Overdrive 1954 – 57 DB2-4, 3 Litre (A.M. No. 250168)
Aston Martin Transmission Rear Overdrive 1956 – 57 2.4, 3.4, “&” 3.8 (except “S” Type) (A.M. No. 250168)
Austin Healey Coupling Flange Overdrive 1953 – 56 100 – 4 Cyl.
Humber Rear Extension Overdrive 1939 – 56 Super Snipe, Pullman MK1, 11, 111, 1V
Humber Rear Extension Overdrive 1954 – 57 Hawk MK1 – V1
Humber Gearbox Rear Overdrive 1957 – 67 New Hawk Series 1, 1A, 11, 111, 1V
Humber Gearbox Rear Overdrive 1958 – 67 Imperial, Super Snipe Series 1 – V
[gview file=”https://bristoloda.org/wp-content/uploads/2013/01/Oil-Seal-Fitting-Instructions.pdf”]

Wheel Bearings



2 litre Air filters

New replacement air filters for the 2 litre cars can be the K&N part number E4530, (it’s a tight fit, but it does fit! – Adrian Berry)
K&N Filters are available from a number of suppliers.

Specifications: 5-7/8″OD, 3-1/8″ID, 3-3/8″H
K&N Small Engine and Light Industrial Replacement Air Filters – K&N manufactures many direct replacement elements
for small industrial engines, utility lawn and garden units, air compressors, generators, chain saws, etc.