Carburation

[peterg]

Alternator working, new battery fitted and another step closer to the open road.

Next problem is carburation. The engine is a 100AB2 and it has triple SU down drafts. Not sure which model but they have the bistarter (choke) control.

I can get the tick over down to 1,500 rpm . It sits there for a while then drops to 950 - 1,000 and cuts out. The other problem is that when the clutch is depressed tick over drops by 300 rpm.

Any wise words would be appreciated!

[Geoff Dowdle]

peterg,

Assuming your engine is a standard AB2 it should be fitted with three 32mm Solex downdraft carbs , not three downdraft SU's .
Those SU's are probably originally fitted to a 400 with a 85A engine .
They could be made to run correctly on your AB2 but would limit the revs much above 4500rpm. They might need re jetting to suit the AB2 and would need a balance tube fitted between each to get them to idle smoothly.
If you decide to retain them you should have the exhaust mixture check by a expert or you might burn a hole in a piston if it runs too lean.

I'd recommend removing them and fitting either three rebuilt original 32mm Solex or three new 34ICH Weber carbs .

The erratic idle is probably due to the carbs being out of balance and no balance tube cast into the head.

Please post a photo of the engine on the Forum so I can see what it looks like .

Regards
Geoff

[peterg]

Will try to, they probably are Solex carbs - it was a Bristol Cars conversion. My mistake. Looking at the work shop manual the drawing for the Solex 32 bi matches what I have.

[Geoff Dowdle]

peterg,
Assuming your carbs are Solex , your erratic idle can most likely be cured by tuning . How much mechanical experience do you have ?
You need a device to measure the air flow of the carbs to get the three throttle butterfies opening together .
Float levels must be set at 16mm below top face of main body .
As a starting point check that all jets and venturi are standard and not modified/drilled out , butterfly shafts are not worn, three carbs are a matched set (they need to be removed from the engine and stripped down to check that) .

It is quite difficult to get 60 year old carbs running correctly when various previous owners and so called " carby specialist" have attacked them in the past .

If you don't have confidence in tackling the job don't go any further and take the car to a Bristol specialist .

If the carbs are in poor condition it might be better fitting new 34ICH Webers with 27mm venturi ,jetted correctly they idle nicely and perform well.

What type of Bristol is the AB2 fitted to ?

Regards
Geoff

[peterg]

Geoff,
Played with them yesterday. Unscrewed the throttle stop screws and screwed in the volume control and have got it down to 1,000 rpm and firing evenly.

On closing the throttle it drops to 1,500 holds and then drops down to 1,000. I assume there is a damper in the carb. I can't go below because when the clutch is depressed engine speed drops by 300 rpm and at lower tick over speeds the engine stalls.

The real pain is finding where the battery drain is. Cables that disapear into harness' bear little relation to wiring diagrams! Especially when the electrician used generic cable colours, not that it is easy to identify what the wire colours are on the diagram in the first case.

[Geoff Dowdle]

peterg,

It sounds like you have improved the tuning slightly but it still needs more work . Have you balanced the airflow , so all three carbs are the same??
If not you will never get them right .
Float levels must be even at 16mm
Butterfly shafts must be in good condition
Unless those three are checked and correct you will never get it tuned correctly .
Before you tune the carbs ,I should have mentioned the valves should be adjusted , ignition points set & ignition timing set .

Re the electrical problem take it back to the auto electrician who fitted the alternator .

I've owned and ran Bristol powered cars for about 40 years and never needed or wanted a alternator . The original Generator and correct large battery will be sufficient for what a Bristol needs unless you have a bank of spotlights on the front and air conditioning fitted .

My 400 has additional tail lights , electric radiator fan , original valve radio and have never had a problem in 30 years of use .

Regards
Geoff

[peterg]

The electrical problem has now been solved. The Bristol regulator has left and right side 'A' terminals. It appears that the right side are fed by the white ignition switch cable rather than the dynamo. One snip and reconnection of the white cable has eliminated the drain.

I burnt out the dynamo and regulator using one of those emergency start chargers. Rather than shell out £130 plus I thought a brand new Bosch alternator for £25 could be substituted. Took a while to get a secure rear mounting.

After having the engine running for twenty minutes plus I thought I might try it on the road today. Got a few hundred yards down the road and it cut out. On choke it would run up and then cut out. Had to be towed back - oh the ignominy.

I think I will have to look at the petrol pump. If an electric pump is installed does the mechanical pump need to be removed?

[Geoff Dowdle]

Peterg,
The original AC fuel pumps work well for many years.
I suggest you just re kit it rather than fitting a electric pump ,
the 450s raced at Le Mans with the AC mechanical pumps.

Many modern electric pumps produce too high a pressure for the Solex float needles and would require a pressure regulator.

Over many years I have had numerous Bristol engine cars , all with the AC pumps and never had a problem with them. Some owners don't like having to prime the carbs by stroking the lever on the bottom of the pump , they are probably too lazy to open the bonnet and do it . But I always check the oil & water at the same time , which is essential when a car is not used regularly.

Geoff

[peterg]

Thanks Jeff,
The pump was fully checked over when the engine was rebuilt but I pulled it apart when trying to trace a fuel starvation problem (traced to a fractured ferrule in front of the pump).

As said I have had the engine running for prolonged periods before risking it on the road. The pump has to be primed after a few days of non use before the engine will start.

Never too lazy to open the bonnet. Possible that is my problem!

Regards, Peter