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

[Geoff Dowdle]

peterg,

When you had the pump apart did you remove the top casting to expose the diaphragm ??
When re assembling it is important to hold the priming lever in a position to pull the diaphragm down from its central/natural position , otherwise the tension on the diaphragm will be too great to allow it to work correctly at higher revs.

I bought a simple pressure gauge and adapted it to a spare banjo bolt to check the running pressure of the AC pumps when using Solex carbs. From memory pressure should be about 2.5 - 3.0 psi . I once had a problem with a engine lacking fuel under load , at idle and cruising it pumped about 2.5 psi but once under load going up a steep long hill the pressure dropped away to about 1 psi.

Having to prime the carbs after a few days of not running is common,
it is often caused by the emulsion tube holders being loose in the carby body (originally a press fit , but I carefully remove them and Loctite them in , to seal them) , where they enter the body is below the fuel level so over time the fuel in the bowl leaks out through the carby throat into the engine .
Over a longer time fuel evapouration occurs to what fuel is left.

Geoff

[peterg]

Not sure what you meant by holding the priming lever in position Geoff as the position of the diaphragm in relation to the operating lever is fixed.

Replaced a valve that didn't appear to be working and refitted. The car started in a cloud of smoke and settled down. But after about 5 minutes of driving around, just as I was about to back it into the garage it cut out and once again would only restart if on full choke and primed. Then in runs for about 8 seconds and cuts out.

I am beginning to wonder whether the diaphragm spade is locating securely in the operating arm as the petrol level is only at the level of the gauze filter. Is this a common point of wear in AC pumps?

[peterg]

Latest progress. Checked the pump and the diaphragm spade is still locating securely in the operating arm but after use the petrol level is only half way up the glass bowl.

Will replace the petrol filter and check the flexible line. Not much else I can think of.

[flyinghorse1]

The diaphragm can be attacked by ethanol & perish--is it intact? You would soon know as petrol would enter the oil system & oil pressure would drop.
You can get ethanol proof ones now.

I had problems getting a good idle and I eventually made new gaskets for the stubs that the solexes mount onto the head,using hylomar on all joints and surgically rebuilding my carbs.

I cant fully recall but the float needle valves may be worn /sticky. I got new rebuild kits from gower & lee.

Note the only level setting is by means of varying the packing washers under the Needle Valve and as Geoff says the 16mm is critical.

I also found that a leaking water pump meant erratic firing on front cylinders,and I had 2 duff plugs as well.

[peterg]

No the diaphragms are good.

I did change the fuel filter today as it had some crud in it. No improvement.
Final check. Undid the union, turned the fuel tap on. Nothing. As the fuel gauge reads half full and is probably correct the fuel fuel line is blocked.

Makes sense. But how to clear it? It's a long way from the fuel tank to the isolation tap.

[flyinghorse1]

Peter,
You really have to drop the tank & sort out why crap is getting through.

I have had this as a student in a Hillman hunter and used to carry a foot pump to pump the crap back to the tank from the engine side . However it always returned.

I also have a vintage Swift that did this-and cut out like you said. The motion must be mobilizing fines & clogging the line.

You may get lucky by draining the tank & starting afresh.

Frost do a slosh kit that should fix the issue if you get the tank out--I have used it in my Swift & a friend in an AC Ace and it works.

POR-15 Car Tank Repair Kit | Frost Auto Restoration Techniques

Graham

[Geoff Dowdle]

peterg,
What model Bristol are you working on ????
If it is a 403 (AB2 engine as you advised) the bottom of the tank has probably got a lot of corrosion in it and might need replacing .

The fuel lines often get squashed where they go under the chassis , just forward of the LH rear wheel area . Foreign matter can get blocked in this restriction.

If you want to fix the problem properly I suggest you inspect for leaks , remove the tank , initially clean & inspect it internally , if a alloy 401/3 check for corrosion holes , replace the fuel line & filter (I assume you have a proper disposable filter between pump & carbs).

You should then have a clean fuel system to the carbs which will be a good position to then start a correct tune up.

Geoff

[peterg]

Got to the tank today and there is a layer of sediment at the bottom that wasn't there two years ago and a partial block on the elbow into the tank.

Cleaned the blockage up and the engine is running. So well that it doesn't stop when the ignition is turned off! Another new one on me.

I will need to rig up a fuel filter in the boot as mine is in the engine compartment and there is plenty to clean out.

Can't get the fuel gauge to operate. It either reads as full, empty or just over half full depending upon where it is wired in.

[peterg]

Replaced the small filter before the pump and fitted a large VW filter in the boot. Put in some new petrol and a bottle of Forte petrol treatment

After a false start I took the plunge and drove the car home. Took about three hours to cover 45 miles. The car broke down thirteen times! Twelve for fuel blockages and finally an electrical problem. On the tenth breakdown (Boxhill dual carriageway) I put the left side of the car onto the verge. At that angle the petrol washed down to the side of the tank leaving the pick up pipe high and dry. I decided to push the car back onto the carriageway and about 100 yards down the road to a side road & safety where I found the pipe was submersed again so I carried on to the roundabout and filled up, only to breakdown again just out of the petrol station!

I'm getting quite a taste for petrol! Blowing seems to clear the pipe better than sucking and avoids the inevitable mouthful. However, it does mean that the sludge has still to be sucked up at some point rather than be trapped in the fuel filter.

Took the car back on Saturday and it only failed three times. So I feel that a lot of progress has been made. Think I will replace the filters when I next go down to see the car.

When it has enough fuel it goes well except for a lot of explosions in the exhaust at small throttle openings. Tickover is 1,000 rising to 1,500 rpm when hot.