Setting Solex Float Levels on the Bristol Engine
The Bristol six breathes through three Solex carburettors, and one setting quietly governs how well they all run: the float level, which fixes how much fuel sits in each chamber. Oddly, neither the AC and Bristol handbooks nor the Solex manual gives a figure for it, the theory being that genuine Solex parts self-set the level. In practice, when the parts are worn or a carburettor has been apart, you need a way to set it. Here are the two methods AC and Bristol specialists actually use, and the faults to check when the level will not come right. This covers the Bristol-engined car, the AC and 2.6 engines use different carburettors.
Confirm against your car
This covers the Bristol six's triple Solex carburettors specifically. The figures here are the working settings of named specialists, not a factory specification, the manuals give none, so treat them as starting points, confirm with your engine builder, and set the car up and test it. Petrol is highly flammable: work in ventilation, away from ignition sources, and clean up spills. A wrong float level can flood a carburettor, which is both a running fault and a fire risk.
Why there's no figure in the manual
Unusually, neither the AC and Bristol handbooks nor the Solex literature quotes a float level for these carburettors. The reasoning, as owners and the Solex documentation explain, is that with genuine, correctly-specified Solex components the level is effectively self-set, the right needle valve and float give the right level from new. That is fine until parts wear or a carburettor is rebuilt, at which point you need a method. Two are in common use in the Bristol world, and both aim at the same thing: fuel shutting off at the correct level in each float chamber.
Method 1, the fuel-level method (Geoff Dowdle)
This measures the actual fuel level in the chamber. The specialist Geoff Dowdle's approach uses a modified main jet holder, the Gg, the main jet carrier, with a length of clear tube, so the fuel level in the chamber shows in the tube against a scale. Read the level and set it to Dowdle's figure, around 16mm from the top of the float bowl.
It can be done with the engine static if the car has an electric pump to prime the circuit: rest a bar across the carburettor intake, tape the clear tube to it, turn on the fuel, and measure the settled level with a rule or vernier. Some describe doing it with the engine idling; owners report the static method works well where an electric pump is fitted.
This mirrors the classic Solex level-check, in which the main jet carrier is removed and the fuel allowed to rise so its height can be read. Dowdle's tube simply makes it easier to read precisely. Note that it is the main jet holder (Gg) that is modified, not the starter jet.
Method 2, the needle-valve method (Tony Byford)
This sets the needle valve rather than reading the fuel. The specialist Tony Byford, who has run this on Bristol engines in Aces, Acecas, Greyhounds, Cooper-Bristols and Arnolt-Bristols for decades, inverts the carburettor top, screws in the needle valve, and lays a straight edge across the face. The needle valve should just close off the fuel at his set dimension: around 3.3mm from the tip of the valve to the face of the carburettor top. If it does not close fully, a thin aluminium washer under the valve brings it closer to the float so it seats properly.
It is quicker and simpler than reading a fuel level, needs no running engine, and, set correctly, gives the same end result: fuel cutting off at the right point. The same top-cover-and-straight-edge principle, adjusted with washers, is a recognised technique across the Solex 32 PBI family.
When the level's still wrong: check the float itself
If the needle-valve height is right but the level is still off, the fault is usually in the float assembly, not the setting. Worth checking:
- A worn float, lever or pivot pin can give the wrong level even with a correct valve height.
- A leaking float takes on fuel, loses buoyancy and lets the level rise, flooding the carburettor. Shake each float and listen for fuel inside, and compare the floats' weights; a heavy one is suspect. A leak can sometimes be found by warming the float and watching for bubbles, and cured with a little solder, or by fitting a new float.
- A needle valve that works loose and rotates can stop cutting off the fuel altogether; and grit on the needle seat causes flooding. Both are worth a look if a chamber overflows.
Which method to use
For a carburettor known to be sound, the needle-valve method is the quick, repeatable choice. For problem-solving, or to confirm what the fuel is actually doing, the fuel-level method shows the real level in the chamber. Owners who have used both report they arrive at the same place. Whichever you choose, use one method consistently across all three carburettors so they match, and confirm the result with your engine builder: this is a fuel setting on a valuable engine, and small changes have a large effect on running.
In short
The Bristol's Solex carburettors have no factory float-level figure. Two proven methods fill the gap: Geoff Dowdle's fuel-level reading (around 16mm, via a modified main jet holder and clear tube) and Tony Byford's needle-valve setting (around 3.3mm, adjusted with washers). Both aim at fuel shutting off at the right level; if the level still will not come right, suspect a worn or leaking float. Set all three carburettors the same way, and confirm with a specialist.
Sources. This article draws on the experience of AC and Bristol owners and specialists in the AC Owners' Club community, and on published Solex carburettor literature.
A general guide to a fuel setting on the Bristol engine, not a substitute for the workshop manual or your engine builder. The figures here are named specialists' working settings, not a factory spec: confirm them for your car, set all three carburettors alike, and remember petrol is flammable: if a carburettor floods or will not set, stop and consult a specialist.
About the author
A lifelong classic-car enthusiast in North Yorkshire, near Thornton-le-Dale, who owns and maintains an AC Aceca, writing the single-model reference he always wished existed. Read the full bio.