Floating an idea
QHow does the float level in the carburettor affect things like starting, flat spots and so on? You often mention it during your technical features about diagnosing problems with such things, but I’ve never quite understood why it’s so important. Does it vary from one type of carburettor to another? Miles Rowland, King’s Lynn
Nigel says:
AConsider a tube halffull of fuel. Above it is air. A hole exactly halfway up it will draw 50 per cent fuel and 50 per cent air. Raising the fuel level slightly will make the mixture drawn off richer; lowering the fuel level makes it weaker. A fixed jet carburettor operates on exactly this principle: raising the float level makes the carburettor run richer throughout its range of operation and lowering it makes it run weaker. The idle mixture is adjustable by a screw, but everything else is set only by the float level. Tolerance (see manual) varies: it may be as much as +/-3mm or as little as +/-0.5mm.
This is important: it tells you how much you can reasonably adjust the level to try to correct any issues.
Older variable-jet SU and Stromberg carburettors have a jet that can be screwed up (weaker) or down (richer) to adjust the mixture strength across the whole range of operation. In this case, float level is not so critical, as the jet can compensate for any inaccuracy in setting. Later types have less or no jet adjustment (for emissions control), so correct float level is essential.
A low float level gives the following symptoms: hesitations from rest and/or at points in the accelerator travel; a loud induction boom with no further acceleration for the last portion of the pedal travel; pinking; runningon after shutting the engine off; ghostlywhite spark plugs. Note that inadequate fuel supply to the carburettor will cause the float level to drop when the engine is working hard.
A high float level causes a strongsmelling exhaust; chugging or shunting under light throttle; lethargic performance; bad fuel economy and black spark plugs.
Hot starting may be slow or difficult.