16 replies to this topic

[ivanhoew]

[Ethel]

That's a nice diagram,

it'd be interesting to see what data it's based on. The A Series might be a bit more tolerant than other engines, it may be a poor breather, but it's a very good burner.

The fuel/air ratio gets richer if you hold the piston down on an SU, hence the damper to slow its rising on acceleration. It works by the "venturi effect"; the faster the air flows over the jet, the lower pressure that sucks the petrol out of the float chamber.

Since the carb works on air velocity it needs help to get the mixture right with (blow through) forced induction, the usual method is put a restriction in the carb throat and feed the slightly higher pressure from before the restriction to the float chamber ( & topside of the piston in the SU's case) - the greater the boost, the higher the pressure difference across the restriction.

Ideally, you want the SU's piston to reach maximum lift just before you reach wide open throttle, that way you always have some control over the mixture and get the maximum airflow through the carb.