John, would I be correct in assuming that you are referring to the air correction jet, which is the only replaceable air jet in the Amal, by being able to adjust the jet orifice diameter, the fuel curve can be modified for cleaner running toward top end revs.
Air moves with ever increasing velocity through the carb as engine revs escalate, in doing so it loses pressure and progressively reduces in density; fuel density however remains constant. The result of this imbalance is that fuel mixture becomes richer by proportion with the increase of air flow velocity. This effect requires correcting or the engine would just simply bog down with excess fuel, so the air jet introduces an air bleed around the needle jet to maintain a more consistent air/fuel ratio, and can be used as a tuning aid in its own right.
All jets, slide cutaway and needle overlap in their mutual transitional influence, and the corrector air jet can be changed in diameter to increase or reduce air flow that adjusts main jet fuel and air mixing.
Overriding this is of course is the nature of engine breathing, in particular pipe influence towards peak torque and into over rev, and the effect this has on the demands made of the carburettor!
It might be worth illustrating the 125 RS Honda, failure of the electric solenoid power jet to switch off correctly prevents the engine from revving much beyond 12,200rpm, with the jet off revs increase to 13,500, such is the effect of over rich fuelling at high engine revs.
The Aprilia has a more sophisticated arrangement in that its power jet has linear switching that is progressive with rpm and pulses fuel several times a second making for a refined mixture curve all controlled from the engine management CPU!
Trevor
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Mon Mar 21, 2016 5:10 pm