When you open the throttle the valve opens to full travel and it should open enough so it never regulates fuel. Pop off comes into play in the braking zone. If you have a full speed straight into a very tight corner the crankcase will have more time to load up as the engine winds down. This is why some will blip the throttle as they brake. If you like your low needle setting for power vs cooling but find you're sluggish getting back on the throttle after a long braking zone you can go up to 12 psi to shut the valve sooner.
Sundog,
I don't "think" a diaphragm carb works entirely like that. The inlet needle *must* regulate the fuel flow into the space under the diaphragm, because if it didn't... the mixture would be at the mercy of the volume and pressure the pump provides... which is not consistent.
There is of course no air under the diaphragm, only fuel. The engine is drawing a certain amount of fuel through the 2 fuel circuits... based on airflow through the carb and jet position. Assuming the kart is in a corner where the driver was off the throttle, the volume/space under the diaphragm fills with fuel and expands, and at some point, the inlet needle is fully closed. I believe, a
combination of: no airflow (pressure drop) past the high speed orifice, the fuel filling the space under the diaphragm and flexing the diaphragm upward, and the spring pressure under the fulcrum arm... are what determine how much fuel is in the chamber and the position of the diaphragm when this happens.
So let's skip right to going down a long straight at fairly stable rpm. Fuel flow into the venturi is "steady state", which I believe will cause the diaphragm to also be in a fixed/static position. That position will be determined by some combination of spring pressure and fuel draw. If the jet gets opened further, the diaphragm flexes downward a bit more, and opens the inlet needle just that slight bit more to match the incoming flow (past the inlet needle) to the outgoing flow (into the bore of the carb).
The transients (constantly changing fuel demand due to rpm and load) are what is super interesting. The carb is "dumb" though, and only knows how much air is going through the bore of the carb and consequently what the pressure drop is past the low and high speed orifices. Coming off a corner full throttle, is where (I believe) spring pressure makes a difference in "feel" on the track. Assuming the above is correct (yea... could be a big assumption, I admit, LOL)... it seems logical to assume that if the spring pressure is high(er), more fuel needs to be drawn from the cavity
before the inlet needle gets lifted off the seat (and I'm sure this all happens very quickly). Depending on a LOT of other things (like how "draggy" the clutch is, what type of exhaust pipe, blah blah), the spring pressure acts (to me) like it influences the rapid "transients" (transition from full throttle to off, or off to full throttle).
fwiw: I built a KT100 for a friend for the last VKA race we had out here on the west coast, and I set it up with 16 psi pop-off, and arm about .050" below the top of the gasket. Idled in the pits, was responsive to jet settings, and was very fast.
I've never been a big believer in the 2 turn low speed with the high speed just barely open. As you know, the Walbro has an angled hole that *should* lean the high speed fuel circuit at high air flow (dilutes it with air). Pretty much any 2-cycle engine will over-rev much better if it's on the lean side up at really high revs, and that "feature" gets negated to some degree if more of the required fuel is run though the low speed circuit.
Anyways.... great topic, and always fun to discuss.
PM