well when mine quit working, really had no issues except at the lower mid range and throttle roll on, it sorta did not like it, now if i dropped some gears, the extra rpm would make it go away. the issue is flow is low and when the throttle is rolled on, the manifold pressure will drop and flow falls off rapidly and the engine leans out and bucks the process. with a little squirt of fuel, the engine doesn't drop out. due to my configuration, i have to run on the rich side because of high speed lean out, for that reason i also do not run a vacuum petcock as sustained high rpm and manifold pressure drop will pinch it off.

 

 

well mr. j if you are talking absolute pressure, you are correct, the term should have been it will go atmospheric. there must be a diff'l in the venturi in order for the fuel to rise. when you slam the carb fuel flow will be altered and any fuel already mixed will have a tendency to drop out, on a short runner like the sportster or bt not so much of a big deal but with the lean out, there is a following of a rich mix when the fuel that drops out gets pulled into the engine. the accel pump helps over come the issue. now throw in a vac slide, other issues can arise. the reason i like the VM series carb. the engine is a positive displacement pump altered by rpm it is finite, not infinite. unless of course you have a variable displacement engine. in days of old, we used to cork the cranks on two cycles to boost the charge.