Try our new info resource - "Aladdin's Cave" (Main menu)Just added a separate link to Ash's Dropbox thread (shortcut)
There should be a non return valve in the accelerator pump body (the removable cap) that prevents fuel flow until the diaphragm is pumped, could that possibly be stuck slightly open. They are still available from CMS. https://www.cmsnl.com/honda-cb750f2-super-sport-england_model14689/diaphragm-pump_16021393004/#.Xw6z8puSmUlThere are no restrictors as such in mine just small bore brass tubes inside rubber tubes (I assume to prevent the rubber tubes collapsing). There's a small air vent in the side of the carb bodies, if one or more is blocked it's impossible to get even running. The vent is hard to see and cannot be cleaned with carbs assembled. Also, I have never seen that vent mentioned in any manual. My F2 runs spot on after the second carb rebuild to clean the vents, where previously it would not balance up at all. And don't forget, the air screws work in the opposite direction to earlier 750's.