So I have this 1977 Honda CB750.  I’ve totally dismantled and rebuilt the carbs.  The bike starts and runs fine.  At throttle tip in off idle, it stumbles or just quits.  A lean-out condition caused be the accelerator pump circuitry not working.  It simply doesn’t provide any fuel to the tiny nozzles in the cabs. 

There are two tiny check valves in the circuitry.  One is in the supply passage to the pump diaphragm chamber attached to the bottom of the float bowl.  This allows fuel to pass one way to the pump.  The second check valve is in the top of a passage at the top of the float bowl.  This allows fuel to flow one way to the nozzles in the carbs. 

I have the float bowl supply check valve working properly, but the distribution side check valve only lets a tiny dribble to pass through.  I’ve tried soaking the bowl in various mediums, combustion chamber cleaner, gasoline, carb cleaner.  I’ve vibrated it for several hours in an ultra sonic clean (simple green/ water mixture) but it still won’t work.

Any suggestions?  Does anyone have a #2 float bowl to fit a ‘77 CB750 that I could obtain?

Is the check valve a little ball bearing that sits in a hole?

Sometimes a gentle whack on the ball with a pin punch will reshape the seat it rests in and make it seal again.

No, it’s a small sorta plastic looking thing pressed into the passage.  No way to remove it without destroying it.  It’s really small, like 1.5mm square.  Cannot hear a ball rattling, even in the one that is working. 

I’ve just dropped off the bowl at Burnaby Kawasaki.  Rick is gonna vibrate it overnight in their ultrasonic cleaner.  Barring that, I’ll be trying to source a #2 carb float bowl.

I think I mentioned this before.... at some point, Honda drilled an extra circuit in those accelerator pump covers to bleed off some of the accelerator pump pressure, resulting in more of a 'burble' than a 'squirt' - probably for emission regulations or to improve MPG.  I would have thought that bowl update was applied to carbs later than your 78 but maybe somebody has swapped the carbs?? You can see the difference in bowl covers in the attached image.  In any case, a common practice for bikes that had the lower pressure accelerator pump covers was to swap in a bowl cover that didn't have the 'bleed off' valve or to JB weld the valve hole to get increase the pressure.  I can't remember if it's my 900 or 1100 that came with the 'bleed off' style bowl but I swapped it for a bowl that didn't have it and honestly, even though a cleaner squirt was definitely visible (on the bench) my seat-of-the-pants dyno couldn't notice any difference.

Mine is like the bottom one in the picture.  That check valve is working.  It’s the one in that float bowl itself that isn’t allowing any fuel to get past it to the carbs.

The ones in your picture are on the supply side.  That valve allows fuel into the pump, then prevents back flow as the pump diaphragm is actuated.  This forces the fuel up a passage in the float bowl past another check valve before being distributed to the four carbs.  In order to effect a decent spray into four carbs, a pretty decent amount of fuel needs to get past that check valve.  Barely any gets past mine.  Not enough to even spray one nozzle.

So I just picked up the float bowl from Bby Kawasaki.  They have much better cleaning solutions than I do.  At any rate, now carb cleaner sprays through unimpeded. 

I’m waiting for the coffee to be finished then I’ll go out to the garage and reassemble/reinstall the carbs.  And we’ll see.

Well, I got it back together.  It’s better than it was, so I guess that’s something.  Ripped it up and down the lane a few times, only managed to get into 2nd gear.  We have speed humps in the lane so you can’t get much speed.  Plus, with the header that’s on it, it’s frickin’ loud.  Runs really good though.  I like it.  I may just keep it.  Just waiting now for a weather window so I can run a couple of tanks of fuel through it.