Carb Vacuum Question

1979 KZ1000st Shaftie
Mikuni VM28ss

Can anyone confirm that the #3 carb will provide vacuum for my fuel petcock? The thread below succeeded in confusing the sh*t out of me. From the airbox side of the carbs, numbers 2, 3 and 4 all have brass nipples. I am assuming (yeah I know) that 2 and 4 are vents. I've removed the emissions stuff as it was falling apart. The carb intake boots (engine side) are capped currently. Clymer can't help me on this one.

www.kawariders.com.au/showthread.php?170...i-28mm-Carbs-Queries

You'll probably need the block off plates for the reed valves(on the head, above the exhaust valves) www.kzzone.com/billet.html

Plug/cap off any open vacuum tubing/lines as this can cause a vacuum leak, making the engine run lean, possibly overheating, burning pistons or valves.

Those ports on the air intake side of the carbs, the end facing away from the engine, are vents. You block those and the engine runs like crap and not for very long. On the engine side of the carbs, you will find four more ports, 1 each on the boots. Use one of those.
Rule of thumb, if it's on the airbox side of the carb, it's for air (or atmospheric pressure). If it's on the engine side, it's vacuum. Gotta have that difference in pressure between the two sides of the carb to get fuel into the cylinder.
All you need is a small amount of vacuum on the backside of the diaphragm inside the petcock to allow atmospheric pressure to force the petcock valve to open. Connect your petcock vacuum line to one of those previously mentioned ports and, if the diaphragm is in good shape, as soon as you turn the engine over there will be enough vacuum on that port to open the diaphragm.

DesertKZ wrote: 1979 KZ1000st Shaftie
Mikuni VM28ss
Can anyone confirm that the #3 carb will provide vacuum for my fuel petcock?...From the airbox side of the carbs, numbers 2, 3 and 4 all have brass nipples. I am assuming (yeah I know) that 2 and 4 are vents. I've removed the emissions stuff as it was falling apart. The carb intake boots (engine side) are capped currently....

Any vacuum nipple may be used for a vacuum petcock.
All unused vacuum nipples should be capped.
Vent nipples must exit to open air.

Good Fortune!

Okay sounds good gents.

On another note, I rebuilt the petcock and carbs, added an inline fuel filter. I can't seem to get fuel to want to feed into the carbs for some reason (I hope I didn't mess anything up).

With no fuel line connected to the tank, fuel runs out of the petcock freely in prime but not in "on". When I connect up the fuel line to the carbs, fuel won't flow out of the petcock anymore. Probably something really simple. This would make me think the vents are clogged?

If the petcock is set to ON or RES -->> Fuel should not flow when the engine is off, as a spring keeps the diaphragm in the closed position. When the engine is running, vacuum pulls open the diaphragm to allow fuel to flow.

PRI bypasses the diaphragm assy. Use PRI to prime or fill the carbs.

As for the vents, you can probably cap the #3 or #4 vent nipple.

martin_csr wrote:

DesertKZ wrote: Okay sounds good gents.

On another note, I rebuilt the petcock and carbs, added an inline fuel filter. I can't seem to get fuel to want to feed into the carbs for some reason (I hope I didn't mess anything up).

With no fuel line connected to the tank, fuel runs out of the petcock freely in prime but not in "on". When I connect up the fuel line to the carbs, fuel won't flow out of the petcock anymore. Probably something really simple. This would make me think the vents are clogged?

If the petcock is set to ON or RES -->> Fuel should not flow when the engine is off, as a spring keeps the diaphragm in the closed position. When the engine is running, vacuum pulls open the diaphragm to allow fuel to flow.

PRI bypasses the diaphragm assy. Use PRI to prime or fill the carbs.

As for the vents, you can probably cap the #3 or #4 vent nipple.

Turns out I crushed my fuel filter tube with the worm clamp. Whoops. All is well folks, all is well.

Thanks for the info on vent tubes/vacuum tubes.