Leaky petcock -- how does vacuum leave petcock?

I've never seen a check valve between the carb and the petcock.
Steve

Hmm, well then, I suppose I'll try clearing out that passage. At worst I'll need a new back plate and in the meantime it'd turn into a non-vacuum petcock.

If the diaphragm has a leak, gas will get into the intake port. You can block off both passages and run on prime. I'd buy a new one.
Steve

Just an update on tracking this problem down. It turns out there is a check valve that's supposed to be there. It seems, according to the KZ750 'bible', that it's supposed to allow neutral pressure back in, just not positive pressure. I'm thinking that's worn out, and buying the whole back piece, since I can't buy it separately.

:huh:

The check valve is just a little ball or BB, so air flow thru the square vac plate is uni-directional or one way.

Remove the square vacuum plate, attach tubing to the vac port, then suck on the tubing --- you should be able to suck air. Then try to blow air thru the tubing --- the BB should block the flow of air. I would go ahead & clean the thing, such as spraying carb cleaner thru the little holes.

Then clean & inspect the rest of the petcock. If no pitting or wear scratches or anything, get a rebuild kit. Some guys prefer oem. I installed a K&L kit (made in Japan).

My 550 had a vacuum petcock. I replaced it with a manual On/Off/Reserve petcock. I think I paid less than $15 for it.

The check valve definitely should not hold a perfect vacuum in the chamber. It needs to equalize somehow.

I believe it's a plastic disc in this case, but I'll have a look today.

It's a plastic disk. never bothered to check before. no need.
If air flows one way, then the problem isn't the vac cover.
The diaphragm can look good but still be bad. After cleaning the petcock body n stuff, I buffed the mating surfaces w brass wool. steel wool would work.

I had to go out and take one apart to see this check ball deal. Sure enough, there is one in the vacuum chamber inlet. That being true though, I would look elsewhere for the problem.

The real problem is having a vacuum operated petcock, :blink: find a manual to swap it out with, problem solved for years..... :woohoo:

ezrider714 wrote: The real problem is having a vacuum operated petcock, :blink: find a manual to swap it out with, problem solved for years..... :woohoo:

+1 My '77 KZ650-C1 came with the manual petcock and I would never swap it for a vacuum one; they simply have too many problems. Ed