TOPIC:

Hello all. My 1978 KZ650C seems to be having fuel starvation problems. I have a vacuum assist type petcock and when it is turned to on I don't seem to get adequate flow and the engine seems to be starving at higher revs. I pulled the petcock out and it was clean. The fuel appears to flow when it is turned to prime judging by the amount of fuel in the filter. Is it possible that I do not have enough vacuum to open the diaphragm enough? I am running pod air filters and a short pipe exhaust. Any reason why I can't just leave it turned to prime and run that way? Seems like it would be the same as if I had a manual petcock. Thanks,

Was this symptom present before installing the pods and shorty exhaust?

It takes very little vacuum to open the valve. Perhaps the diaphragm needs to be replaced.

Yes, you can just run it on prime as long as the float valves are working perfectly, but that is not solving the problem.

You can definitely switch it to prime. It's a good way to rule your tap out as a problem. It's not the best long term solution though. It's not going to damage anything, but if you forget to turn it off after a ride and your carbs aren't 100%, they can overflow.

Switch it to prime and go for a ride. If that fixes your problem, first check the vacuum hose. If they get a bit old they can reduce the vacuum, restricting fuel, and make the carb it's attached to run a little lean. Changing the hose fixes this.

If you do use it on prime (like a manual petcock), do switch it off after riding.

You won't have reserve. When you run out I could hear your yelling from here.
Steve

Perhaps dirt or crud inside the peacock is clogging operation of the diaphragm o'ring spring mechanism. Would remove, disassemble, and inspect the parts, clean, replace any damaged parts (especially o'ring or diaphragm). Kits and parts are available. Check with Z1 Enterprises (see vendor button at top of screen) and ebay and other sources.

Can suck carb end of vac hose to provide vac for testing whether there's fuel flow thru petcock set in a non-prime position.

Good Fortune!

TexasKZ,
The pods and short exhaust were on the bike when I bought it. It was sitting for awhile when I was dealing with some clutch issues. It ran fine prior to that. I did put on a new in-line fuel filter. When it's on prime you can see the fuel run into the filter but when it's turned to on it seems to just run dry. I did disassemble it and it looked clean inside. I'll check and see if I have enough vacuum. Thanks,

Your carbs need cleaning, pay special attention to the pilot circuit.