Fuel Starved - Is this a Petcock Problem?

Just started experiencing a problem with my otherwise perfect running '78 KZ650.

The bike seemed as if it was being starved of fuel - it would intermittently "cut-out" under normal acceleration, but when I accellerated hard, it would be fine. This condition lasted for a few miles, then eventually I would have to hold the bike nearly wide-open to get a max of 50 MPH. After a few more miles, the bike stalled and would not restart. I switch over to PRIME and then RESERVE - the bike started and ran under the same lean fuel conditions. It would then eventually stall again and not restart.

I disassembled the vacuum petcock and checked for blockage - found none. When I poured gas directly into the fuel line, the bike would start and run, but would not start and run with the petcock in place.

Is this a condition that exists when the diaphram in the petcock fails? Igition system seems fine - it runs when there is gas, but the petcock does not seem to be delivering the gas.

PLEASE HELP!!!!

chopstar wrote:

...Is this a condition that exists when the diaphram in the petcock fails? Igition system seems fine - it runs when there is gas, but the petcock does not seem to be delivering the gas.

PLEASE HELP!!!!

I don't know anything about vacuum petcocks, except that the vacuum line must be vacuuming...
But I have one other question: Have you tried running it with the tank cap open to eliminate the cap vent as a source of starvation?
Also since you checked that the carb works when fuel is poured in, and the petcock seems to flow, have you checked that fuel comes out the lines (when removed) just before the carb (on prime or whatever)?

Post edited by: Biquetoast, at: 2006/08/21 10:20

I'll do some more research tonight. I just thought it was odd that the bike started and ran fine when I poured gas directly into the fuel line - sans tank and petcock. Hmmmmmm...

Post edited by: chopstar, at: 2006/08/21 10:42

SIMPLE QUESTION: Can a vacuum petcock fail and cause the lack of fuel flow to the carbs?

As with any technical issue, try troubleshooting to eliminate potential issues. The easiest way to troubleshoot YOUR problem is to place the petcock in the PRI position which essentially gravity flows gas to the carbs. If your fuel filter is clogged, if the line is kinked, if the tank has grunge in it... any of these things will cause the bike to run lousy regardless of whether it is in PRI or ON (or whatever it says for normal operation). If the bike runs like a champ with the PRI position selected, you have a problem with the petcock. Could be the diaphragm OR it could be that the vacuum line has lost integrity and is leaking, etc.