Petcock not keeping up?

When running at an idle for extended periods (stuck in in-town stop-'n-go traffic), I can see the fuel level in my fuel filter is much below where it should be. If I continue to stay in that traffic and don't change anything, I basically run out of gas.

This does not happen when running at any kind of speed or if I run on PRI, so it has me wondering if my petcock's going (again), if I'm running too rich at idle and the petcock simply can't keep up (wasn't able to color-tune when I synched the carbs), or if I simply don't have enough vacuum on #2 at idle.

Any thoughts on where to look first?

 

Try put it on pri & see if you expereince the same problem. If it is okay, you may have a leak in your vac line from the carbs to the fuel valve. My thought is you are pulling enough vac when accelerating/normal driving to compensate for the vac leak. Check this out & let us know what you find.
Gregg

 

Check your float levels and make sure you don't have a vacuum leak.

When you said your petcock might be "going again" what did you have to do to it before?

I've noticed that symptom too on occasion but I know my petcock is "lazy" at times (needs a new diaphragm) so I just run it on PRI for a while. A complete rebuild is on my to-do list.

 

Greg, I ran on PRI for a bit, and it's fine. Fuel levels in the filter are right where they normally would be. Short of replacing it, how would I check to see if there's a leak in the vacuum line?

Fitz, float levels are perfect (courtesey of Mn-Maxims). As for a vacuum leak, I replaced all of the seals in the carbs including the throttle shaft seals. I'll see if I can grab a propane torch and check to see if the boots have a leak (didn't think to do that yet). Now that I think about it though, if this was a vacuum leak, wouldn't it be revving higher and/or running leaner?

When I bought the bike the only setting for the petcock that worked at all was PRI. I picked up a rebuild kit right away and pretty much replaced everything in it. After that, it's been working flawlessly. My thought that it might be needing a rebuild again is from a possible concern about rust/sediment getting trapped in it, or that I didn't clean it out thoroughly enough the first time I had it apart.

I wonder - could having my Idle Mix screws too far out cause this as well? I wasn't able to Color Tune it after rebuilding the carbs, but otherwise the bike seems to be running perfectly fine.

 

Short of pulling the vac line blocking one end & sucking on the other I don't know how you would check it. Youcould try that or just replace it & see what the results are.

 

the unlit propane method should work on your vac line as well, perhaps just not as instantly. try it out, but go slow. if you have a small leak, it'll be sucking (since it's a vacuum) the ambient air into the hose, and eventually, into the cylinder. when this happens with the propane instead of air, you'll notice the engine react to it, usually by revving up a bit.

 

doubtful. A vacuum leak in a petcock vacuum hose big enough to have the propane trick work is gonna have to be so big the petcock probably wouldn't be working at all.

You might want to go ahead and REPLACE the vacuum line. It's possible someone replaced it with just "hose" and it's collapsing on itself.

When you rebuilt the petcock you did replace the diaphragm, right?

 

Fitz, if the line was collapsing on itself, wouldn't that also happen at higher revs, since there would be less time for the line to resume it's normal shape?

Yes, when I rebuilt the petcock, I replaced all the o-rings, the diaphragm, the black disk and I think even the spring ('s been a while so I'm not positive on the spring...). Petcock rebuild kit was OEM from my local Yamaha dealer.


Ok, any idea what to use for replacement hose and where I can get it? I figure the one that's on there is pretty soft to begin with, so low RPM distortion could partly be responsible for it. That, and I definitely think the line needs to be longer - I can barely lift the tank before the line runs out of slack.

 

Get vacume line from your local parts store.

 

when this happens with the propane instead of air, you'll notice the engine react to it, usually by revving up a bit.thread insert helicoil

 

You are also describing a vapor lock. If air would leak into the fuel line, then gas would leak out. Try another tank of gas from another station. (jmho).