Petcock problem?

I have a problem with the fuel petcock. Bike ran fine one day, the next day wouldn't started. Fuel flowed nice on Prime, turn it on res and on, crank engine over, and it just dribbles out. Took petcock off, cleaned it, put it back on. Fired up, rode it about 25 miles and it felt like it dropped a cylinder. Then it got worse and felt like it dropped another. Pulled into a parking lot as it stalls and started figurin out why. Took line off and had the same problem. Put it on prime, fired up. Rode it home on prime parked it turned it back to the on. Two hours later took it around the block a few times (Few miles) and outta no where it started runnin rough again. Put it back on prime while riding and it went away.

I had the petcock off three times, and its clean as a whistle. Only thing I can think of is the o-ring is sticking, or vacuum isn't strong enough to unseat it? Everything is put together right (99.5% sure) and don't know if it could be something other than the petcock? Just wondering what you guys thought I should do next besides buy a new petcock. Thanks in advance

 

I run mine in prime and use a $7 inline shut off. Cheaper than a new one and I'm not too stressed about it.

 

It sounds like your not pulling the fumes from the tank good enough, try going for a test ride with your gas cap popped, if it keeps running with no problem, your vacuum line from the tank to the front carb boot must not be pulling the fumes out through the breather of the fuel screen / petcock going up into the tank.

If you have good vacuum on that hose at the end that connects to the underneath of the tank , then you may need to drain your tank and un bolt the petcocks full filler neck and screen out of the tank and clean it.
Also your cap might not be breathing, blast the key hole and everything with some carb or brake parts cleaner, and air blow out the cap real good.

 

If it is running on prime, then there is no problem with the gas cap. The vacuum line also does not suck fumes from your gas tank, it simply operates a valve in the petcock. Since you say you cleaned your petcock, my best guess is that there is a vacuum leak in the vacuum hose between the gas tank and intake boots.

 

I had the same problem when I first got my XJ until I figured out I was low on gas and it ran fine on prime or reserve. I have a bad habit of looking for the most serious problem first and it is usually the simple solution.

 

I do the same thing every time I accidentally hit the emergency off switch.

 

Yeah the vacuum line is fine I checked that out. It had enough gas in it, it seems to anyways I will fill her up and see if it continues to have the same problem. It only ran on prime though without problems, on and reserve it ran the same. I may just end up runnin it on prime when I ride, and turn it to the on when I'm not if thats the case. But I'm sure to forget. Thanks for the input I'll try her out when I get home

 

Ok let's review. Gas cap vent is clear, petcock has been cleaned, and vacuum line is not broken. Fuel dribbles out except on prime. So there has to be a fuel restriction. Inline fuel filter would cause problems at prime setting as would petcock screen in tank or kinked fuel line so that's not it.

Small hole in petcock diaphraghm would cause the problem. Bent plunger shaft on petcock would also. I would hold the diaphraghm up to a bright light and look closely for a small pin hole. Anyone else have a guess?

 

Yeah I will have to check the plunger shaft and the diaphram ..... I didn't look at it to close when I cleaned the petcock out. Thanks, because I didn't think of that and its a possibility.

Its not the fuel filter because I checked it before the inline filter