Leaky Petcock after rebuild xj650

Hi everyone,
Petcock on xj650 testing it after rebuild with kit from chacal. Petcock installed on tank. Clear vinyl hose from fuel port to my gas can, and another from the vacuum port to my hand vacuum pump.
At pri, I see fuel flow.
At on, nothing, then when I apply vacuum I also get nothing - it's because my fuel level in the tank is below the reserve level.
At res, with vacuum I see fuel flow. Release the vacuum and fuel flow stops - almost.
I can see slow accumulation of fuel in the line.

Is this an issue with the seat next to diaphragm or wave washer petcock arm side, , or both?

Bob

 

So the issue is that when it should not be flowing (ON or RES, no vacuum) it is still allowing flow?

There is an issue with the tiny o-ring on the diaphragm peg not properly sealing against the seat in the body of the petcock. When you rebuilt the petcock, did you pay special attention to the condition of the "seat" in the petcock; polish it, etc? Did you install a stronger diaphragm return spring?

Have you read through this: https://xjbikes.com/forums/threads/rebuilding-the-yamaha-vacuum-fuel-valve-w-pics.25058/ ? It covers what I pointed out above, in detail. Keep in mind that if the seat in the cast body of the petcock is pitted beyond being able to be polished out, you'll just need to find another one to rebuild.

 

Hi Fitz,
Your summary is correct.
I did read your writeup, The condition of the seat looked good aka no pitting. I did install an M6 washer In the spring pocket to increase spring pressure, still had the slight dribble at no vacuum.
FYI I also read the comments to your writeup that say to follow the procedure EXACTLY so I have no excuses. Lol
I shall get to polishing it and report back
Bob

 

For the oring in question does it benefit from silicon lube (like orings/seals in card rebuilds) or best the leave dry?

 

I left mine dry, 7 years ago. Still doesn't leak.

 

I might have spoke too soon. I left the clear hose connected overnight with a big loop back up, to measure how much fuel leaked overnight, and there was none.
I am thinking that, once the vacuum is not present and the oring seats, the fuel between the oring and the fuel barb dribbles out over a minute. So it seals, and I am seeing the fuel that was already past the sealing point. Comments?

Anyways, having outed myself not polishing the seat, I took care of that.

BEFORE




After



Will test again with the hose tonight but I think I'm good.

Correction on earlier post- I used an M4 washer, not an M6 washer, in the spring seat.

For fun, zoom into the first pic to see yours truly.

Bob

 

That seat looks much better than it did. There is no benefit to greasing the o-ring, it will just get washed clean anyway as gas flows past it.

It sounds like you're on the right track. Be careful increasing the return spring pressure too much, if the vacuum can't overcome it the petcock won't work. I generally use an original-spec spring unless there is still an issue (usually intermittent) even after doing everything right.

 

I had a very similar issue and seem to have it resolved as well but the leak filled the crank case with gas before I got the leak off the bike. When I change the oil should I open up anything else on the engine to check for fuel where it shouldn’t be. Had drips coming out of the drive shaft boot at the rear of the engine. Popped that off and it was wet but not crazy. Want to make sure i don’t miss anything and cause damage after it’s “fixed”.

 

If all else fails you can try what Chacal suggested that I do. Place the o-ring side of the diaphragm inside a small socket. Then take a something like a socket extension and place on the spring side and very, very, very lightly tap it. The idea is very, very, very slightly deform the metal plate on the diaphragm making it seal a bit better. It worked for me about 5 years ago and I've had no issues since.

 

You just need to change the oil. What little contaminated oil is left won't be enough to cause harm. The remaining fuel will evaporate out of solution during the next ride.

 

I'd recommend another oil change after a couple hundred miles.

Also, keep in mind that a drippy petcock alone won't fill your crankcase with gas, the gas has to get past the floats as well. This does not necessarily indicate a float problem; floats are quite busy and depending on what they are doing at the exact moment the motor is shut off, they don't always slam shut. In a perfect world, we wouldn't need a petcock, we could just (theoretically) trust the float needles all the time. But the world is far from perfect, so here we are.

My point being: if you experienced a crankcase full of gas, don't implicitly trust your floats OR your petcock. Before every ride, take a quick sec to check your oil level. As long as it doesn't suddenly increase you're fine.