Fuel Leaking from Carb Bowls

Finally finished bobbing by 1980 XJ650 Maxim several weeks ago (pics to come), to find myself with a fuel leak - with the carb bowls as the source. The issue arises only when the petcock is left on PRIME for more than several minutes, and seems to subside if set to vacuum. My guess is bad bowl gaskets - as they've never been replaced to my knowledge - so I'll be rebuilding the carbs here shortly. Nevertheless, should I be considering at any other potential sources of this problem?

Background - I cleaned and re-jetted the carbs (42 fuel pilots, 120 fuel mains, to account for intake and exhaust modifications) over the summer, and all floats appeared in good condition and (I believe) set at proper height.

Thanks guys.

 

If the floats are in order I don't think it`s the gaskets. Make shure the gas dosen`t come from the connections. (But yeah replacing the gaskets is probably a good idea anyhow)

The reason it only happens when you have it on prime is because you bypass/open the vacuum operated valve in the petcock when in prime, so it`s the same as leaving the water on in the shower.

Wipe clean the carb rack and everything around it, remove the airbox connections, or pods, and turn on prime and take a good look where the gas comes from,

 

One or more of the float valves and needles are leaking. You can try polishing the valve seats and the edges of the needles if the rubber tips appear to be soft, pliable, and not deformed. Or just buy new valve seats and needles.

I just did that to mine and it stopped the leak while the bike is parked, although I'm not sure it stopped the leak while running since my gas consumption is still high. I also cleaned the rubber tips with a q-tip and carb cleaner, but if you do that immediately clean the tips in gas to prevent any damage. I'm running a tank of gas now just to check the consumption and then will replace all the valve seats and needles if the consumption is not up to normal.

 

My 2 cents - I thought I had a similar issue however after several attempts to solve leaky needles and poor float bowl seals I discovered that one of the o-ring seals on the small tubes that interconnect the fuel supply to the carbs was leaking. Bummer because the only way to really fix this is break the rack up....

The kicker was that I had already wet set the floats and they were dead on and upon rechecking this - again right on, the logic of what was actually happening escaped me...

 

how did you find the leak? Did you smell gas or actually see gas leaking.

 

This was tricky, one day it was just a faint smell of gas in the garage, then a couple of days later I noticed a few drips after a ride, then a couple of mornings later I was about ready to start the bike to head to work and I had a puddle of gas on the driveway. Needless to say I didn't ride that day. After several frustrating attempts to find the leak, I ended up hooking the carbs up to the gas tank with the carb rack completely off the bike. Thats the only way I could see where the leak was. With the carbs on the bike, it's really hard to see in between the #2 and #3 carbs.

 

That sounds like a MAJOR PITA! How did the rack breaking go?

 

Yep it was a bummer but breaking the carbs wasn't as bad as I was expecting. Lots of good info on the site and a great tutorial on doing it. I did have a friend help me and that person also had access to a homemade vacuum gauge, something I will build one of these days....