Fuel leak

[JE4570]

Hello all,
Ok so now I need to tackle a fuel leak.
My bike runs great (of course when it’s electrical system isn’t frying coils and ignition systems). I have had a slow gas leak for a while usually ok while riding but definitely to the point where I need to shut off the petcock after parking. The other day a fello 2 wheel enthusiast pulled up beside me at a red light and informed me I was leaking fluid which was no doubt my fuel leak. I also notice my gas tank drains a lot quicker than it ought to be.

Before I blame the carbs which seem to be running the bike just fine, I do notice a hiss from the gas cap. Could a gas cap seal vacuum leak be causing my issues?

My petcocks (one on each side) appear to be the non vacuum type and I believe I have BS38 carburetors.

I mostly want to rule out vacuum problems from gas tank before I have to take the carbs off since it runs well right now. But obviously my next place to look is the float and shut off needle.

Overall I really just want to know if a vaccume leak from gas cap could be causing fuel to leak into air boxes through the carbs even if the carbs shut off valve was working properly?

 

[WBacon]

Hi JE, I'm certainly no carb guru but I'm sure there's no way for your gas tank is causing your leak. It's a simple gravity feed system. I think you need to look in the float bowls.

 

[gggGary]

Blocked gas cap vent path is a known issue, IF the rubber cap/tank seal has been replaced and isn't still a sieve of air leaks.
The vent path through the cap is a labyrinth type deal, the small cross section hole/path passes though a gasket, blockage is common there. This can create both pressure and vacuum issues. The steel on brass BS38 float valves are a bit of a known problem area as well. I have some viton tipped needles on order that "may fit". Rubber on brass like used in the later BS34's is more likely to stay drip proof. When checking float valves pay attention to the springs in the needles, weak spring action is a "toss the part" condition.
Shutting off petcocks when parked is SOP on all manual petcock gravity flow fuel systems. The smallest amount of particle contamination can easily cause any float valve to leak.
Any old rubber fuel lines MUST be replaced. Fuel leaches the elastomers out of the rubber line causing the now brittle inside to shed in fine slivers, the perfect shape to wedge float valves open.....

 

[Goldenboy]

Blocked gas cap vent path is a known issue, IF the rubber cap/tank seal has been replaced and isn't still a sieve of air leaks.
The vent path through the cap is a labyrinth type deal, the small cross section hole/path passes though a gasket, blockage is common there. This can create both pressure and vacuum issues. The steel on brass BS38 float valves are a bit of a known problem area as well. I have some viton tipped needles on order that "may fit". Rubber on brass like used in the later BS34's is more likely to stay drip proof. When checking float valves pay attention to the springs in the needles, weak spring action is a "toss the part" condition.
Shutting off petcocks when parked is SOP on all manual petcock gravity flow fuel systems. The smallest amount of particle contamination can easily cause any float valve to leak.
Any old rubber fuel lines MUST be replaced. Fuel leaches the elastomers out of the rubber line causing the now brittle inside to shed in fine slivers, the perfect shape to wedge float valves open.....

I've always shut the petcocks and went so far as to disconnect the lines from the tank , but I was always under the impression that changes in atmospheric pressure dumps the bowls with the Venturi system, because they wouldn't leak right away, and sometimes just dump the bowls all at once. I rebuilt my petcocks and I just replaced the fuel lines, tank to carbs. After all those years, they were like hard plastic. Also just replaced the cap gasket, but it's a flat washer type. Are you referring to the toilet bowl float looking cap gasket on newer bikes?

 

[DaveO]

Seems logical to me that if the gas cap breather is blocked it can cause increased pressure if you are riding on a nice sunny day.

 

[5twins]

First and easiest check would be on the petcocks. Remove the fuel lines and see if the petcocks drip or dribble in the "Off" position. If you have a bad float needle and seat, you can still get a small amount of fuel leakage with the petcocks off after the bike has sat for a while. All the fuel in the lines could end up draining into the bowl(s) and that may be enough to overfill them. I had a bike that did that.

 

[JE4570]

I just pulled and cleaned the carbs. The floats were in good shape too. I raised the float height 1mm so they shut off a hair bit sooner. It seemes to have stopped the leak so far. I have a paper under the carbs. we will see how they fair. They werent so bad inside and were relatively clean. I just soaked the the float valve well. Time will tell. Of course now I probably need to tune my carbs up again which I cant do until I get my ignition parts in the mail.

 

[JE4570]

Well shoot I spoke too soon. I think I may replace the valves and see if that helps. I also will check the petcocks

 

[Goldenboy]

First and easiest check would be on the petcocks. Remove the fuel lines and see if the petcocks drip or dribble in the "Off" position. If you have a bad float needle and seat, you can still get a small amount of fuel leakage with the petcocks off after the bike has sat for a while. All the fuel in the lines could end up draining into the bowl(s) and that may be enough to overfill them. I had a bike that did that.

This sounds just like something else...

 

[JE4570]

Update on this issue: I just replaced the float needle in the leaking carb. So far no leaks, but we will see with time again. Dont want to speak too soon like last time.