Yamaha fuel leak problem

 
I have a 1968 Yamaha 500 single, fairly extensively modified.  I've owned it
for over 22 years and it's been pretty trouble free, but now it has a problem
I can't solve.

It has a Mikuni VM36 carb.  A while back, it started leaking out of the
overflow outlet.  I took it to a mechanic who had some experience with
modified SR500s (he used to race them) and he replaced the float shutoff
valve (twice) and adjusted the float level down a little.  Still leaked.

He said it was probably siphoning.  That didn't sound to likely to me, as why
would i suddenly start that, but just in case I experimented with various
lengths and positions of the overflow hose. Still leaked.

I completely rebuilt the innards of the float system,  Still leaked.

At first the leak was occasional, but over time got worse to where I couldn't
ride it a city block before it started.  It doesn't leak at idle or at low
RPMs.  A few times when it did this, I pulled to the side of the road and put
the vacuum petcock on prime so gas would freely flow into the carb.  Didn't
leak under those circumstances so I think I can rule out the shutoff valve.

Finally, in desperation, I broke down and bought a new VM36 carb.  Guess what?
It does the same thing.  OK, it cost me over $150 but now I KNOW it isn't the
carb.

My thought at this point was that the gas tank was pressurizing and forcing
the shutoff valve open.  I drilled a 1/16" hole all the way through the gas
cap and when that didn't work I rode it with the gas cap open.  Still leaks.

I'm just about out of ideas.  The entire system consists of gas tank, petcock,
fuel line, vacuum line, inline filter, and carb.  Since the tank, lines, and
filter are passive components, and I've ruled out the carb, that leaves the
vacuum petcock.

Is it possible for an aged (28 years old) vacuum petcock to cause this
problem?

Does anyone have any other ideas?

I've got a classic bike in great condition that I can't ride and it's driving
me nuts.  Please help me solve this problem.  Please!

PS:  If you'd like to see the bike, look at:

http://www.intergate.com/~lard/motorcycle/SR500.htm

--
Homo Sapiens is a goal, not a description.

--
It's turtles, all the way down

 
Did you re-use the bowl from the old carb?

If so, the standpipe in the bowl has rotted through or has a
hairline crack in it.   This allows the fuel to drain out from
the overflow.  

Jim


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 jim rozen wrote:


No, I did not.

--
It's turtles, all the way down

 

If this is really true AND it is consistently repeatable, then.........

1) I don't know why but the problem is with the valve and/or vacuum line.

2) It hurts nothing to ride in Prime (except you probably lose the reserve,
if you have one).
    If that IS the problem, then it is also likely that the fuel is NOT
turning off completely regardless of what position the handle is in.

 Ken Abrams wrote:


Could you please explain how the vacuum line could overcome the shutoff valve?
 

I guess I wasn't clear on that.  I wasn't riding with the petcock in prime.
I'd be riding down the road and the gas would start leaking.  As soon as I
saw that, I'd pull to the side of the road and idle, whereupon it quit
leaking.  I then put the petcock in prime on the assumption that if the
leaking had been caused by the shutoff valve not closing, i.e. something
stuck in it, that the prime setting would cause further overflow.  It didn't,
so I'd put the petcock back to normal and ride off.  As soon as I got to
about 2500 RPM or better, the leak would reoccur.

However, just for the heck of it, I'll try riding in prime and plug the vacuum
line.  It's worth a try.

--
It's turtles, all the way down