carb leaking

 I've got a problem with a carb on a '78 SR500 leaking through the
overflow. But it doesn't appear to be the standard ones of the valve not
closing or the floats not floating.

The bike was modified when I bought it (25 years ago!).  It has a White Brothers
kit in it with
a different piston, cam, and exhaust, and the carb is a Mikuni VM36,
not the original.

There were no changes to the bike prior to the start of the leaking.

It only leaks while running down the road.  If I pull over and leave the engine
idling it
does not leak, even if I put the petcock on prime.  And yes, prime works
 - I tested it :-).  That would seem to rule out the valve.

When it first occurred, I did the standard things of replacing the valve, twice,
and
eventually rebuilt the entire float bowl.  Still leaked.

Someone suggested that there might be a crack in the float bowl.  I dried out
the float bowl
and filled it, first with gasoline and then with alcohol, and left it sit
overnight.  No leaks.

Someone else suggested that the overflow hose was siphoning gas due to its
placement in the
airstream.  I tried moving it to various locations and finally put it in
a bottle through a rubber stopper with a hole in it.  Still leaked.

Another suggestion was that the gas tank was pressurizing and forcing the valve
open so I
drilled a hole through the gas cap.  No effect.

The final suggestion was that somehow vibration was causing the gas in the bowl
to cavitate.
I removed the air cleaner and tied the carb to the frame to change any
vibration frequency.  That didn't work either.

In desperation, I bought a whole new carb.  Guess what?  It leaks the same way!

I've just about given up and decided to junk the bike or part it out on Ebay.  
But I thought I'd see if anyone on this group has had the same problems or
knows a possible fix I haven't tried yet.

Help!  Please!

 


Fascinating.
Back to basics.
If the overflow tube has a hose on it (that presumably extends down beyond
the frame) and it does NOT leak when standing at idle then:

1) How do you really KNOW that it leaks going down the road.......I mean
really.
2) Does it exhibit the same behavior with the hose OFF??
3) What happens standing still and revving the engine?  Revving in gear with
tire off the ground?

Seems like I remember a case of the leak being at or near the petcock,
running down the hose then around the back of the carb and finally dripping
off the end of the overflow hose.

 On Thu, 13 Mar 2008 18:08:44 -0500, Just Me wrote:


I can look down and see it coming out of the hose.  After I
proved that the hose wasn't siphoning, I positioned the hose where I could
see it without being a contortionist.


I haven't tried that, but what would it prove?


It won't leak.

  Revving in gear with

I haven't tried that yet but I will.  Thanks for the idea.  If it leaks
then, what have I proved?



Unless it's magically penetrating the overflow hose that scenario is not
my problem.  The gas is flowing INSIDE the hose.

Thanks for the response.

 


When you are grasping at straws, you don't want to miss ANY, no matter how
small.
Often a tiny piece of the puzzle may reveal the hidden image.
IOW, I don't know; just a wild idea. !!!  ;-)


Well, that's kind of the point.  Appearances can be deceiving.

OK, so plug the hose firmly for a while.
Why?  I don't know, just another wild idea.

 
Maybe the bowl is getting overheated? Maybe air is getting pushed into
the bowl vents? I am curious to know what you find... My 69 CD175 does
a bit of this, I think it is just the float bouncing around. Maybe
your float is too high?