Topic: Carb Overflow Question - Why is My Leak Intermittent?

[B. Alec]

I have a question about the specific way one of my carbs is leaking - I know I can get in there and make sure everything's seated right, clean, and the float is set right (which will be rechecking work I did recently of course) but I'm curious to see if folks have an idea why the leak behaves the way it does so I can target my efforts.

I only notice the leak if I open the petcock while the bike's on its side stand.  It doesn't appear to leak when upright although it may and I just haven't noticed it. 

The leak is pretty heavy - a steady pour out of one of the overflow tubes, but it's intermittent. It leaks out for a few seconds heavily and then stops completely. Then after five to ten seconds it starts up again, then stops.  This would probably go on and on if I let it. 

So I'm imagining what's happening is the fuel level in this one bowl increases to the point that it starts overflowing, then once the level drops below the drain tube it stops until the level gets high enough to drain again.  But I'm confused as to why there would be this delay - why wouldn't it just be a steady leak that doesn't stop if it's just a matter of the float height or a stuck/dirty valve?  Wouldn't it even out so the constant amount of fuel flowing into the bowl would mean a constant amount of overflow?

Any thoughts? I feel like I'm missing something obvious here.

'76 550K...

[Gordon]

That sounds pretty normal.  Do you have a drain tube connected to the overflow outlet?  If so, there's probably a slight uphill curve to the tube before it goes over the back of the engine and down in front of the swing arm.  The fuel has to fill the lower part of the tube before it can spill over the curve. 

It's also possible the fuel level is rising just barely above the top of the overflow standpipe so the pressure has to build up enough to push the fuel through the little hole.

[TwoTired]

While on the side stand the float pivots slide over and rub on the float pivot posts.  If there is a burr on those the float pivot operation is erratic.  Also, the pivot pins may have have burs on them from tool marks near the ends.  Side stand use would make them side load over toward the burrs/marks.

How's that for visualization?

Cheers,

[B. Alec]

Both of those things make total sense.  It's likely the pivot posts - my overflow tubes are pretty straight up and down. 

I may just never turn the gas on unless I'm sitting on the thing.  I am so very not interested in opening those carbs up again any time soon. 

Thanks y'all.