Gas in crankcase? How is this possible?

[Pendulum]

Ok, so I was thinking about this issue this morning, and it occurred to me that this shouldn't be possible unless I'm not understanding something.

I've got VM carbs on my GS550, and when I was rebuilding them they were overflowing due to the float heights being off. When this happened, gas poured out of the overflows on the bottom of the carbs. So, how does gas end up in the crankcase?

[chuck hahn]

Petcock doesnt fully shut off gas as designed..the carbs fill up and slowly the gas trickles into the cylinders and seeps past the rings and into the crankcase.. just follow the arrows.

[Pendulum]

Petcock doesnt fully shut off gas as designed..the carbs fill up and slowly the gas trickles into the cylinders and seeps past the rings and into the crankcase.. just follow the arrows.

See, where I'm having trouble is the whole overflow tube aspect. I understand what you're saying, but... Why wouldn't it just come out of the overflows?

[mike_of_bbg]

CV carbs don't have overflows...

The other possibility is of course that there's a leak in the petcock diapragm and the fuel is being sent directly down the vacuum line.

[Pendulum]

CV carbs don't have overflows...

The other possibility is of course that there's a leak in the petcock diapragm and the fuel is being sent directly down the vacuum line.

Ok, that makes perfect sense! I think the chance of it going through the vac line are slim in my case since I can disconnect both lines and move pull the tank off, move it around, carry it, etc without more than a tiny drop coming out when I 1st disconnect the actual fuel line. Now I understand why gas in the crankcase is mentioned so often around here... The newer bikes just don't have the overflows.

[mrbill5491]

install a clear vacuum line and fire up the bike and watch to see if the to see if the gas is getting in that way.

[btbarb]

may only leak while running

Ok, that makes perfect sense! I think the chance of it going through the vac line are slim in my case since I can disconnect both lines and move pull the tank off, move it around, carry it, etc without more than a tiny drop coming out when I 1st disconnect the actual fuel line. Now I understand why gas in the crankcase is mentioned so often around here... The newer bikes just don't have the overflows.

I'm VM carbs too, my petcock never leaked while disconnected and the fuel port worked fine and didn't drip at rest. Once I put the clear line in, I could see about a teaspoon worth of gas in the vacuum line. I could tell my oil was thinning after hard running, but thought it was the brand. I never noticed a high oil level because I changed frequently enough trying to solve the non existent brand problem. Once I got on the right trail, I could smell it in the case too. Also explained the one rich plug I couldn't get right. New petcock solved it all. After the new petcock, I still sometimes see a little vapor in the vacuum line, probably temp or weather related, but nothing measurable.

If we all agree that 30 year old petcocks fail, the leaking into the vacuum line is just the first failure sign. Once the diaphragm fails bad enough, it won't cut fuel off when you're not running, which is when you'd notice a bad petcock for sure. Since VM carbs do overflow out, it seems a leaky diaphragm that still basically works could do more damage over time than one that just totally fails.