gas smell in crankcase (78- GS550)

[rthor]

Ever since I read about getting gas in the crankcase I've been on high alert hoping to prevent a potential problem. I've rebuilt my petcock (stock vacuum type), installed new needle and seats, set float heights to 25mm as per factory specs (with gasket in place). The bike is running great, but....here is what I'm noticing...
When the bike is stone cold I take the oil reservior cap off and take a whiff from the crankcase; smells like oil and nothing else. After a good long ride, (hour or so) I take a whiff from crankcase; major gas smell... After a complete cool down I smell the crankcase again and it's back to the smell of oil. Do I have a problem here or am I looking for one?

[spyug]

I doubt you have a problem.

Check the oil in the sight glass if it exceeds the maximum line then you could have gas in the oil. To be sure, drain off a little into a cup and stick your nose in . Rub some between finger and thumb it should feel thinner than virgin oil.

If it does seem contaminated you should replace the petcock. Don't try a rebuild, they seldom work.

If there is gas in the oil it will stink cold or hot so I'm thinking you're smelling gas from carbs that are too rich or unburned gas from exhaust.

Check a little bit deeper and see where you stand. Let us know what you find.

Cheers,
Spyug

[GavinJuice]

sounds like blowby

[rthor]

It's been a while since I checked compression, but my last check yielded these numbers, (100, 110, 105, 120). The valves are adjusted to spec so I'd say my lousy compression is likely due to rings. When I aquired the bike it had been poorly stored for some time, (probably around 15 or more years). I would expect that there's a pretty good chance that the rings are seized up in a few places. Blowby seems quite likely. Maybe this winter I'll do a top end tear-down and free up some of those sticky rings.

[rthor]

I drained a bit of oil into a small container and verified the gas contamination. The oil is still viscous, but somewhat thinner than virgin. So after all of my efforts to stop the contamination, looks like I'm going to have to open up that petcock again - this time I'll be looking for pin holes in the diphragm. In the mean time I'll change the oil (again), plug the vacuum line from the carb and run it on prime.

[alke46]

You mention that you set the floats to 25 mm "with gaskets in place".

I thought my manual stated the float height is set with the gasket off of the bowl. That would change your float level. Could that cause a problem getting gas in the oil?

[rthor]

In the Clymer manual it states a 26mm float height without gasket and 25mm with gasket. (I kind of like to go with the factory specs). And in my limited knowledge of carbs and float heights, I woudn't think that an incorrect float height will cause gas to flow into the crankcase. I would expect that an overly low setting would result in a fuel overflow out the bowl overflow tubes... (but let me emphasize that I am no authority on this matter).
But with my new needle and seats, and everything else more or less up to specs, I'm going to try other experiments to isolate the problem. I'll have to report back on this later however, as it's way too cold for shadetree repairs up here in the great white north.