Quote:
Originally Posted by tpelle
Yes, I know that the petcock has a vacuum valve that will shut the fuel off when the engine stops, but this might fail and possibly overflow the carburetor float bowls, which could cause gasoline to eventually contaminate the crankcase oil and cause serious damage to the engine. So getting in the habit of switching the petcock to "off" on shut-down is a good habit to acquire.
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The first point you made actually happened to me after a long ride with my friend. We parked our bikes in a parking space that was set at a slight incline and went into a McDonalds to get some grub. We came back out and I tried to start my bike - it kept cranking but didn't start. My buddy said the bowls were flooded and told me to turn the petcock to "Off" before starting it again. Luckily it started and I was able to ride off without issue.
I don't understand your second point about oil contamination though. The only way it could get into the oil is through a combination of bad rings/seals and gas accumulating in the cylinders via a chronically over-rich condition, but the latter would be unlikely as the excess gas would get blown out of the exhaust at the completion of each power stroke. If the former happened, the bike would run poorly and would show outward signs of that. Maybe I'm missing something though.