I just pulled my XS out of storage after 8 years, started going through it. I checked the oil, had the right amount of clean oil, did a tuneup, and ran it for about 10 minutes. I decided to change the oil and found the oil mixed with gas, a LOT. I went through the carbs, cleaned them, nothing wrong there. Went through the gas and vacuum lines, no problem. And .... I'm stumped; how can gas get into the oil? What am I missing?
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gas in oil
- Thread starter JaysXS650
- Start date
How does your engine sit, front to back? Is it level? Or do the carbs slope down towards the front?
Like William alludes to above, there's only one path for for gas to get to the oil... through the carbs.
If you leave the fuel petcock on, or it leaks past when off, then you have a steady supply of fuel to the carbs.
If either float valve leaks, the bowls will overflow.
If the engine is pointed downhill, even ever so slightly, that overflow will run down the intake, past a partially open intake valve and into the cylinder.
From there it leaks past the piston ring gaps and down into the engine sump.
It's never happened to me but I've read that it will fill an engine overnight.
So to sum it up, you have one or both petcocks leaking when off... and one or both float valves leaking/stuck open. It takes both conditions to get to the oil.
If you leave the fuel petcock on, or it leaks past when off, then you have a steady supply of fuel to the carbs.
If either float valve leaks, the bowls will overflow.
If the engine is pointed downhill, even ever so slightly, that overflow will run down the intake, past a partially open intake valve and into the cylinder.
From there it leaks past the piston ring gaps and down into the engine sump.
It's never happened to me but I've read that it will fill an engine overnight.
So to sum it up, you have one or both petcocks leaking when off... and one or both float valves leaking/stuck open. It takes both conditions to get to the oil.
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Thanks so much for the info. I've worked on bikes for years, this one was a real head scratcher.
I'm going to rebuild/replace both petcocks.
I did check the floats for buoyancy and cleaned the float valves.
The carbs are as level as I can make them. The engine's never been moved, but it does seem to tilt forward a bit.
And I have one more question: I have a '78 XS650 with two lines to the petcock, one gas, the other (I think) to pull gas fumes from the tank to the engine, after the carbs. Is it possible for this small line to siphon gas directly to the engine? Will capping this small line make a difference?
I'm going to rebuild/replace both petcocks.
I did check the floats for buoyancy and cleaned the float valves.
The carbs are as level as I can make them. The engine's never been moved, but it does seem to tilt forward a bit.
And I have one more question: I have a '78 XS650 with two lines to the petcock, one gas, the other (I think) to pull gas fumes from the tank to the engine, after the carbs. Is it possible for this small line to siphon gas directly to the engine? Will capping this small line make a difference?
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Best check for floats is to push 'em down into really hot water and see if any air bubbles stream out. Heat expands the air inside the floats and if there's a leak, you'll know almost immediately. Do a site search here for Viton tipped float valves.
And double check the float levels. Don't remember off the top of my head what the level is for your bike, but that info's in the book.
We're actually referring to the stance of the bike. Parked downhill is what causes any leaked fuel to flow downhill into the head.
If there's two nipples on the petcocks, they're vacuum operated. Engine vacuum pulls 'em open, engine off closes 'em. The smaller of the two nipples is the vacuum line.... goes to the nipple on the intake boots. I suppose it's possible the vacuum diaphragm can leak causing fuel to enter that way, but I've never heard of that happening in real life. Do a site search for vacuum petcocks. There's info here on rebuilding 'em. I rebuilt mine about 4 yrs ago and they're still holding up.
Finally, click on the Tech button at the top of the page and get to know it's contents. There's a wealth of knowledge in there... all written by us forum members, based on real world experiences. Not much will go wrong on these bikes that isn't covered there.
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Oh... and welcome to the madhouse!! 
