Topic: Where did my fuel go?

[xruss]

I've held off asking this question for some time for fear of being informed I'm an idiot, but the time has come. I went through the motor last year - carbs, electrical, etc. - to make sure she'd run before doing a full tear down to get it all in proper shape. I threw about a gallon of gas into the tank to test it. I ran it for just a bit, but not nearly enough to burn a gallon. I left the gas in it and began working everything else over the last six months or so. I pulled the gas tank to move it and found it was bone dry. I never saw a single drop of leaking fuel in the time I picked it up with gas from the PO (about a year), nor did I see any in the time I had the tank on the bike to work on it with fresh gas. So, where did it go? Is it possible the seal on the filler cap is bad and it evaporated? Is there a slow leak that I just didn't notice? The PO lined the tank, although there are a few spots where the lining appears to be coming up. This may be from a gas nozzle scratching it off directly under the filler. Thoughts? Is there a way to pressure test the tank at home?

[ekpent]

Seems if it evaporated out there would be some sticky brown gummy substance in the bottom of the tank. Do you see any old tar down there.Since you never noticed or smelled any leaking you might be able to rule out the petcock leaking. Do you turn it off ? Somebody maybe 'borrow' the gas that was in there ?

[Johnie]

Yes, you would see something in the tank...discoloration and smell. I think someone needed gas for the mower.

[bollingball]

Could it be in the oil?

[xruss]

Bike is in the garage and I'm the only one who would mess with it. Petcock was closed. I'll see about taking a look down into the tank to look for remnants. If it did evaporate, that should just require a new gasket on the cap, correct? It is still the original cap with the pin still intact, so I know it isn't an issue of an incorrect cap.

[Gregorymoto]

Your roommate stole it?
Don't make it complicated man.
Put gas in it see if it leaks, change your oil. If it has gas in it, well that would odd, but you can go from there if there is.
No need to pressure test the tank.

[xruss]

My roommate is my wife and she certainly has no interest in touching anything moto related. I've drained the oil since to pull out the motor...no separation that would show gas.

[Gregorymoto]

My roommate is my wife and she certainly has no interest in touching anything moto related. I've drained the oil since to pull out the motor...no separation that would show gas.

LOL. That was a joke. Why pull the motor?

[thirsty 1]

If it was say a 1/2 gallon I guess it could evaporate. I occasionally  leave the petcock on my 400f on and it makes a nice little spot on the ground (overnight). If you had a left your gas on and one of your carb's doesn't wanna seat it could have run onto the ground?

Either that or you ran it dry during your last ride and you didn't realize it? 

[Johnie]

I don't know man...I had gas in the tank of my K3 that was in there since 1986 when it was run last. It did not all evaporate out. There is no way a gallon would evaporate in 6 months with the lid closed even if the gasket was bad. Leak yes, evaporate no. Let us know how you come out and what you find.

[xruss]

Gregory, I'm pulling the motor out, but not breaking it down. I was happy to get it running again after it sat for a few years unloved. No sense messing with what's working, engine wise, but I want to paint the frame to stave off the rust. And, I've been replacing just about everything else that's worn over the years.

[DH]

I'm gonna guess here....if you have a fuel petcock that doesn't shut off correctly and allows a slow drain, AND one or more carbs with float valves not shutting off completely for whatever reason...
It is possible for the fuel to slowly flood the engine due to high fuel levels. Sometimes fuel will weep into the airbox (750) or plenum chamber (500/550) or into the intakes(?) in which case it'd end up in the engine and cause a high oil level. Usually flooding carbs will piss out the overflow tubes that are attached to the bottoms of the
float bowls. but if the overflow tubes are clogged, well,, its gotta go somewhere.  Again, just a guess. Any other thoughts from the experts?

[bollingball]

I'm gonna guess here....if you have a fuel petcock that doesn't shut off correctly and allows a slow drain, AND one or more carbs with float valves not shutting off completely for whatever reason...
It is possible for the fuel to slowly flood the engine due to high fuel levels. Sometimes fuel will weep into the airbox (750) or plenum chamber (500/550) or into the intakes(?) in which case it'd end up in the engine and cause a high oil level. Usually flooding carbs will piss out the overflow tubes that are attached to the bottoms of the
float bowls. but if the overflow tubes are clogged, well,, its gotta go somewhere.  Again, just a guess. Any other thoughts from the experts?

No expert but see reply #3

[xruss]

If it were to drain into the oil, would the fuel and oil separate? I drained the oil just a couple of weeks ago and have it sitting in a jug in the garage. It hasn't split into layers. I changed the oil before going with a test firing of the bike and to help run the nasties through in anticipation of putting in another fresh batch when she's all back together.

[dougan]

I've had

I'm gonna guess here....if you have a fuel petcock that doesn't shut off correctly and allows a slow drain, AND one or more carbs with float valves not shutting off completely for whatever reason...

x2.  In case you think this situation is unlikely, I'll just comment that I have had this problem with a moped of mine before.  If your petcock isn't fully sealing, and there's a continuous flow of gas to your carburetor, your float only has to be a tiny bit off to drip a drop every now and then, and that's all you need.  Not out of the question.

[DH]

If it were to drain into the oil, would the fuel and oil separate? I drained the oil just a couple of weeks ago and have it sitting in a jug in the garage. It hasn't split into layers. I changed the oil before going with a test firing of the bike and to help run the nasties through in anticipation of putting in another fresh batch when she's all back together.

The gas and oil will combine if they are mixed together. Your engine oil level
will be higher on the stick than normal, and the oil itself will be much thinner (in a severe case), more like "oily gas". If you drained out more oil than you put in originally, you'll know where your gas went, as bollingball suggested in post #3. You might also check for signs of stains inside the airbox if its a 750, or plenum chamber if a 550 or 500. That might be evidence of previous carb overflowing
while the bike sat.