Fuel IN exhaust - Star Motorcycle Forums: Star Raider, V-Max, V-Star, Road-Star Forum
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post #1 of 7 (permalink) Old 05-31-2019, 12:19 PM Thread Starter
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Exclamation Fuel IN exhaust

Had a strange thing happen this morning with the 650. As I backed it down the hill out of my garage and went to close the door, I noticed a trail of liquid on my driveway that matched the path I had just taken with the bike. It was a thin liquid and there was nothing on the garage floor, so I thought maybe this was just moisture in the exhaust (like you see sometimes coming out of the tailpipes of cars). So before I started the bike I reached into one of the exhaust pipes with my finger and felt some liquid in there, it was gas. So I cautiously started the bike and it seemed to run normally. I drove it on to work with no problems. My question is, if this was gas, how did it get there? Do I have a problem I need to address? (I just had both carbs rebuilt at the local Yamaha shop this Jan). I should disclose one other thing. When I parked the bike on Wednesday, I had just filled the tank and I filled it more than normal because I was pretty sure I was going to ride some more in just another hour or so. Turns out that I didn't. So it sat in my garage will and unusually full tank of fuel for 2 days. I know that you aren't supposed to overfill the tank because warm fuel will expand and make a mess. Could this overfilling of the fuel tank have anything to do with the fuel in the exhaust? I'm hoping someone has seen this before and has a better idea of what's going on than I do. Thank you!

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post #2 of 7 (permalink) Old 05-31-2019, 01:01 PM
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Gas expands with heat. In a hot garage fuel could have expanded, path of least resistance could have been through the fuel lines into carb, past floats and into combustion camber. Smell your oil, fuel smell, don't start anymore and change oil. If the fuel did make it into combustion chamber some could have made it past the rings into oil. If fuel petcock was in off position this would not have happened. There maybe other issues but I've seen this happen before. Let us know about fuel smell in oil. Good luck.

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post #3 of 7 (permalink) Old 05-31-2019, 04:21 PM
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I never shut off the fuel petcock, until I went in the garage one day and said "hey it smells like gas in here".
Since then I always turn the gas off. And like Les said , checked the oil and had to change it . Not sure how well diluted oil lubricates ,but it can't be good for the engine. It's just too thin. Our bottom ends are plain bearing not roller like on the two strokes.
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post #4 of 7 (permalink) Old 05-31-2019, 08:35 PM
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the big question is whether you had the petcock on or off?

Is your bike a California model by any chance? They have the closed fuel system, if the tank is overfilled Im not sure where it would overflow.

On my 49-state VS650 when I overfilled the tank on a cold day, when it got warm it came out thru the gas cap, and covered the top of the gas tank, and dripped down on the left side of the bike.

the other possibility is you got a stuck float. with the petcock left open most of the gas would come out the overflow tube, and spill on the top left center of the engine block (if the tube is attached). Whether that could find its way onto the exhaust pipe and look like it was coming from there, I cant say.

The exhaust on your bike has a heat shield fake outside everywhere, and its not air tight. I would think if you had fuel flooding into the exhaust from the cylinder being filled, the exhaust inner pipe would be sealed, and the only place it could leak out would be the opening (tip) at the end. The bike would have to completely fill the pipe for it to get that far, and when you started the bike it would shoot burning gas out like a flame thrower.. You did not mention that happening.

Like others have said, dont start the bike again till you get to the bottom of this. A little bit of gas in the oil will dilute it down from 20W40 to 2W5 - it will be too watery to build enough pressure for the bearings to float on a stream of oil, and it wont take long to eat your bearings.

The really safe thing to do is clean the pan you use to drain your oil till its spotless, then drain your oil out, and then measure it. With the oil still trapped in the filter you should only get about 2.7 quarts out (measure it)

if you drain out more than that, and esp if you drain out more than 3 quarts, then you got gas in your oil. You should also be able to tell just by smelling it.

If the oil is the right amount and does not smell like gas, you can put it back in the engine, or if its about time anyway replace the oil and filter.

If its got gas in the oil, take the filter out too, let the bike drain overnight with the plug out and the bike on the kickstand, and you will be ok to put in new oil and filter.

Also trace down the carb overflow tube and see if anything is coming out of it with the petcock on and the ignition key on so the fuel pump will run.

Again: chances are the tank overflowed and spilled out, and it just looked like it was in the exhaust. The fact that you rode it to work and it ran normally also makes it doubtful the carbs were overflowing.

But repairing the engine because the bearings are destroyed would be so expensive its worth a few hours of your time to make sure the carbs are not flooding the engine oil sump with fuel.
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Last edited by KCW; 06-01-2019 at 05:50 AM.
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post #5 of 7 (permalink) Old 06-04-2019, 02:17 PM Thread Starter
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I don't smell any gas in the oil and from the sight glass the oil still appears good and thick - it's not thin like it would be if it had gas in it. Like Skypupbob has said, I too have never shut off the petcock. However, I will try to instill that into my routine. Ran another whole tank of gas through the bike since then and haven't had a problem. Thanks for the input!
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post #6 of 7 (permalink) Old 06-06-2019, 06:38 AM
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My mechanic has advised me that when I'm finished riding I should shut off the gas supply via the valve and let the bike run with the gas off for 90 seconds. This removes any pressure that may be on the carbs.
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post #7 of 7 (permalink) Old 06-07-2019, 08:50 PM
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^ I dont know if running the bike with the petcock off is a good idea

on a fuel injected bike the high pressure fuel pump will be sucking a vacuum on the closed fuel line - might collapse it, and if nothing else it will be pulling for all its worth to suck junk out of the fuel filter

on carbed bikes there is still a fuel pump, but its lower pressure. it will not pull enough vacuum to collapse the fuel line, but running any fuel pump dry cant be a good thing

On both my bikes I have a pretty good idea when the main tank will empty and the engine will start to stumble (around 200 miles on my 650 and around 170 miles on my Royal Star). I usually let the bikes run close to that point, but I switch them to reserve before the engine starts to run lean

I dont want to get in a situation where I need to crank the throttle hard, or to be in a nice sweeping curve, and have the engine drop out on me unexpectedly.

By switching over a few miles early I can be confident the 650 is still getting about 58mpg, and will go about 230 miles before it runs dry on reserve, and the RS is getting about 45mpg, and is good for 220 miles on reserve. When I flip the lever to reserve I start looking for a good place to refuel.

If the bike is not running right, and it starts to run out of fuel before the normal flip over mileage on the odo, then the bike will stumble early, and I will know the engine needs attention (or I have been riding into a serious headwind).

With a carbed bike you always want to shut the petcock off everytime you park the bike. If for no other reason than if someone bumps your bike while its parked and knocks it over, the float in the carb could tip on and fill your crank case with gasoline. If the bike is not damaged you stand it back up, start up the engine, and your oil has been diluted with gas to be thinner than water - you ride off and 2 miles down the road every bearing in your engine is destroyed. There is also the very small chance that when you park a carb'ed bike a float will stick and flood the case with gas - not likely, but why take the chance?

I really cant think of any benefit to running a four cycle engine till the carbs run dry, unless maybe you are putting the bike into storage for months.

Last edited by KCW; 06-07-2019 at 08:55 PM.
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