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post #1 of 13 (permalink) Old 11-23-2013, 04:10 PM Thread Starter
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gas boiling...

yea, you read it right.

Today, after about a 45 minute ride with a couple stops of about 10 mins total time, I parked in the local shopping mall and went in to kill time (needed to be away from a house full of kids for a while).

Now, engine was obviously fully warmed up (fan cycling normally, temp never over 1/2 on gauge).

ambient temp around 80-84, sunny, no clouds, light breeze, but nothing to write home about.

came out from mall after around 45 mins, walked up to the bike and hear a hissing.. Knowing the cap will vent if pressure is built up, didnt think much about it. until I realized it was doing it after about 5 mins while I was getting ready to fire up the bike. Opened cap, yup, pressure, and I can see the gas boiling! Yes, bubbles and all. needless to say, when I closed cap, hissing started again.

Looked around for any gas on ground/engine/carbs, didnt see any, so when ahead and tried to start. didnt start as easily as normal (didnt want to start at all really). NOT like the no start when hot I normally get (which includes the backfires), this was like I was out of gas. after a couple tries over a couple of minutes, it sputters to life, and ran pretty normally.

stopped about 1/2 way home (20mins of ride) and check it again (not boiling, so the air flow cooled the tank enough to stop the boiling). let it soak in sun for about 20 mins, and checked again.. no boiling.

I am assuming the issue was just a combination of temps/sun and fully warmed engine causing it. Anyone else have a similar experience?
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post #2 of 13 (permalink) Old 11-24-2013, 03:11 AM
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Gasoline boils at 185 degrees F or 85 Degrees C, that is only 27 degrees cooler than water. If your gasoline was really boiling inside the tank, you wouldn't have been able to stand the heat between your legs. I am more prone to believe that the fuel tank vent lines are clogged or pinched under the tank, and the hissing you are hearing is the gas cap doing all the work for venting. The Bubbles (boiling) would also suggest this. One other possibility is your vent lines are going some wherre other than to the atmosphere.
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post #3 of 13 (permalink) Old 11-24-2013, 12:29 PM
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^^^ Pretty much what I was thinking too. My guess was that the bubbles weren't actually from gas boiling but from air trapped/getting in someplace and bubbling back into the tank. And the hissing was the air escaping as it is supposed to.

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post #4 of 13 (permalink) Old 11-24-2013, 12:55 PM
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x2 about there not being actual "boiling" of the fuel. That would need to be a very high temperature and the vapors coming out of the tank when the cap is opened would be pretty intense. I agree that starting with all the vent tubes including tearing apart the cap to clean it would be the first logical step. Good luck.

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post #5 of 13 (permalink) Old 11-24-2013, 02:12 PM
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Agree with what's been said about gas cap venting. Doubt seriously the gas was boiling. Any heated liquid under pressure will release some "bubbles" when the pressure is released.

I'd also like to mention if you normally have hot starting issues with backfires your battery may be the cause. Guessing you don't have an AGM type battery?

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post #6 of 13 (permalink) Old 11-24-2013, 04:01 PM Thread Starter
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will check into the venting situation when I get a chance..

KM, as far as my hot start issue.. I do have an AGM battery, my problem is surely just the pickup gap. I havnt pulled the cover to make the mods.
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post #7 of 13 (permalink) Old 11-24-2013, 04:36 PM
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Yes 185gas would be more than uncomfortable.i get burns from soups and sauces all the time and heatin temp id 165 and holding temp is 140 so if your gas was boiling you would have roasted seatnuts.

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post #8 of 13 (permalink) Old 11-24-2013, 08:01 PM
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If it does get hot enough to boil the gas you could always use your tank to heat your supper.

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post #9 of 13 (permalink) Old 11-24-2013, 08:28 PM
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Don't know about the boiling, doesn't seem likely. But the gas cap should never make a hissing noise. There should never be either pressure or a vacuum in the gas tank. If there is, the tank is not venting properly. Take the cap apart, and remove the valve form inside. Make sure the vent hose on the tank by the front of the seat is open and not connected to anything. If you still have venting issues, check the little rubber grommet at the back of the gas cap. Ethanol gas may have melted and blocked it.

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post #10 of 13 (permalink) Old 11-24-2013, 08:52 PM
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My gas cap has made a slight squealing noise the last 2 years (since I bought the bike)(both before and after the earshave). Especially noticeable in hot weather after a good ride. I know that I need to clean the vent but it runs great and since I rebuilt the petcock always starts easy when hot.

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