Gas cap vacuum


Information and questions on GL1100 Goldwings (1980-1983)
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velvetrider
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Motorcycle: 1983 GL1100 Interstate

Gas cap vacuum

Postby velvetrider » Wed Sep 23, 2015 5:01 pm



Although I am in the middle of a whole upper rebuild on 'Blackvelvet' 83 GL1100I . I thought I'd see if someone has any insight into vacuum hiss when I remove my gas cap? This is usually after a good ride, when I am going to refuel.



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Aussie81Interstate
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1981 GL1100 Interstate (sold)
1988 GL1500

Re: Gas cap vacuum

Postby Aussie81Interstate » Wed Sep 23, 2015 5:13 pm

Usually after a longer ride, I believe the fuel tank will get warmer from radiant heat from the engine - therefore the fuel expands, giving a hiss when you open it - completely normal.

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velvetrider
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Re: Gas cap vacuum

Postby velvetrider » Wed Sep 23, 2015 9:32 pm

Aussie,,thanks, makes sense, however it seems as though the air is going in, not out. I have even put my nose close to see if I could smell something, Maybe my sniffer is broke also...

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Oldbear
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Motorcycle: 1983 GL1100A Aspencade

Re: Gas cap vacuum

Postby Oldbear » Wed Sep 23, 2015 10:02 pm

I had to soak my cap in Seafoam for a day. My vents were mostly plugged. It was creating a strong vacuum after a fuel fill - strong enough to starve the bike of fuel flow. I've added a clean to my 2-3 year maintenance schedule. Just my two-cents...
My wife is the greatest - she won't let me sell my bike - I'm less grumpy when I ride...

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Johnyy Smoke
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Vetter. "Its like Deja Vu all over again".

Re: Gas cap vacuum

Postby Johnyy Smoke » Thu Sep 24, 2015 4:40 am

Yep- Soak it overnight in white vinegar and some compressed air to blow things out will help.
Clogged gas cap vents can lead to frustrating problems, carbs can overflow, and the chances are your fuel pump will fail over time.
How did the Athena gasket kit work out for you? Regards, Johnny

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velvetrider
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Re: Gas cap vacuum

Postby velvetrider » Thu Sep 24, 2015 9:38 am

Thanks guys, I'll try that when I get her back together. Just now trying to get carbs out. Not as easy as the book makes it sound..1st #3 vacuum cap screw head stripped, then throttle cable has to be removed from twisty grib to get enough slack to reach carb connection. On & ON..
Geoff.

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WingAdmin
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Re: Gas cap vacuum

Postby WingAdmin » Thu Sep 24, 2015 9:53 am

velvetrider wrote:Aussie,,thanks, makes sense, however it seems as though the air is going in, not out. I have even put my nose close to see if I could smell something, Maybe my sniffer is broke also...


When the bike uses gas, air has to enter the gas tank to replace the used-up gas. The gas cap has to allow air into the tank for this to happen.

Similarly, when the gas tank has air in it and is heated, the air (and fuel vapor) expands, pressurizing the tank. The gas cap has to allow some of this pressure to escape.

There is a vent in the gas cap that allows this to happen. You don't want just a hole in the gas cap, as this would easily allow air to migrate in and out of the tank, causing a strong fuel smell, and allowing damp air into the tank where it would eventually condense into water. So there is a valve in the cap that opens only at a specific pressure differential. This is why you hear the hiss when you open it - there is a bit of pressure differential (be it vacuum or pressure, depending on the situation and temperatures).

The problem happens when this vent gets clogged. The engine keeps using up gas, but the clogged cap can't let any air in to replace it. Eventually the vacuum becomes so great that the fuel pump is unable to overcome it, and the bike dies of fuel starvation. Opening the gas cap and letting air in instantly fixes the problem...for a while. Letting the bike sit for a while, allowing a tiny bit of air to leak into the tank over an hour or so will also alleviate the problem temporarily. The best solution is to clean the gas cap (soak it in Seafoam overnight) or replace it.

Incidentally, the same problem happens to small airplanes, although it is a bit more serious. Many small airplanes have what is called a "wet wing" where the wings, made of thin aluminum, are the fuel tanks. If the gas cap is clogged, the engine fuel pump causes a vacuum in the fuel tanks just like it does in motorcycles - however, the vacuum in the aluminum tank is enough to cause the tank to collapse inward. Remember, the wing IS the tank, so when it collapses, it destroys the structural integrity of the wing, and the wing folds up in flight, causing the airplane to crash! All because of a clogged gas cap vent.

indianakid
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Re: Gas cap vacuum

Postby indianakid » Thu Sep 24, 2015 9:11 pm

ok i dont want to get too far off topic but the door was opened......... i have been in the aviation / aerospace industry for 30+ years and have not heard of a collapsed fuel tank caused by an in-op vent causing a structural failure in flight. i did a little interweb searching and could not find anything there either. could you please cite an example, incident or aircraft to which it has occurred. Thanks !

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WingAdmin
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Re: Gas cap vacuum

Postby WingAdmin » Fri Sep 25, 2015 10:21 am

indianakid wrote:ok i dont want to get too far off topic but the door was opened......... i have been in the aviation / aerospace industry for 30+ years and have not heard of a collapsed fuel tank caused by an in-op vent causing a structural failure in flight. i did a little interweb searching and could not find anything there either. could you please cite an example, incident or aircraft to which it has occurred. Thanks !


I did a search, and couldn't find it either, although I know I've heard of it. I do recall seeing a picture of an airplane with a collapsed fuel tank and a wing bent as a result.

It's a powerful force - check out this video: http://www.pipingguide.net/2009/04/rail ... lapse.html

f1xrupr
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Re: Gas cap vacuum

Postby f1xrupr » Fri Sep 25, 2015 5:30 pm

There are some guys on YouTube that implode (emplode?) steel 55 gal drums by putting a little water in, putting fire under it till boiling (with the cap off), then putting the cap on and cooling the drum fast, and it crushes!
This is kinda silly I guess-when I was a kid, we had a goat on a rope. Back then, you could get a square 1 gal metal unvented gas can. One day I came home from school, and found my gas can all messed up...like it'd been stomped!....I was so mad at that poor goat! Years later, I realized what happened.
I had an old truck, and if you filled up the gas tank then sat the truck in the sun, after a while it would overflow bigtime The tank was behind the seat.
Gas seems to be extremely sensitive to temperature!....was that off topic?


My exercise bike is a goldwing.


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