Gas tank venting

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biknut

biknut

Well-Known Member
Sep 28, 2010
6,632
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Dallas
I thought I would bring this to everyones attention.

I have 4 brand new stock gas tanks. I just checked them to make sure they vent.

2 of them don't vent at all, and will hold a vacuum. One vents, but pretty slow. Only 1 tank out of the 4 vents well enough to call it good.

These 4 tanks came from 3 different manufacturers kits.
 
GearNut

GearNut

Active Member
Aug 19, 2009
5,104
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San Diego, Kaliforgnia
Sadly, a common problem. Some folks will drill a tiny pinhole through the top of the cap to let it vent. That can lead to fuel spitting out of it and a couple of extra holes need to be drilled into the bottom of the cap to reduce this effect. The extra holes in the bottom allow any fuel or condensate collected inside the hollow cavity of the cap to drain out.
Don't top off the tank completely if you do this though. You need air space between the fuel and bottom of the cap.
 
G

Greg D

New Member
Apr 17, 2013
2
0
0
San Carlos, California, USA
How are these gas tanks supposed to vent?
Just got my motor running today and found that after a few minutes with the engine running there's such a vacuum in the gas tank that gas doesn't flow into the carburetor!
I don't see any sort of hole in the gas tank or in the gas cap that could act as a vent, but these are advertised as "vented". I haven't been able to find anything on the Web that explains or shows how these "vented" tanks or caps are supposed to work, so if you know, please educate me!

In advance, thanks for your help!
 
biknut

biknut

Well-Known Member
Sep 28, 2010
6,632
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Dallas
The cap is supposed to have a vent hole in the underside. A lot of them come without one, or one that is too small.
 
2door

2door

Moderator
Staff member
Sep 15, 2008
16,302
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Littleton, Colorado
Take a look at these two caps. The one on the right doesn't vent. The left one works fine.
Compare where the peen is in the edge of the cap. You'll see the right one is lower, closer to the edge than the left one.
The peen is there to distort the seal between the inner and outer parts of the cap where they're crimped together. When the peen is high enough it allows a tiny gap where the parts meet. If the peen is too low the inner and outer parts are snug against each other and make an airtight seal.

You can either peen the cap just above the factory one or drill a very small hole just above it.
Or you can order a new gas cap and hope for the best.

I hope this explains the non venting gas cap issue.

Tom
 

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JustAnotherBozoOnABike

JustAnotherBozoOnABike

New Member
Mar 31, 2013
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RI
...a most excellent explanation and i thank you for it, now i know WHY as well as how!

your posting ought to be a sticky to make it easy to find for all the future noobs yet to vapor lock.

thanks again, bozo
 
scotto-

scotto-

Custom 4-Stroke Bike Builder
Jun 3, 2010
6,505
24
38
Ridin' inSane Diego, CA.
I vent my caps in one of two ways. One is to install a barbed hose fitting into a gas cap and use a length of tubing with or without a check valve as shown.



The other is to drill a 1/16" hole through the cap, then drill a larger hole 1/2 way through the underside in the same hole. Install a small ball bearing in the larger hole and peen the cap so the bearing won't fall out.





I posted this to help people with custom tanks, bungs and caps.......they need venting too ;)
 
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G

Greg D

New Member
Apr 17, 2013
2
0
0
San Carlos, California, USA
Excellent posts! Thank you, both!

I think the ball-bearing valve works best for overpressure in the tank, but the real problem is under-pressure (vacuum) in the tank. I suppose that vibration when the bike is rumbling down the street is enough to open up the valve often enough to relieve a vacuum though. Or maybe the ball bearing doesn't seat all that well and there's always a small path for air to travel to equalize the pressure.

Love the pics of your bike. Beautiful gas tank! And i really like your solution for the rear sprocket! Our next step is to add a disk brake up front like you've done.

Thanks again!
 
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