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Thread: Gas Caps

  1. #1

    Gas Caps

    I've read some posts here about vented gas caps, some posts speaking of taking the cap apart to vent it. The caps on my 46 chief are reproduction - not sure which ones. they are vented in the sense that they allow fuel to be drawn onto the carb, but when pressure is created in the tank (from the engine heat), they do not allow that pressure to escape. they are vented only in one direction. when i shut the bike down after a ride, i can hear the caps hissing as pressure tries to escape. is this ok? should i have caps that vent in both directions? i posted pics of my caps - i dont know how these would come apart or if i should even bother based on how they behave.
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  2. #2
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    Re: Gas Caps

    Those appear to be automotive caps for sealed systems, way wrong for your chief. If they are hissing when you stop your system is likely pressurizing and fuel mileage and performance will go to hell. Stock vented caps are designed to allow the pressure to equalize at all times, letting gravity feed the fuel, and prevent starvation due to stoppage of fuel flow. Get the right caps.
    Friends help friends move,
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  3. #3

    Re: Gas Caps

    based on the pictures, they look identical to what greer or starklite is selling (part # 41821) and they also match the stamped "OIL" cap on the bike. So I believe they were sold as being the right caps for the bike, but perhaps incorrectly designed with a one way valve. these are older reproductions, so hopefully this was a mistake that has since been rectified.

    as i said, it feeds the carb just fine as the caps allow air to draw in, but does not allow air out. seems like the pressure that builds will eventually find the weakest link to escape - which may be a petcock or a weak tank seam.

    any thoughts on modifying these to breathe in both directions?

  4. #4

    Re: Gas Caps

    Easy enough to test your theory, install the cap, put some air pressure on the fuel line coming from the tank. Why would an engineer want a one way pressure valve?
    64 Duo-Glide

  5. #5

    Re: Gas Caps

    Quote Originally Posted by linckeil View Post
    any thoughts on modifying these to breathe in both directions?
    Linckeil, have you tried blowing compressed air through the vent, from the underside of the cap? Like Panacea says, hard to think those caps started out being one way valves, so maybe thereís dirt or varnish clogging the venting mechanism, and perhaps a good soak in solvent will free it up.
    1946 Chief - take apart and put back together
    101 Scout - building stage

  6. #6

    Re: Gas Caps

    yes, i tested them. i didn't use compressed air. the "nipple" on the underside of the cap allows air in easily when you suck, but does not allow air out when you blow. this is consistent with the hissing sound i hear after the bike is shut down. when i remove the cap all the built up pressure quickly escapes.

    glad i asked the question - seems the consensus is it should be venting in both directions as i suspected. modern vehicles use a pressurized gas cap, so maybe they did get a one way check valve mistakenly put in them (might be a stretch). hopefully the are just clogged like you suggest, but if they breathe one way and not the other, i am not too hopeful that's the issue. but i will soak in solvent and see if that makes a difference. i will also use compressed air to help blow anything out.

  7. Re: Gas Caps

    Hi,
    Yes, you need your caps to release gradual pressure increases in the tank, but to close if the bike tips over. The floating valve of the original gas cap had that feature. One problem that can happen with an outwardly unvented cap is that the excessive pressure can rupture the long lower inner tank seam; if you are running soldered tanks. A second problem that can happen - and I had this happen to me, is that the excessive pressure can force the float to unseat - flooding the engine. I was running a Keihin carburetor, so the flooding didn't go into the engine, but out onto the ground via the overflow tube. My fix was to drill a small hole all the way from the outside to the inside of the cap. That fixed my venting problem, but it created an extreme hazard when the bike tipped over once with the engine running. Luckily the engine stalled and there was no fire, but there was a stream of fuel coming out of the drilled cap. All of this reminds me, I need new caps that are vented in both directions and also have the tip-over protection feature.

  8. #8

    Re: Gas Caps

    30 years ago I drilled my repro gas caps with a 1/16" drill bit and have never had a problem.

    Dave

  9. Re: Gas Caps

    Hi,
    Drilling a hole in the cap will allow it to vent in both directions, but there is no tip-over protection with that set-up. I have never had a bike catch fire, but from photos I have seen of motorcycle fires - it isn't pretty. My bike has silver-soldered tank seams, so the tank wouldn't rupture in a fire, but I have some regular solder fixing some holes in the bottom, so if my bike catches fire, the solder will melt, and there will be a full flow of gasoline out of my tank onto the fire. That really isn't a gas cap issue, but anything that reduces the chance of a fire is a good idea. I should probably replace my gas tanks, but for now, I'm just taking the chance that things will be O.K.

  10. #10

    Re: Gas Caps

    sorry to sound ignorant, but when reference is made to drilling a hole in the cap, are we talking about a straight hole right threw the cap? i see how this would of course solve any venting concerns, but as stated, would cause a fuel spill should the bike go down. this doesn't sound like a good option to me.
    who sells the caps with venting in both directions and the tip over feature? it sounds like not all vendors if people are drilling holes in them?

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