Clarke tank vapor lock - one way vent tube?

Saturday, it was warm (in the 80's) and when I was riding over some whoops, my bike bogged a little. I've never had that happen, so I stopped for a minute. Then it happened again... but, I didn't do anything about it (wasn't exactly sure why it occurred). It didn't happen anymore, but while I was riding down the road (stop and go traffic- top speed was maybe 40mph- bike was probably hot), it really started bogging, like it couldn't get fuel. So, I pulled over and removed the gas cap (I've got the 3.8 gal Clarke with a shorty vent tube)... gas flew out like a geyser! I've only read about vapor lock- never in my life have I experienced it. I thought maybe the vent line was clogged, so I removed it and and rode on, and it happened again, but not as bad. When I got to my garage, I expected to find the vent in the gas cap clogged, but it wasn't... so, I'm unsure how to prevent this from happening again.

I'm thinking about replacing the crappy cap that came with the tank- it just feels cheap. Although I've never had a problem with a plastic cap, I may go with billet.

And, should I be using a one-way vent tube? I've used them before and had no problems. I've also used quite a few plain-jane vent tubes and had no issues.

I don't think this is a Beta-specific problem, but it's a new one for me.

Well, after too much web surfing, I decided to buy a Tusk billet cap and I'm going with an old-school fuel line for the vent tube (no valve).

We'll see how it works.

I ditched the one way valve in short order after getting the bike due to tank pressurization issues. Been fine since.

Standard for riding in hotter weather.


I've ridden a couple times in pretty warm weather (yesterday was mid-80's), and I've had no problems since switching to the Tusk cap and valveless vent tube.

I don't think it's actually vapor lock. Mine would do the same on a hot ride. I think the tank pressured up so high that it was forcing fuel into the carb, right thru the needle and seat. This raises the fuel level until it hits the overflow level. Mine would just sit there running gas out the overflow,even with the bike off. With the level that high it will run rich and bog. I fixed mine the same way you fixed yours, eliminate the check valve on the tank. I think I am getting better gas mileage now also. No telling how much fuel was being wasted.

It happens with the standard tank too.

I'm thinking of drilling a bigger hole to the tap (it's the beta billet one)

Or maybe glue some heat absorbing sticker panel underneath the tank :thinking:

I have had similar problems as listed above. I have had this issue slow me down in two races now. The first time I figured it was just a bad check valve and replaced it. The second time I realized that there was way too much pressure on the tank. Both incidents happened when it was 80 + degrees out and after two or three hours of straight riding.

I will do as others have with just running a tube minus the valve. I would also like to cure the root of the problem too. I will be installing a comparable material to Dynamat on the bottom of the fuel tank and putting a fan kit from Beta. Hopefully doing these things will help the situation. Glad to hear Im not the only one experiencing this symptom.

As indicated, do not use one way valves on your gas cap breather. Even though some of my riding buddies still continue to use them, I have heard and read about many bad things happening because by these devices.

:thumbsup: Glad you figured it out, mine never had it so it never did it. I can tell you this, It's a one way valve so it sucks air in and eliminates gas from coming out in case of tip over, but can also build air pressure. In fact, I wouldn't put it passed being a CARB thing invention. Like the gas cans we need to buy here in CA, they spill more than they get in the tank. We need to be protected from ourselves. :goofy: Edited by PALMER84ONE


I've ridden a couple times in pretty warm weather (yesterday was mid-80's), and I've had no problems since switching to the Tusk cap and valveless vent tube.

LMAO!! I have the Tusk cap as well on my Clarke tank. Next ride I will be ditching that little swivel thing.

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