Won't run with gas cap on...

[anachronism]

Anybody have this problem? I built one of what I am assuming are known as the "happy time" kits.

It has been running great, except for one issue- With the gas cap on, the engine will stutter after a few minutes. Opening the gas cap resolves the issue. It seems that the gas cap is making an airtight seal, creating a pocket of suction at the top of the tank that keeps gas from flowing down the line.

Any suggestions to resolve this?

 

[HoughMade]

You have correctly identified the problem. It needs a vent of some sort. A 1/8" hole in the middle of the cap works, but some fuel may splash out.

 

[BSA]

Hello anachronism and welcome to the forum. Please introduce yourself in the correct section so that we can all say Hi.

 

[Large Filipino]

I have never heard of a stock happy time gas cap doing that. That must be a really good seal. It's supposed to vent. On your cap do the thongs that hold the cap in place give way to the springs inside?

 

[sparky]

Is it possible to pressurize the fuel system on a happy time? If you got a new carb or something, would it be fairly cheap & easy?

 

[fastboy9]

Ye this was happening to me as well, I just drilled a small hole in the top of the cap as a 'breather', most dirt bikes have these. That will sort out the vacuum problem out straight away