please help, 1967 cub I have pressure in the gas tank when r

10ring

New User
please help, 1967 cub I have pressure in the gas tank when running.
Zenith carb I cleaned it but still has pressure when running I can put my hand over the open gas hole and can feel it building up hot or cold.
tractor runs fine. it just seems weird I see no bubbles coming from the gas port in the tank or sediment bowl.
how can I have pressure build up but gas still gets to the carb??
when cutting grass gas comes out of the vent hole in a mist.
I think the obvious thing is to rebuild carb. but it runs fine.
Im baffled
 
The issue is not the carb, it has no way of making pressure. The issue is heat. If you are using fuel that was sold this winter or early sprng, it might be the vapor pressure of the fuel. Summer fuel is less volatile. You can get a free replacement multi baffle gas cap. It always astounded me that heat from the close engine proximity and little heat shielding causes many tractors to blow a steady stream of fuel vapor out of the vent. I will bet that substantial fuel savings could be achieved with reflective insulation between the engine and the tank. Jim
link
 
Jim's right, there is no way for the carburetor to "run fine" and pressurize the gas tank. I mean, if the intake were completely blocked off, and the carburetor was dry, and the engine was spinning backwards, yes, it would pump the tank full of air.

Has to be heat. There are some tractors that have the gas tank sitting right over the engine, and when you work the snot out of them, the gas can boil. A Cub is not one of those tractors. The gas tank is BEHIND the engine, so engine heat contact is limited, and there is airflow. The engine would have to be getting incredibly hot.

You also say this is happening COLD too right? Cold engine, but are you in the Arizona desert or a similar hot climate?
 
You are absolutely correct about the reflective insulation. We had a good chunk of a roll of self adhesive reflective coated fireproof fibreglass matting left over from a engine cowl upgrade on a Cessna. I installed it on the gas tank shield and have never had fuel vapour issues again, and yes in hot weather the fuel usage is better than before the retrofit.
 
Thanks for a different direction in thinking. I researched gasoline and the additives such as ethanol, oxygenated additives...ect. The gas will separate and also draw in moisture.
I Drained the tank and got fresh gas. I added gas stabilizer and found my problem stopped. I knew it was something stupid that I was over looking.
 
It had nothing to do with heat. It was in the 60's here in Michigan last night. I had pressure in the tank as soon as I started it this morning and could feel the pressure build as soon as i put my hand over the gas tank opening. Not a little pressure it was quite significant. I changed the gas and added stabilizer and the problem stopped.
 

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