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Previously owned a 9N and an 8N now a new to me, Jubilee. Converting to 12volt and have started the work including new radiator. While it was uncowled I noticed the gas cap was very loose so bought a new one. Battery is disconnected, has not moved in a week or so. There was a strong smell of gas after a day with the new cap. Opened the cap and there was obviously a reasonable amount of pressure that had built up. Noticed the exterior of the carburetor was damp. My question is: whether the carb needs attention or not, where did the pressure or vacuum come from without the tractor being started or moved?
 

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The cap you bought apparently is not vented. Rather than replace the old cap, why didnt you get a new rubber seal for the cap?
No the carb doesnt need attention.

What happened is the new cap on it would have eventually given you the headache of the engine stalling. Then it might cough and sputter until you open the gas tank. Of which you would have heard a hiss of sorts as you would have broken the vacuum you would create with a non vented cap.

That non vented cap was for a tractor or vehicle that may have had the carbon canister. The cars from the 1980's used carbon cannisters to recover fuel vapor. The idea being that the fumes from the gas tank would expand into the canister. When you started the car it would purge the fumes to the carb keeping the then gas of the time from going bad.
 

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I agree with everything you said but still wonder where the pressure or vacuum came from as it just was sitting there. I'll have to dig that old cap out of the trash and buy a gasket or figure out how to vent the new one. A 1/16" hole through the cap might work. By the way, thanks very much for the reply.
 

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The heat of the sun, even on a cold day will make gasoline get hot enough to vaporize. As gasoline vaporizes, unless vented, will create excessive pressure in the gas tank. Since the float in the carb is only strong enough to resist a few pounds of pressure. The carb over flowed with gasoline. Again, put the old cap on there and it will vent the excessive pressure.
 
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