Re: gas tank pressure question
I agree with Professor in that if the tank cannot vent it can, and will build up pressure if left in a warm environment and the petcock is closed. Opening the petcock will flood the engine. The float/needle and seat in small engines with gravity flow fuel systems are not designed to hold back much over atmospheric pressure. Increasing tank pressure much over that will result in severe flooding and leaks. Vent the tank for both positive and negative pressure. I wouldn't be too concerned with explosive pressures; something else would probably leak and lower pressure before the tank ruptured.
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