Originally Posted by GameRV
Ran systems for 2 hours no problems removing the gas cap after first 30 minutes. Gas cap clearly states non vented. Will replace. Have no idea where to look for vent that may be clogged.
It was a hunch - I had heard of fuel starvation due to vacuum before. I have even heard of automobile fuel pumps permanently creating a vacuum strong enough that the outside air pressure permanently collapsed the sheet metal tank (but I have never seen it firsthand).
Old time fuel caps had vents to allow air in to replace the liquid that was being drawn off. Today the vapor that might evaporate through the pinhole is considered an environmental crime - ever see the gasoline pumps with the vacuum fitting to suck up the vapor being displaced by filling the tank?
When the engine is running, there is probably a system on the engine that would vent the fuel tank in such a way that any fuel vapor is drawn in and burned. Since your genny ran fine for a while, I had figured that it was either overheating or fuel starvation due to vacuum. I guessed that the vent system through the engine probably was not designed to vent the tank when the engine is not running. If your bus engine does not cut out when driving, the original system is probably OK for that purpose.
So that means you will need to either find an old vented cap, or create some other air intake (filtered for pesky insects?) that will allow you to draw liquid from the tank without running the bus engine. I'm glad you found your problem.