Join Date: Sep 2006
Location: Southern Minnesota
I've seen a plugged gas cap vent act like this. It's an easy test. When acting up, loosen the cap, and see if the problem changes.
Other than that, I'd be suspicious of a gummed up carb. Pump gas is terrible, and it doesn't take more than a few weeks of sitting to have an issue.
I also don't have much faith in the stock petcock. It has an auto shut off, that has proven unreliable. Beyond that, it's completely unnecessary for the petcock to automatically turn on and off with the engine. A normal petcock from a Raptor 660 is a common swap, and one I recommend.