The weird thing is that before and after that, it starts like normal. Open the petcock, wait 10-15 seconds, twist the starter for a few seconds, it is running. It was only that one time I had fuel in the bowl where I actually had it start with difficulty.
I wouldn't be surprised if there might be another issue... the generator, a 3000 watt Yamaha (3000 iSE, no battery boost) has over 2500 hours on it, and I've mainly done oil changes and other upkeep, as well as use a fuel preservative with each tankful.
Maybe I should change the fuel preservative from Star-Tron to something else.
