Hey everyone, I am having a strange issue that I *think* I know the cause of, but I wanted to run it past the grizzled veterans here

I have an '09 GS500F, and this year she apparently only runs in reserve.  The other day I thought I was being quite conservative in letting her run in the driveway for a few minutes after changing the oil, but sure enough I only made it two blocks before she started bogging down and then died.  I got her started again at the stop sign, but barely made it 100 yards before she quit again and I had to coast into a neighborhood to park.

After that she just would not start at all.  I did some searching and saw suggestions to try switching to reserve, and sure enough it worked.  She started up (roughly), but then kept running fine, and I even managed a decent hour-long ride.

Of course the next day I went and started her up in reserve and all was well, but the debugger in me couldn't resist trying to re-create the problem, and I switched to ON after she was warmed up.  Sure enough she ran for a bit and then started running rough and bogging down.  I kept her going by adding throttle, and noticed many backfires.  She died when I let off the throttle, and then would not start in ANY petcock position.  I even waited an hour or two before trying again, and nothing.

That was yesterday, and this morning I went and tried her again (in reserve) and she fired right up like there's no issue.  So I think this rules out a carb issue, and I'm thinking that somehow my ON position is causing her to run too rich and she eventually gets flooded and won't start.

I think this would require that reserve and ON have their own diaphragms, which I am not sure of with this design.  At any rate I think I'm about to buy a new petcock on ebay, but I would like to know if this theory makes sense or not.  A full assembly is only $20 more than a parts kit, so in any case I will probably not be able to resist tearing apart the old one to see what's wrong.

I have read that these petcocks do get clogged, so it's possible that the ON setting is clogged (she did sit for a while), but if that was the case I'd think that I would always be able to start on reserve or prime.  So I think there must be some other problem, like the diaphragm getting stuck open or something.  I don't have gas leaking all over the place, though, so that also seems questionable.

Thanks in advance

I agree with you, replacing the petcock is a good idea. Your story reminds me of a similar situation with my bike when I first bought it and it was made right with a new petcock.