if its overfilled by a fraction of a quart I dont think that would make the engine idle fast
If you sit on the bike a hold the front brake with your right hand, and reach down with a small mirror, you can see the oil level in the window. Leaning the bike a bit to the right will make it drop, to the left will push it over the top of the window.
Normally when I fill my VS650 with 3 quarts of oil, after riding a few miles to fill the new oil filter, the oil looks a bit high in the window, maybe a 1/8 inch above the top mark when the bike is full. Leaning the bike just 10 degrees to the right brings it down in the middle, so I know I did not put an extra quart in by mistake. Ive never had any idle issues with the 650 in the last 35,000 miles I put on the odo.
If the bike is a quart over the crank shaft will be spinning hard into the oil surface, and stirring it up into a froth. That could end up blowing out the crankcase breather tube on the top of the rear cylinder head, and into the air intake box on top of the carb.
I guess having some oil injected into the air intake could act like additional fuel and make the bike idle faster, or if its blowing back into the air filter and restricting it, that would be like riding with the choke on.
So... that's something to look at: pull the air filter out and if its not white like paper then replace it. Dont bother trying to rinse it off with a solvent or blow it off with compressed air.
If the bike is a quart over filled you could do the messy partial drain with the drain plug, or get something like a turkey baster and a short tube and see if you can suck it from the oil fill cap opening.
Usually if a bike has sat for a while and then does not run right its because of 10% ethanol fuel breaking down. Even if you use a stabilizer, there is a time limit beyond which it will still go bad.
Simply because the bike sat for a year or more, If you are still using the old gas, drain it all out and add a carb cleaner like SeaFoam when you fill it with fresh gas.
If you already burned off most of the old gas I would still go get a carb cleaner and run that thru for at least a couple tanks of new gas, and see if it resolves the problem. I know it seems like its too simple of a fix, but a good carb cleaner and following the directions on the bottle (do not pour the whole bottle in one tank of gas) has fixed many bikes on this forum over the years.
Its only about $8, and it really does work.
Last edited by KCW; 07-02-2019 at 10:22 AM.