Re: Yanmar 2gm20f
My guess is that its simpler than you expect. Check the throttle idle set screw. I think the PO tried to get the engine to run by cranking the rpm so it wouldn't stall due to bad fuel. Now you have good fuel, and the low setting is way too high.
I have done this myself with outboards.
The yanmar is a great engine...and maybe its working perfectly?