ORIGINAL: Shimano

Hmm, yeah that fuel should be just fine, if its a correct mix (there is a rare case of incorrect mixture's from factory).

Id try another batch of fuel, similar to that mix just to verify its not the Fuel..
Also, try a plug with an idle bar on it. I dont think the OS #8 has one.

We use to run these motors for years, but its been a long while now. Being your running a stock muffler, that should be Okay.. but I wouldnt rule out the muffler causing conflict either, at idle speeds. Although thats a long shot too, like bad fuel.

At least that would help rule out a couple things, before you go tearing it down to replace o-rings and gaskets.

Another note on these engines, they are HARD on bearings.. We use to have to replace the main bearings sometimes once a month, front bearings would be replaced about every other time, or so. The ABC piston and sleeve make a higher compression, and pound on bearings. Even loading after run only delay's the replacements.