i had similar problems with the OS 91fx,i bench ran it with the reccomended prop,ran about three tanks of 10% through it,tuned it by the specs,this is one of the newest versions.when i finally got the ring seated in,it started to run a lot better and kept getting better as i set the lower end.this engine was mounted in a VF edge540 and is cowled i have a cut out for the head.i have never had overheating problems and it runs good and decent power.i have had this engine for about a year now and am still using the stock glow plug that came with it on 10%cool power we buy in bulk.i am using a Pitt's style muffler the engine is mounted sideways.