I need some help with my favorite engine, the Enya 53-4c. The engine is about a year old with one and half gallons of fuel thru it. Last month I lost my muffler half inflight. I did not touch any settings before or after I lost the muffler half. Yesterday I installed a new OEM Enya muffler and ran her up on the stand and she was knocking at wide open throttle. I richened it up 500 RPM from max RPM and it was running fine on the club set-up stand but when I throttled up for T/O she was knocking at full throttle. I put here back on the stand richened it up 1000 RPM and it ran rich but fine. Throttle up for take off and it knocked the prop loose on the shaft. Here is my set-up: She is mounted right now on a Spacewalker II inverted with a APC 12-6 prop, Enya #3 plug with Omega caster/synthitic 5%. I was reading my Enya paperwork last night and it recommends 5-10% caster/syn blend and also recomends using a head gasket if the engine knocks with a 12-13 inch prop. Has anyone experienced this ? And what was the fix? Prop down to a 11-6?