ORIGINAL: Dr. Bellows
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?
This may be unrelated, but I've seen it fool some very good engine people over the years.
Ensure that your battery, or other solid object, is not vibrating and creating the noise that you are hearing. Our ears and our brain can be fooled fairly easily, in this regard. Don't ask.
I would have suspected the upper end of the connecting rod also, if you flew the engine for a while after losing muffler pressure when the muffler came apart. You did say that you checked the connecting rod, so this probably isn't the culprit. Enya's are built really well, and it would surprise me if this were true. But, it's just something else on the check list.
Since you had the engine apart, I assume that you checked for carbon build-up in the cylinder and head assembly. A spec of carbon could be acting as an unwelcome ignition source.
Good luck finding your way out of this dilemma. Please let us know the cause.