I have a new GMS 1.20 two-stroke with a Bisson Pitts style muffler installed in a new Extra 330. The engine was broken in with 3/4 gallon of Wildcat Premium Extra 10%Nitro and 18% castor/synthetic blend and APC 16x6. I installed a hotter than normal Enya #3 plug to minimize an overly rich mid-range ( a common thread subject ) and was very happy with the engine and its incredibly low idle. On the fifth flight I started the engine and began to lean the top end using my tach( I've been running it 400-500 RPM below peak during the first few flights-it has a "Dykes" piston ring) when it suddenly quit and nearly seized during the run-up. I went to restart and noticed the engine was VERY tight - similar to an engine that has sat for years with congealed fuel in it. I put some after-run oil in it and turned it over and over until it loosened up enough to restart, which it did with no apparent power loss. After another uneventful flight, I checked it and it was still VERY tight, although it it is equally tight throughout the turn with absolutely no grinding. I don't want to pull a new engine out, but this thing is much tighter than when new and much tighter than any of my 12 other assorted ringed,ABC, ABN, etc.
Any ideas?