This doesn’t relate directly to the OP’s problem, but might be of interest. My red head CXLS came with an excessive compression ratio, IMHO. With only 5% nitro it was pinging under load, and stinking hot after flying (as well as pretty vicious to start). Adding a couple of head gaskets transformed it, and it’s behaved perfectly ever since.
That's kind of interesting to me because I checked both of mine after I bought them and the ringed engine had a compression ratio of 8.77:1 while the red head was 9.3 :1 so essentially the same but both are way below the compression that can be used for zero nitro fuel (which is all I ever use). The thing that really impressed me with the ringed engine was that from the first ever flight (SV-11) it was 4 stroking from release but switched instantly to 2 stroke as soon as the nose went up then instantly back to a 4 stroke. It continued with this 4-2-4 run even in sharp square turns, something I'd never experienced before, and did this reliably for many hours of flying afterwards.
OK, returning back to compressions again with the button head. This made it easy for me to make a much higher compression head to suit the zero nitro. I was aiming for 13.5:1 but on my last pass in the lathe I took off slightly more than I wanted and ended up with 12.4:1 so I figured near enough for now. In flight the extra power was quite noticeable, especially with the still instant switch to 2 stroking, so I was quite pleased with that. However, before flying I'd run the engine on the test stand doing back to back runs first with the standard button then immediately swapped to the modified button recording peak revs both times using the same plug, fuel and prop. Peak revs went from 8800 to 9500 which meant a 26% increase in HP but, probably more importantly, with the standard head the 4-2 break was only evident between 7800-7900 before a clean 2 stroke whereas the modded head went between 7500-8200.
The CXLRS (stunt) version comes with a low compression head button while the RC version (CXLR) has a higher compression button. The difference can be seen in that the low compression button has a small indentation like a punch mark around where the glow plug seats.