It's hard to say what you should do next other than try and clear up what may be a misconception about valve operation. you asked

"Could there be too much back pressure from the muffler or is the exhauste valve not fully closing causing extra pressure on the compression stroke?"

#1- Unless the outlet of the muffler is crushed or pinched closed I t would not be causing too much back pressure and yes it is a small empty chamber.

# 2-valves not closing would result in "less" compression, not more!

I sugesst that you start with the basics and check the mixture screw adjustments as previously mentioned and then adjust the valves using the tools provided with your engine.

If you are either unfamiliar or uncomfortable performing these adjustments, enlist the aide of somone who has done it before
The adjustments are not difficult, however I hesitate to try and explane them for ffear of not being able to tell you accurately enough to avoid adding to the problem

Roger Aka GIFLYRC