Too Tight a crank is a real possibility
Easy enough to check...better with a Thermal heat gun
Run the engine for a minuet or three up to temp. Pinch off...feel or test the nose to see if it is hotter than the aft case
Careful if using finger... I scorched mine first time despite having a thermal gun across the rom
I run a LOT of Norvel .049 and .061 for combat planes... most of them Have a tight pinch and hard to break in initially...most seemed like duds until I saw the too tight crank posts... Quick polishing of the crank is all it took to get over 1500RPM increase
I only run Galbraith head and Nelson standard plugs and 25% Nitro in mine
My method of polishing the crank front bearing area is to chuck up the nose of the crank in my small electric drill...
I bought one of the Pine Wood derby hop up kits from Hobby Lobby one time and it has a Diamond paste for polishing the soft (Nail) Axle
I went on line to find the diamond Paste maker and found a source for the 5 grits they offer
Thats my OCD way...but I have friends that do it all the time with first crocus cloth then 800 grit and then 1200 grit and call it done
Trust me the crank is hard enough that you would have a hard time getting it too loose