Ok ,I see that you suspect a low compression problem due to valve leakage and if that is the case I guess that you may need to lap the valves and valve seats

Not knowing what rpm you are trying to get there is no real way to determine what ( if any ) single thing is affecting the performance of your engine

There are so many variables such as

Fuel/nitro content/age

Prop/pitch/diameter/type

Glow plug/age/type

Ambient tempreture/humidity etc. etc.

How about it Bill Baxter, jump back in here any time
I'm no expert here, but am trying to help

Roger Aka GIFLYRC