What plug should I be using? I am currently still using the plug that came with the engine. The engine is fully broken in now with a half gallon of gas through it.

Use C6TF Nova Rossi plug

C6Tf if using 16% Nitro and C7Tf is using higher Nitro

Oh ok cuz i am using 30% with a shim.

That is the same motor setup i use and it runs really strong

I'm running 7f. Runs good but I could use a little bit more bottom.

there are different ways to get bottom end fuel , pipe, gearing , head clearance, clutch , ect .I think in your case get the motor modded or clutch.

I use 6tc or 6tf plug........

this is the clutch setup I use for MTX4
flyweights drilled to 4mm diameter holes
MSR ultra hard spring at 1.5~1.7mm
clutch gap 0.60mm......
yellow shoe......