I absolutely love the C version.

I run at sea level with the 7F nova plug and shim .30 to .35 and 16% nitro fuel.

Can be tricky to tune and need to change the conrod frequently but the engine has power and revs everywhere.

What brand car and gearing are you using?

What are your clutch settings?

Do you run a lean or rich low end needle setting? How many turns out is your low speed needle?

Maybe I/we can help.

Sam