I would say you got a bad Carb.

A Stihl chainsaw should turn 10 to 12,000 rpm.

Does it feel like it has good compression?
Can you turn it over easily by hand, or does it get real hard to turn when the piston
comes up on the compression stroke?

I have a US 35, which is a low power engine.
It does run strong and I run an 18x8 and or an 18x6.

I still can't find my tach but I have tuned by ear for years and it turns those props with authority.