I'd try 32:1, fresh gas, a good 22x8 prop, (my favorite lately is NX)...check and/or clean/replace your plug as needed, tune the engine properly...and report back. If the exhaust is a question here, run it again with what you had on it, run it with the muffler that came with it originally, and run it with no muffler at all....again, report back. Could just be a combo of oil ratio, mixture settings, prop choice, and exhaust. All things that can considerably contribute.