Originally Posted by Gooroo
What RPM is the engine making the knocking noise?
If it is only at idle, it could be end play in the crank causing the crank to "bump" out and in. This is very common on old bronze bearing motors but much less so on ball bearing motors.
Just a thought, my 80 4-stroke had a strange full throttle knock and I watched the crankshaft bounce in and out with the knocking noise. That engine didn't get run much either, and it turned out the dry cam lobes were causing enough friction to force the crank endplay by the timing gears.
Odd thing was, you could not feel end play, but you could see it with the engine running. Some lube on the cam lobes, problem solved.
A realatively large fuselage shape like a decathalon is a great amplifier for any noise the engine will make.