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Old Oct 12, 2012, 09:45 PM
Gooroo is offline
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Joined Sep 2010
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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.

Do you notice the knock when you let off the throttle at high rpm? This is usually the sign of a worn rod. Normally, you won't hear even a very worn connecting rod at full throttle since the piston keeps constant pressure on it. A really bad rod will knock at low idle due the flywheel effect of the prop pulling the crank between pulses.

Try to find out when, exactly, in the rpm range it is making the sound.

One last possible thing and this might be what it is. Check your MUFFLER. If the muffler is one of the newer cast designs that has some kind of a baffle in it this baffle can get loose and rattle around and make noise, especially in the lower rpm ranges. Higher oil content fuel like the 2nd kind you tried could help to mask the sound due to more or heavier oil coating the internals of the muffler.
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