Engine/Drive Train Noise

[dinkum1]

We recently purchased a 1975 Bristol 27' with an Atomic Four. During
our initial cruise we noticed that at RPMs above 600 there was a
knocking sound. This sound is not heard with the engine running in
neutral or increasing the RPMs while in neutral. With the engine in
forward gear and upon acceleration (more than 600rpms) the knocking
sound becomes apparent and increases as the rpms are increased.

Question: Does anyone know if this is normal for the Atomic Four or
Bristol 27 or if there might be a particular problem that needs to be
addressed? Any input or help would be appreciated. Thanks.

Michelle and Harley

[tenders]

Not normal. What happens in reverse? Could be, in order of severity: something fouled on the propeller, spark plug wires on wrong spark plugs, badly misaligned timing, loose zinc on the shaft near propeller (happened to me once), loose propeller, loose coupling between shaft and engine, misaligned engine causing shaft wobble, bent shaft, broken transmission.

When hearing hooves, think horses before zebras.