81 Marathon 2 pg knock and fuel pump diaphragm blowing

[sbklf]

I feel probably two separate issues. Engine is a Robin 244:

-Recently rebuilt (top and bottom)
-Converted to HEI ignition with new pulse coil
-Using Amsoil at about 80 to 1
-Governed down to well below the max recommended RPM
-The throttle/start limit are adjusted correctly with the proper spring on the cable

After it warms up good it has a knock at full throttle. Not a constant knock and it seems to change between engine loaded and unloaded (like if coasting down a hill). Hard to explain but it sounds like valve clatter. It did it before I rebuilt it but much worse. Seems I adjusted the governor to a lower RPM is maybe why not as bad now?

If the timing was causing a knock, would it be from being too far advanced or retarded?

Anything else that might cause the knock?

The other problem I have is that it blows the engine side diaphragm out of the fuel pump quite often. It will be OK for a while then out of the blue it will blow. It back fires occasionally but not continually. This weekend it seemed to blow before the engine even fired off. After I replaced the diaphragm it backfired a time or two but never blew again.

If timing was causing a backfire that blows the engine side diaphragm on the fuel pump, would it be from being too far advanced or retarded?

Any other reasons I might be blowing that fuel pump diaphragm?

I would rather fix the issue if there is one but is there any other options for fuel delivery on this thing other than raising the tank for a gravity feed (elec pump maybe)?

[cart around]

It seems like an awful lot of back pressure to keep rupturing diaphragms.

I would think that the ignition being advanced too much would be the same thing as pre-ignition.

If it fires while the piston is still low in the cylinder, I can see the flame spreading to the bottom end causing your problem.

You said complete rebuild, so I assume the exhaust is clear.

Is the piston in with the side port facing the intake?

[my1423]

Without hearing it hard to say.

Definitely put a timing light on it and see where you are at.

What is your compression at? Maybe someone milled the head and you are to high.

Check and adjust your micro switch. Make sure it just barely shuts off when you let off the pedal. Have seen them adjusted way to loose and the ignition shuts off way prior to the throttle closing.

Check the muffler for plugging and carbon deposits.

Check the motor mounts. Might be shifting and banging the cradle. Pretty common. Also check the exhaust for dings shiny spots where it could be rubbing under load.

Double check the engine cradle for cracks. Known to stress crack and make an odd knock.

Check both your clutches for tight sheeves. Have seen worn out clutches make a knock sound from being too loose and wobbling about.