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F-20 knocking

3LFarms

3LFarms

Member
Location
Ogden UT
Finally got new tires this week and took it on its first drive today.
When I pulled the head after I bought it last year, I found the engine had been overhauled with high compression pistons. Everything measured out within spec and I cleaned everything up and buttoned it up. Installed an overhauled carburetor and I went through the magneto. Engine starts easily and runs great with the timing retarded. When I advance the timing more than halfway at idle, it starts sounding like one or more rods are loose. Its sounds like something is banging pretty good. Its definitely not a happy noise. Retard the spark and it purrs like a kitten. I am at about 4600 ft altitude. I put fresh regular grade gas in it.
Is it possible it needs higher octane fuel to keep it from knocking or is the timing just set too advanced? I put the magneto back timed the was it came off. Tractor had not run for 30 plus years, everything was intact as if it was parked one day and left to sit, so i just put it back that way.
 
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Put a timing light on it and see where timing is at, a magneto is either retarded (impulse) or full advance.
 
The rods are all at .0025 to .003 inch and the bearings are in great shape.
All I can think is that the timing is just too advanced, but it never kicks back when I start it. It does have excellent compression.
I just dont want it running too far retarded and getting hot.
It doesnt smoke at all except briefly when the throttle is advanced.
 
It could be one or more of a few things. It could be your rods are a bit loose. It could be a wrist pin is not quite tight in either the rod or the piston. It could also be that the timing causing the spark knock condition. Or it could be with the rods loose it allows the piston to just barely touch the head possibly due to the rods, or maybe and this would be stretch timing is causing the piston to rock in the cylinder with the detonation of the fuel/air mixture. I had a piston one time also have the top section above the top ring come loose and unscrew so it hit the head. Sounded just like a rod knocking.
 
It won't kick back at cranking speeds as long as the impulse is set to trip at or after TDC. If the impulse coupling to main magneto shaft alignment is slightly off in the mag, the running timing may not be correct, but the impulse coupling still makes it fire at the correct time at cranking speeds. I suspect that is what is happening here. Put a timing light on it and see where your running timing is at after the impulse coupling disengages. You may have to make your own timing marks on the crank pulley for convenience, the factory ones in that hole in the bell housing are a pain to see. Also confirm your point gap is correct, that will also affect running timing.
 
Because a mag has no advance mechanism, when cranking (had or electric) the impulse coupling retards the spark to allow the mag to make enough
voltage to fire the plugs at near zero rpm. To do this that coupling triggers at or a bit after TDC. This prevents hand cranking (dangerous)
kickback. Once the engine is operating, the mag goes to full advance (internally). Thus your timing adjustment might be too far advanced from
start up speeds. An electric starter will spin it fast enough to hide kickback. At altitude, it should not spark knock on regular fuel. Jim
 
I agree with John M. If it purrs like a kitten with the spark retarded, leave it retarded. Otherwise, adjust the Mag connection 1 one hole as John suggests.
 
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