1935 B John Deere with Wico magneto

I have a 1935 B John Deere tractor with a Wico AP477B. When I hand crank it with a timing light it fires 2 degrees past TDC. With the tractor running it fires 20 degrees before TDC. Is this a magneto problem or is the governor/mag not timed properly?

Sounds like its working just fine to me.

Its supposed to start/impulse time around TDC and if too fast it can kick back. Once the impulse stops functioning (due to centrifugal force) around 200 RPM or so the advance run timing is in effect which is normally around 20 to 30 BTDC.

John T

If you can hand start it with out it kicking back throw that timing light away....

Well your timing advance is work right like it should. IF you want to find out why time it before top dead center and try to start it by hand. LOL

DON"T do that!!!! The magneto is designed to retard the timing at low speed. This make it fire right after dead center when your hand cranking it. This helps prevent the engine from"kicking back" if it does not start. Then when the motor does start it advances the timing to before top dead center so it runs better.

Sam, just to clear some things below up and add more info, the mag DOES NOT have what is actually any timing "advance" (it doesn't mechanically "advance" the time over 200 RPM). When she's running over around 200 RPM its timing is determined by 1) The Drive Cup used,,,,,,,2) Points Gap,,,,,,,,,3) Rotation/position of mag in the governor case,,,,,,,,,4) A few other factors. Its does NOT advance the timing and normal run timing is set, but the timing is RETARDED for starting via the impulse assembly.

However, the mags impulse mechanism PERFORMS THE RETARD FUNCTION so that less then 200 RPM due to the lack of sufficient centrifugal force the impulse mechanism engages and if the adjustment is at say a 20 degree LAG ANGLE (Means how much the start timing lags behind the normal run timing) when its engaged she will fire 20 degrees RETARDED from where the normal run timing occurs. IE start at TDC (20 degrees RETARDED), normally run at 20 BTDC. NO ADVANCE MECHANISM only a retard mechanism (impulse assembly)

Soooooooooooo the mag DOES NOT advance the timing at run RPM. However the impulse mechanism causes it to RETARD the timing when you're starting it, otherwise she would kick back if she sparked before the piston is at TDC.

Lag Angle is how much the start/impulse timing LAGS the run timing. To enable it to start and NOT kick back the impulse mechanism retards the timing slower then what she normally runs at.

Hope this helps

For best hand crank starting you want the start/impulse timing at TDC. Too fast she kicks back, too slow shes harder to start.

John T