Magneto timing

[5 Forward]

My tractor is running very well. The magneto is a Fairbanks Morse. I went through the top dead center and all the timing procedures. To check this, I turn the engine over with the hand crank watching for a spark on number 1 sparkplug. The sparkplug is grounded to the manifold. The magneto trips just short of the TDC mark on the crankshaft pulley in crankshaft rotation probably 2 to 3 degrees. My question is, when I use a timing light, the timing light fires probably 30 degrees prior to number one TDC. What am I missing? Is this something inherit to the magneto? Did I install it incorrectly and just lucky it runs?

[Bill V in Md]

The IH J4 magneto fires at TDC at cranking speed. At running speed it fires at 13 degrees before TDC. Maybe the Fairbanks Morse magneto is set up with a different advance, but I wouldn’t think the advance would deviate that much. Are you running the timing light from a 12v power source?

[John *.?-!.* cub owner]

I am not familiar the the FM magnetos, but the bigger tractors such as the M did use a 30 degree advance as I recall. The magneto you have was probably for one of them. ON the IH H4 mag, the amount of advance was controlled by the plate on the end that tripped the advance

[Jim Becker]

That difference (30 in your case, 13 in a J-4) is known as "lag angle". Many F-M, Wico, etc. magnetos have an adjustable lag angle. Check into the method for your specific model magneto and set it to 13 degrees.