OT - Fairbanks Morse FM Magneto timing

I have Wisconsin AKN with a Fairbanks Morse FM XD1B7 magneto. I got her running but having trouble with lack of power and governor hunting. I check the static timing and the impulse is firing at TDC correctly but the running timing using a timing light is about 30+ degrees advanced. The book says it should be 17 degrees but it doesn"t say how to adjust it... and I sure can"t figure it out!

I pulled the mag off and don"t see a way to adjust the advance like the Wico type mag.

The magneto mounts to the engine with two bolts but timing is not adjustable.

I assume the advance is built into the mag but is it adjustable? Maybe it is fixed and I don't have the correct mag...

The timing mark I am looking at is a very distinct bar | on both gears. I played with moving one tooth each way but the "lag" as Bob explained is always about 30 degrees. Of course when I move the timing marks the impulse doesn"t trip at TDC which I think it should be. I haven"t had the engine open so I don"t know if the cam timing is correct. I will pull the valve cover and verify.

I have a Wisconsin manual that covers a few of the early engines. That"s why I thought it should be running at 17 degrees advanced. It doesn"t talk about specific serial numbers, just that model in general.

What determines the correct angle? Why would the same engine used different settings; 17, 25 and 30? How does that affect power?

Thank you all for taking the time to help me with my little problem.

By the way this engine is used in a homemade log splitter my father-in-law gave me.

(Lag is the amount by which the starting {when the impulse is operating} timing "lags" behind the running timing.)

The weights and/or the hub that carries the weights will be labeled in "degrees lag".

It's been a while since I've worked with one of these, and run it on the Allen mag machine, but if I remember correctly, the number is given in degrees the MAG turns, not crankshaft degrees, so if the mag is driven at 1/2 engine speed, the timing, measured at the crankshaft, will advance 2 degrees for every degree of impulse "lag".

If the mag runs crankshaft speed, the actual advance will be what the number on the impulse states.

Thanks for the great info! The mag fires at the top of the compression stroke and at the top of the exhaust stroke, so it's running at crank speed.

Tonight, I will pull the drive gear off the mag and check the impulse assembly.

I guess the next question is if I have the wrong impulse, where can I get the correct one?

The engine had this mag on it when I got it, it wasn't running when he gave it to me. I am fairly confident it is the correct mag.

So here is the latest. I got home tonight and checked a few things out. First of all, the manual I have been referencing covers the ACN and BKN but NOT the AKN I am working on. My blunder for assuming the information would be the same, this manual says the ACN and BKN should run with 17 deg advance. I checked the timing again, from my measurements its firing 36-37 degrees BTDC. So the first question I need to find the answer to is what is the correct angle for my spec number 67941? I will try to find a dealer to help me tomorrow. I don't find anything on the web.

To answer some of your questions. This does have the Zenith carb which I rebuilt. The impulse spring seems strong and I have a good spark when tripped. The engine runs quite well at idle. Watching time timing mark with a light it is rock solid without missing. Open the throttle a little and still a good smooth, solid running. I can hold the throttle over riding the governor and it runs fine... I think I just realized part of my problem. Must be something with governor or linkages....

I still believe its running with limited power. Would the timing be part of my problem?

Sorry this got some long.

Thanks again for working with me on this problem!

I haven"t touched the valves other than adjusting the clearance. When I rebuilt the carb I had trouble getting it to run without choking. I had set the idle and main jet to 1 1/4 turns out per the zenith book. With these settings it wouldn"t run without choke. I took the carb off several times to make sure the passages were clear. Finally, I opened the needles a couple more turns and now it does run well without missing a beat, other than the hunting. I will play around with the throttle rod adjustments and see if it makes a difference. I am still not convinced there isn"t a carb/fuel problem.

Based on what Bob said the impulse coupling on the FM mag sets the timing or lag. They are fixed so to get a different timing setting you have to replace the impulse for the one marked with the timing setting required.

I measured the timing last night and found it to be at least 36-37 degrees BTDC. I will try to explain how I measured it. Maybe I have a flaw in my process. I counted the vanes on the flywheel = 20. 360 degrees divided by 20 equals 18 degrees per flywheel vane. When the engine is running the mag is firing 2 COMPLETE vanes before TDC which equals at least 36 degrees.

Maybe I am driving this dead horse but it still seems a lot to me. Tonight I will remove the impulse coupling and find out how it"s marked.

Called our local Wisconsin dealer. I talked to the parts man who said the timing should be 17 degrees. I asked a few other questions and he directed me to the service department. I left a message and waiting for his call back.

I totally agree the hunting has nothing to do with the timing. I also feel confident about it because I can watch the timing mark stay rock solid while its hunting...

Because I can override governor and hold the thottle steady I think the governor or linkage is the hunting problem.

When the service person calls me back I will ask if they can test my mag.