Farmall M new mag running rough

I have a farmall m that purred like a kitten until I burnt out the mag. I did something stupid while rewiring and wont go there.... but anyways I put a new rebuilt mag back on it and had it running again but it will not run as smooth as it did before. It just hasnt got that pur to it anymore. The rain cap used to just sit still when it ran but now it flaps. Any ideas on what I should tinker with. Is there some fine timming I need to do and how. Thanks

Did you try playing with the timing of the mag just by rotating it a little? Sometimes just a little move one way or the other can make a big difference in performance.



Andrew

Yes I did and I messed with the carb a bit even though I wouldnt of had to I think. The mag inside couldnt be off right. Other wise it wouldnt run at all right?

Are you SURE you don't have two spark plug wires swapped around in the cap?

With the cap and internal cover off, check the shaft to see if it wobbles any, side-to-side, up-and-down, all around. Any significant wobble will throw off point gap and timing. Also can't hurt to check for an intermittent short/break in the insulation in the ground wire that runs to the switch (with switch in run position, check for continuity between the insulated stud on the mag and a known good ground), especially at places where it passes through retainer clips.

Dont get me wrong here it runs and have been using it. Just planted some corn with it using a 4 row planter and just spread manure last night with a pto spreader. It just doesnt have the same nice little purr. I was just thinking if maybe there was some fine timming that I needed to do and how to do it. can I stick a timming light on it to check or do I go by something else?

I personally have very little experience with a magneto, but I do know that a battery ignition distributor, if the timing is advanced too far, will exhibit the same symptoms as you are describing.

one other remote thought are all the plugs and wires up to snuff??

Andrew

Sounds like a spark timing issue. If the idle timing is wrong a smooth idle cannot be achieved.

With the engine warmed and idling, loosen the mag clamp bolts and turn the mag a little each way. Try to find the spot where the idle is smoothest then retighten the bolts.

Then I’d stick a timing light on the #1 plug and check where the running timing is. The manual says it should be 35 deg BTC – which gives good power and reasonable fuel economy under load. If you find it's not at 35 deg, suspect a problem with your new mag's impulse coupling not giving the right advance degrees in "run".

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Incidentally variable advance is one of the key advantages of a distributor over a magneto: The distributor will vary spark timing to match RPM from idle (0 deg advance) to full governed speed (40 deg). A magneto OTOH has only 2 timing settings: “Crank” (0 deg at cranking speed; impulse is engaged) and “run” (35 deg advance at all RPMs faster than cranking speed).

Unfortunately the non-adjustable 35 deg “run” setting means you may must compromise the timing setting between the smoothest idle and the best power at full RPM/load.

Bob M, Ok I messed with the M last night with a timming light. with the 2 marks on the pully the most I could get by turning the mag was about 19 degrees then going the other way was back to zero or somewhere around there. Now you said something about the mag being set at 35 degrees. How do I check that? I do remember when I line up the gears in the mag there are 2 marks that are suppose to meet. Is that what I am suppose to double check?

Magneto “advance” refers to degrees as measured at the crank.

If the mag is timed properly the impulse will trip exactly at TDC. Check by slowly turning the crankshaft while watching fan pulley. (For safety pull the center wire out of the magneto while doing this!) The impulse should snap just as the TDC mark (the single notch on the fan pulley) aligns with the pointer.

If it does not, loosen and rotate the mag as required until it does. More detail on the mag timing process is in the owner’s manual.

There's no need to mess with the gears inside the mag. (If the mag gear timing somehow gets messed up it usually won't spark at all).

Once the mag is set to TDC, start the engine then use the timing light to check the running timing. It should be 35 deg BTC - which I THINK for the M is the first of the double marks on the pulley. I’m not certain however – it might pay to measure and mark the 35 deg BTC point on the pulley yourself.

I hope this makes sense...

Thanks Bob, I will let you know what happens!