M crank timing marks, Mag impulse

I am trying to time the mag on my M. I can not get it to impulse at tdc. The engine has two marks on the crank about 1/4 inch apart and I have verified that it is at tdc compression #1 cyl when at those marks. I continue to turn it by hand 1/4 turn of the crankshaft to a third groove in the crank pulley. That is when the mag impulses or clicks. The rotor in the cap points to number one post when the engine is at tdc ( the two marks at tdc and piston at tdc.)What am I missing. I have timed the mag on my H using the same two marks at tdc. On the H, the mag impulses at tdc like it says in the mag timing book. I know I can check the timing of the rotor by the marks on the small gears in the mag. But how can I change the impulse position. If I change the mag position by 1/2 turn, It will still impulse at the third mark. I tried a second mag and it impulses at the same mark and the rotor lines up at #1 at actual tdc.

Is this a different mag or has it been running on the M before? I have found the coupling with the lugs are different for different engines.

You certain the mag rotor is timed properly?

The IHC H4 magneto has TWO sets of timing marks on the rotor drive gearset. One is for conventional rotation (A,B,C,H,M,W4 and W6); the other for reverse rotation (W9 gas and gas-start diesels).

If it's set for the wrong rotation the mag cannot be properly timed to the engine.

On the 123 engines i remove a spark plug then turning engine ove using a little mag lite i can see when the piston is at the top and if the impulse is ready to trip then i can check the rotor to see if its lined up corrict. Y7ou should be able to do the same on the M.

Does the serial number on the magneto itself happen to end in the letter 'Y'? If so, it won't work without changing the drive gear relative to the cam gear. It is for another application. The reason I know is that I rebuilt one that was configured differently than the H4 mag on my '47 Farmall H. The oddball mag had an impulse that snapped at 90 degrees after top dead center and was stamped XR by the drive lugs.

If this mag isn't what you have, your drive timing is off. Worst case: Your cam gear nut has loosened and allowed the gear to get out of time.

By the way, the third mark on the front pulley is to line up the pilot bearing Alemite fitting on the perimeter of the flywheel with the hole in the bottom of the clutch housing when greasing the tractor.

Thanks for the replys. I checked the serial numbers on both of the mags that I have tried. The one has no letters after the serial number and the one on the tractor now has an "L" at the end of the serial number. The one that is on it now, came off of a running Farmall H. I had a set of points break in it while plowing snow a couple of winters ago, so I swapped it with the one off of this M. So I have had the mag on a running tractor, but I have not had this engine in the M running. I have already checked to make sure #1 piston is at the top of it's travel with the tdc mark at the pointer. So it sounds like I had better pull the rocker cover and make sure the valve timing is correct for the crankshaft position. I have not checked the compression on this engine but it does blow out air against my thumb on the compression stroke. I have another running M with a distributer and two H's with mags that I can compare the magneto drive gear position with at tdc to see if something is out of time in the front of the engine. I am guessing that you are right, something is out of time. I will check it this weekend sometime when I have more time to get into it. It's caterpillars during the week and farmalls on the weekends. Thanks again for the replys and the help.

I got a chance to work on the M today. I compared the mag drive gear position at tdc to my H. It was off. I removed the valve cover and checked the valve/cam timing. It looked good. I found the mark on the cam gear with a mirror and lined up the mark on the drive gear with it. I got it to impulse at tdc the way it should. One of the mags had the rotor out of time so I had to get that right. I got it to run using both of the mags. Thanks for the help