John Deere B magneto timing

I have a John Deere B with a Wico XH1042 magneto with drive cup 6274. It says 25 deg. on the cup. Is this supposed to be timed to impulse on TDC or is there another timing process to go through?

Tell you how I do my B's. On flywheel impulse should be back
towards the axel and #1 piston should be at TDC Make sure it is on
the compression stroke. The magneto housing shaft (the slot in
there should be horizontal) Hope this helps

First of all, if it has a 25 degree drive cup, the lag angle should be set for 25 degrees. Then you need to time the static start/impulse so she SNAPS/IMPULSES right at TDC, thats (for left cylinder) when the Flywheels Left Hand Impulse (LHI) is at the flat mark on the tractors side located in the 3 O Clock position. Get her so she fires (snaps/impulses) left cylinder at TDC when slowly cranking so she can start good and then if the lag angle is set at 25 once running and over 250 RPM the run time advance kicks in and she should be firing about 25 BTDC which is where you want it.

If you loosen the bolts and rotate the mag forward CW that retards the spark while backwards CCW advances it. Adjusting the lag angle in the rear of mag affects ONLY start timing but rotating the entire mag in the flange affects both start and run timing.

This is ONLY a starting point, Ive never published an entire mag timing procedure. When I give Seminar Workshops on Wico C and X Mags I spend maybe 15 to 20 minutes describing this and thats to much typing for now lol

JD H Publications and Duane Larsons literature pretty well covers all this, get some manuals n study them and the literature I just mentioend then post back any questions..

SHORT ANSWER, ADJUST LAG ANGLE, THEN YESSSSSSS STATIC TIME IT SO SHE SNAPS N FIRES THE LEFT CYLINDER WHEN LHI IS AT 3 O CLOCK

John T

Bring #1 up to tdc compression stroke. Mark on flywheel should be at 3 oclock to correspond with mark on 5th/6th cover on side of trans. Make sure Mag is ready to fire top hole on cover. Slot in governor housing for Mag should be horizontal. Install mag (leave loose enough to turn on bolt slots.) rotate forward on bolt slots. rotate flywheel 1 full turn, tap top of mag toward rear of tractor until impulse trips. Tighten mag mounting bolts. (Top wire to flywheel side plug). Hope this helps, I don't have my manual here in the house, but gave the procedure from memory. If I left something out, someone will correct me. Dale

Good short procedure, I tend to get too long winded and never have typed up a canned response (for copy n paste) to this one. A rookie has a 50 50 chance but getting it so the correct plug will fire next (wires right top n bottom) takes me a page to describe if I type it lol. Myself and the late n great Duane Larson used different methods but he was a PERFECTIONIST which I admired. Pat Browning is also dead now but I think his website may still be up it has a timing procedure also

John T

John
I have Duane Larson's method. I couldn't find anything about setting both the running and impulse timing anywhere but in his method. I was wondering if I needed to do both or just what the JD-B service manual said. His sounds a little complicated but if I need to do it I sure will.

Id have to look in his manual, but when he and I both gave Seminars on this his "long" method was to first precisely set the run time advance and then go back and set the start timing. My shorter method was to set the start timing, stick on a timing light and so long as the run timing was from 20 to 30 BTDC Id consider it good enough and NOT do more.

John T

That's what I wanted to know. I studied his process and wondered if I really needed to do all that. Sometimes this tractor is hard to start and other times it starts easily. It seems to be an ignition problem. I am having the mag rebuilt because it doesn't seem to have a very hot spark. Being no expert by any means I decided to have it professionally rebuilt. I wanted to make sure I would be able to time it correctly.

Once you get the mag and carburetion BOTH perfect, which is absolutely essential for easy starts, GET THE MAGS STATIC START IMPULSE TIMING RIGHT (The SNAP) AT TDC (LHI at 3 O Clock) ,,,,,Too fast she kicks back, too slow harder to start......Forward CW retards, backwards CCW advances.......

John T