Magneto timing

After working through several problems the ’42 B roared to life yesterday. It is in need of several adjustments – brakes, clutch, etc – that for the most part shouldn’t be a problem. Not having worked previously with magnetos, however, I could really use some good advice on setting the timing.

Before it was running I set the timing using the procedure outlined in the IT manual. It must have been close as it started on the second crank, but now I need to make the final timing adjustments while running. I’ve identified and marked the flywheel timing mark and I have a good timing light that can be advanced. What I don’t know is; do I set the timing at low idle or top speed? Is there any advance? Any short-cuts or pitfalls I need to know about?

If anyone has a good procedure for this I would be most appreciative.

You DON'T need to set the timing on a properly operating MAG while running.

Set it so it impulses AT (NOT before) TDC, and leave it there.

Earlier and you risk "kickback" while starting.

Slappy -

There is no "running" timing adjustment with a magneto. Like Bob says below, set the mag so the impulse trips at TDC then accept whatever advance it provides when the engine is running.

If the mag is working right timing be steady at around 35 deg BTDC from idle to max governed RPM.

You may mess up what you've already done by trying to time your mag the way you would a battery fired distributor. The timing job you did was to get the impulse coupler to trip right at TDC with the engine stopped. That's for starting purposes, and you don't want to wind up with that timing point advanced any or you run the very good chance of having it kick back and catch you when hand cranking. Same risk with advancing a battery distributor too far.

Now to your main question, as far as timing advance, speed is not an issue with a mag. It doesn't have the variable advance of a battery ignition. Timing is either at TDC when stopped or fully advanced when running at any speed. For that reason, you'll see folks around the board every now and then looking for a battery distributor for a puller -- they want the advantage of the variable advance that they can't get froma magneto.

If you were just going to check (or even fiddle if you decide you want to) the timing, I can't answer exactly. The H4 mag on your tractor lists a "lag angle" of 35*, which would be an AWFUL lot of advance. Full advance on a battery distibutor on a B would be 20* and you reach that at 800 rpm, so I suspect, where the mag is geared for the rotor to turn once for every two turns of the motor, that the 35* lag angle would translate into an advance of 17.5* when running. That's speculation and reason coming in on my part. Perhaps somebody will jump in to confirm or correct that.

Thanks everyone for the quick reply"s - I"m slow but I"m catching on!

Thanks again and have a great day.

Scotty,
The H4 magneto impulse coupling (where the lag angle is determined) turns at crankshaft speed. So 35° advance for the impulse will fire the spark plug at 35° BTDC. One thing to watch for: Some H4's were designed to only have a 15° lag angle. There will be an X or Y stamped at the end of the serial number and on the impulse cap on those.

Thanks!

I know my BN is a stong runner on a mag but, jeezum, 35* seems like an awful lot of advance. Ain't gonna argue with success, though!

You mentioned the 15* mags off the M, 6 and 9 diesels. I'd think it would be interesting to rig one of those up onto a 113 just as an experiment to see how it'd run with the timing closer to what the battery distributors run at.

Scotty - According to my data sheet the stock B distributor runs even MORE advance than the mag (40 deg vs 35)

Would then logically expect that running a B on a mag with only 15 deg advance would sacrifice performance at speed. (It might idle a little better however...)

Besides it would tough to adapt a diesel mag since diesel mags turn the opposite direction from the gassers.

The impulse is supposed to be set to snap at 5 degrees after top dead center. All H and M's have that second mark right after tdc on the front pulley and it is the 5 degree mark, this gives you a 30 degree advance while running and it also accounts for a not very smooth exhaust sound at low idle with certain combinations of heads and pistons etc. Not sure if the little tractors flywheel were all marked that way or not.

Well, I'm just livin' and learnin' today.

I did a little looking around after you posted and am finding some disparity between my own manuals. I took the 20* battery igntion full advance for the BN from the table in the I&T manual, yet yours is showing 40. (What are you looking at by the bye?) At the same time that same table in the I&T shows 15* for a SuperC, but the specs in the IH owners manual for that tractor calls for 30*, the same factor of 2. Hmmmm.

I was aware that the H4 on the diesels ran CCW, but thought that was due to the extra idler gear, not anything internal to the mag. Could you reverse the direction of your plug wires to have it fire right?

"Not sure if the little tractors flywheel were all marked that way or not."

Nope. The only reference mark on the 113/123 is on the flywheel itself and at TDC, which is where it should be sitting when you snap the mag into time.

The IH Blue Ribbon manual for mags has a section in it explaining how you can swap out four parts (I think) and turn a clockwise mag into a counter-clockwise mag.

Scotty - My data is from page 7 of the I&T #IH-8 manual. Doesn’t mean it’s right however – I’ve found several other errors in this publication over the years…

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I suspect the “factor of 2” difference in timing advance values listed in some other sources may come from semantics and/or poor fact checking by the writers.

Specifically spark timing can be expressed in crank angle (which the engine tuner cares about) OR distributor rotor angle degrees (which the distributor designer must build for). And since 4-stroke engine's crank angles are DOUBLE the engine's distributor rotor angles (rotor to crank drive ratio is 1:2) it’s easy to see where a “factor of 2” listing degree error might creep in.

(Now imagine if we had both SAE and metric units for angles....!)

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“Could you reverse the direction of your plug wires to have it fire right?”

Theoretically yes. However a backward-turning magneto provides only a weak spark - or more frequently none at all. Also mag impulse couplings work only in the direction they are designed for.

So no – swapping plug wires around won’t work.

Aha! My 20* is coming from Table 2 back on p. 49 of the same book. Odd. I do see now that the heading on the table says the data is in distributor degrees and rpm. (I've just circled that statement and put a couple of !! beside it!)The 2:1 works just fine, but that table turns out to be kind of useless for the guy in the field as there's no real handy way to accurately check rotor angle. Huh! Going back to my SuperC calling for 30* in the owner's manual, and showing only 15* in Table 2, I guess I oughta rely on the tables in the front of the I&T for that data going forward. Good catch on your part.

And thanks for clearing me up on the CCW mag. I didn't have any plans to try it, but have always kind of had it in the back of my head. I have in my memory having heard of the kits to convert them that Slappy mentioned. I'd guess it has something to do with the polarity of the magnetized shaft in relation to the coil among other things like the degree of retard and direction of the pawls on the impulse??

Thanks, again, Bob!

Scotty - FWIW the IHC H4 mag can indeed have it's rotation reversed! Involves retiming the distributor drive gear to the rotor pinion (timing marks for both right and left hand timing marks are provided on the rotor gear) then rearranging (or maybe replacing?) a couple impulse coupling parts.

The process is covered in detail in the H4 magneto service manual.

Thanks, Bob. That's about what I thought.

As things worked out, the mag was about the only thing on my BN that still worked, so it's one of the manuals I haven't HAD to get for the various tractors and implements. Probably wouldn't hurt to have one around for when the day comes. But then we've got a pretty good mag shop up in the northern end of the state, too, and I think he's younger than I am, so . . . ;8^)

Thanks!

Scotty - You can download the complete IHC "Blue Ribbon" magneto service manual from the link below.

The manual is stored by page - the H4 is completely covered on pages 39 thru 55.

Have fun!
IHC magneto service manl

There are a few parts (2 or 3?) in the impulse coupling that can't be simply flipped over for the opposite rotation. They need to be replaced with "mirror image" parts. Given those parts, reversing an H-4 is no big deal.