Magneto impulse lag angle

Can somebody help me understand lag angle? I think the base mount IH mags are 23° lag angle. which means the magneto lags behind (retard) the spark 23° when the impulse trips? But how does it do that? because when the impulse trips it speeds the mag ahead a quarter turn and makes a spark right now. Or does the motor actually turn 23° while that is happening? I'm just trying to get my head around this so I can clear up any mis-conceptions about spark advance.

Cowman, I'm gonna stick my neck out on this one because although I give Seminar Workshops on Wico C and X Mags at Natl John Deere shows, IM NOT FAMILIAR WITH IHC MAGS, but believe this to still be true in theory BUT NO WARRANTY

Yep, by definition Lag Angle is how much the impulse start timing Lags the advanced run timing. The way it works is at very slow cranking/starting RPM little dogs are extended outwards so they wind up the impulse spring and it trips to fire at TDC (if so adjusted). But over 250 RPM or so centrifugal force keeps them inside so theres no wind up and trip so the timing depends on the relationship of the drive cup to when the points open, and if the drive cup and the mag is configured and fine adjusted for say 25 BTDC, thats when she fires once the impulse is no longer working......

If it runs at 25 BTDC (function of drive cup and mags points geometry and adjustemt) but you want it to start at TDC THEN YOU ADJUST THE LAG ANGLE FOR 25 DEGREES WELL DUHHHHHHHHHH

On the Wico Mags the drive cups can be selected for maybe a 20 or 25 or 30 run timing and then you simply adjust the lag angle to match the drive cup used. Even if the drive cup is a fixed unit (IHC ???) the lag angle still can use some fine tuning adjustment due to points gap and cam lobe wear and slop and wear in the impulse assembly

NO WARRANTY, THATS SORT OF HOW WICO MAGS DO IT and why the lag angle can be fine tuned and adjusted so she starts right at TDC then runs maybe 25 BTDC........

IHC MAG EXPERTS EDUCATE US PLEASE!!!!!!!!!!!!

John T

Without the impulse it would fire 23 degrees BTDC. So when the dogs catch, the engine is ahead (mag stopped) by 23 degrees and should be at TDC when the dogs release & the mag catches up again. Pretty simple now but pretty ingenious in its time. John T has a good explanation too.

You've got the right idea Nebraska Cowman.

A magneto fires when the points open. When properly timed this happens at 23° BTDC. With the retard mechanism engaged the magneto stops turning while the engine continues to turn. In the case of the IH magneto the engine turns 23° before the dogs in the retard mechanism are released. This causes the spark to occur at TDC rather than 23° BTDC.

The spark control is just backwards from most battery ignition distributor system. The magneto has a mechanism to retard the spark while distributors use a mechanism to advance the spark.

As other have already posted, and correctly I might add, lag angle is simply the number of degrees of rotation on the crank that the impulse will prevent the mag from firing. The concept is the same for most mags. As is stated, some are adjustable, some Re not. The IHC mags are not adjustable to my knowledge without physically modifying the impulse.

Hey John T - I'm not an H4 mag expert (all my tractors have battery ignition). But I have fooled with friend's mags from time to time.

Anyway the IHC H4 is functionally pretty much the same as your Wicos with couple minor differences:

1 - The "missing speed" - speed where the impulse cuts out - is 120 RPM (early production mags) and 150 RPM (later production).

2 - Lag angle is fixed at 35 deg for carbureted engines and 15 deg for gas-start diesels.

Neither the missing speed nor lag angle are field-adjustable. Rather, parts within the coupling (body cup, spring, pawls?) determine the missing speed and lag angle.

...Bob M

Thanks old friend, I dont recall seeing any lag angle adjustments when I look at the IHC mags torn down on an assembly board like Grant has at the Florida Flywheelers. Still, if you rotate the mag in its flange that changes BOTH timings so I see how a fixed lag angle can still work to set the start at TDC. The Wico C and X etc were mdae to be used on a ton of different tractors and can work in either direction so they needed to be able to adjust the lag angle.

John T

Bob IH also made some impulse couplings complete for use with fire crater pistons. 25 degree 371894r91, 20 degree 363851R91, 17 degree 377037R91. For A through 6 series.

Indeed! I'm certain my ancient (and well-worn...) "Blue Ribbon" H4 manual was written way before fire craters were even thought of.

John - Similar to the Wico C/X the H4 was used on all manner of different IHC engines - everything from the A to the W9 diesel.

Also an H4's rotation can be reversed by changing a couple impulse parts and retiming the rotor drive gear.

...Bob M

Got it now. So when we time a mag so the impulse trips at TDC in starting the tractor will actually run 23° sooner. And on the older base mount mags there is an additional advance as you time them with that lever fully retarded.

Thats how I understand it, if the start time lags the run time 23 degrees (23 degree lag angle), and she starts at TDC, then she will run at 23 BTDC. All this timing stuff is subject to points gap and wear n tear n slop in the impulse assembly and other wear factors I reckon. Thanks to the gents who helped me understand IHC mags also. If youre ever at Florida Flywheelers, Grant has a beautiful under glass breakdown with all the parts and labels for an IHC Mag and a Generator, a pictures worth a thousand words and a complete breakdown of all parts must be worth ten thousand words........Yep, I'm more of a John Deere guy, but they let me hang out over there on the "dark side" lol and I even gave a brief workshop/seminar on IHC Charging Systems last year......

John T

Cowman, here is a link to a fundamentals of Mag operation.
Covers some very in depth info on mags, magnets, wires, and plugs. Lag angle is on the last page (shown in an illustration)
http://www204.pair.com/bbg46/FM Mag Manual/Mag Fundamentals & Ign Req(1-22).pdf

Well that was a first, post shows up in preview but then blank
when submitted

Cowman, here is a link to a fundamentals of Mag operation.
Covers some very in depth info on mags, magnets, wires, and plugs. Lag angle is on the last page (shown in an illustration)
http://www204.pair.com/bbg46/FM Mag Manual/Mag Fundamentals & Ign Req(1-22).pdf

Hey 36F30, do I have to get out my secret decoder ring to see your post? LOL. Poking a little fun.

Thanks, I got it saved

(quoted from post at 04:13:56 08/06/13) Hey 36F30, do I have to get out my secret decoder ring to see your post? LOL. Poking a little fun.

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Post is blank on modern view, I tried everything to get it to post.
Shows up in classic, I never use classic