Mag timing with a timing light

Anonymous-0 · Dec 11, 2008

What is the proper way to use a timing light to time a mag? Do I disreguard the fire mark on the flywheel, and use piston location to set timing? Thanks stan

Bob · Dec 11, 2008

Most tractor mags fire at or very near TDC while the impulse is engaged (cranking), then advance all at once to about 25º BTDC (the exact "spec", "lag angle" is specified by the engine manufacturer).

So, the flywheel timing mark is likely to be @ TDC.

You CAN'T advance the mag earlier than that or the engine will "kick", destroying the starter (which could be YOU)!

So, if the mag fires @ about TDC @ cranking, and the actual running timing is not "early" enough, you CAN'T advance the mag, but will have to rebuild the impulse and/or switch it to a unit with a greater "lag angle".

Anonymous-0 · Dec 11, 2008

I agree with Bob below. I set the timing on my old Allis engines the same whether they're mag. or batt. ign. I turn the mag. or dist. to get the fire or advance mark(as opposed to the TDC mark) in the window at high engine speed. Of course the mag. impulse and the dist. advance need to be working properly.

John T · Dec 11, 2008

I time a mag (static start impulse timing) so she SNAPS and fires just at dead TDC. That allows it to start cuz if its too fast before TDC it can kick back yet if too late after its harder to start. You dont really need a timing light to set the static start impulse timing because you can hear it SNAP and set it so that happens right at TDC. Sure a timing light will flash then when she fires at that TDC but you cant have it cranking too fast or running to set the start timing cuz once running the mag will automatically be firing at its advance run time.

Then you can use a timing light to see where shes firing at once the impulse is disabled (around 250 RPM on Deeres) n shes sittin there running at idle or faster.

A Mag doesnt have a variable advance timing curve like a distributor (just has a start and run time ONLY), she starts at one time and then over 250 RPM or so the impulse stops working and she fires say at 20 to 30 BEFORE depending on the mag and drive cup and points gap and the lag angle setting and wear n tear.

John T

John B. · Dec 11, 2008

Some IH magnetos have a hole where you can disengage the impulse locking pawl and turn the mag over whitout tripping the impulse. Then you can use a meter or battery powered test light to check exactly when your points are opening.

RAB · Dec 12, 2008

Their eyes were better in those days.

Initial set, after mag had tripped, to a fraction after TDC (to prevent kick-back) - engine was turned backwards past TDC after mag tripped, then forwards to TDC, for the static set.

When running, designated plug lead was flashed in timing hole near to flywheel to illuminate the timing mark at rated speed.

Timing was then adjusted to optimum for fuels of the day. They often recommended a hat, or similar, was held over the area to improve visibility of the timing marks by reducing the ambient light level.

Just remember those fuels are not available now, so optimum timing may well be changed.

No reason why a modern timing light should not be used instead of the spark flash.

Regards, RAB

Anonymous-0 · Dec 12, 2008

If its an I-H It trips at TDC then when engine runs it goes to 30 some degrees advance. You have to line the drive on the mag in the slot on the gear. So a timing lite and trying to set advance is not possible. The older ones on hand crank like an F-20 ther was a lever to retard for starting. A timing lite would show the advance when running but there is no advancing to adjust.

Mag timing with a timing light

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