Timing UT Magneto

skycarp · Jul 15, 2008

I am setting up a newly overhauled 1945 MM UT. Hopefully not a big issue, but in reading my operators manual on timing the magneto, it refers to tractor fuel or 70 octane gasoline. For 70 octane the mark made on the flywheel is to be 1 1/2 inch PDC. This will cause the mag. to fire 3 1/2 inches BDC. (This is quoted from my memory as I don't have the book in front of me.)

My question is, if running 87 octane, do I want to retard the spark even more, or does it even know the difference between 70 octane and 87 octane?

Thanks for your insites.

Dean

Anonymous-0 · Jul 15, 2008

I suggest use the 70 octane settings. Haven't set a mag on a U, but you don't want the retard spark before TDC when you start it. If anyone hand cranks it could kick back and break an arm if the spark occurrs before TDC.

The way a magneto's advance works is the trip arms on the impulse trip at or just past TDC. (1-1/2" past TDC in your case.) Then when it starts, the trip arms swing inward on grooves or tracks that actually advance the timing to the specified amount of advance built into the impulse coupling. The specified amount of advance is almost always stamped on the impulse (Sometimes you have to take it apart to see the number.)

There is only that one timing advance on most tractor magnetos. If you want more power, you may want to install a battery ignition distribuitor.

Anonymous-0 · Jul 15, 2008

I use the high octane in my 1940 ut. Gas is not the same as in the 40s and it runs great.
L A

Bob · Jul 15, 2008

You posted:

"Haven't set a mag on a U, but you don't want the retard spark before TDC when you start it."

I believe you meant to say do not ADVANCE the spark to before TDC, or ir may "kick back" when cranked.

Bob · Jul 15, 2008

87 octance vs. 70 octane gas will allow you to ADVANCE the spark, you certainly DON'T need to retard it.

HOWEVER, the "no spark before TDC under impulse or it will kick and break the starter" (you or the electric starter) rule still applies.

Anonymous-0 · Jul 16, 2008

Bob, In a way yes.

Most of the magnetos used on tractors have a spark at one initial position when cranking and after the engine starts and gets up to speed the spark then advances and occurs at an advanced position determined by the design of the impulse coupling. There is no variable advance to them as far as I know, like there is in battery ignition distribuitors that have spring loaded flyweights to do that job. The spark is either retarded or advanced.

The spark at that initial position (The "retard spark") should never be allowed to occur before TDC because of the hazard of hand cranking. Its hard on starters too.

Bob · Jul 17, 2008

Isn't that what I said in my post???????

Anonymous-0 · Jul 18, 2008

Bob said:

"I believe you meant to say do not ADVANCE the spark to before TDC, or ir may "kick back" when cranked."

Yes, if you are referring to the spark at that initial position when cranking and not in the advanced position the mag acutally fires at when running. You knew what you were talking about. This guy may not.

Timing UT Magneto

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