Magneto timing and play in rotor drive gear

Don Yoder · Oct 18, 2010

I have overhauled my Farmall A -1940 - reassembled it and upon trying to start it found the Magneto to be out of timing even though it was aligned with the mark on the gears. In order to get it in time with number one TDC cylinder I had to off set the drive gear by two teeth. It now seems to rotate and align up with correct firing to TDC on number 1 cylinder. Any one have any experience in this before. Also the rotor drive gears have about 1/8 inch play when you rotate the rotor by hand. Doesn't sound right to me - should these gears be replaced, or will it run with that much play in rotor rotation ??

Any one have any knowledge in this area??

Zachary Hoyt · Oct 18, 2010

I have no specific knowledge but I have a 49 H with the same mag and it has some definite slop in the gears. I rebuilt it and it worked fine after Janicholson helped me with how to do it and how to time it. I think you want to turn the rotor back as far as the play allows and time it there since it is not likely as far as I can see to get ahead of itself.
Zach

gene bender · Oct 18, 2010

The point of the rotor is wide and so are the pins in the cap so that will leave for some error. Is the play most in the gear or does the rotor have some play in the slot where it fits.If you have the rotor lined so it fires at the pin on the cap it will probably run just fine remember the spark does jump pretty good. I would try it a good test is remove all the plugs and then with the wires on them you can turn the engine over and watch the fire and with finger in the hole you can tell easy if its firing on TDC.. I have the end assy thats in new shape if you think you need it.

Anonymous-0 · Oct 18, 2010

Don, I seem to recall those mags(H4)have two timing marks on the gears. You time it depending on the shaft rotation. It is also used on the MD WD6 and WD9 and runs in the opposite direction as on the Gasoline engines. If you are useing your mag the impulse coupling will be right, if you get a used one make sure it's setup for your rotation.
Later Bob

Bob · Oct 18, 2010

Dan, you KNOW there's not THAT much play in the gears. That's IMPOSSIBLE. The rotor may fit a bit loose though.

If you have ANY knowledge at all about Kettering ignition systems you will realize what is important is the time at which the breaker points OPEN, making the spark, NOT the exact position of the rotor.

If the rotor is even CLOSE to where it needs to be at that time the spark will make it's way to the proper high-tension terminal.

WHY would you take it upon yourself to screw with the timing of the rotor, rather than setting it "by the book"???

Here's a link "to the book":

http://www.cleancomputes.com/Cub/Blue%20Ribbon%20Service%20Manuals/GSS-5035%20Service%20Manual%20Magnetos/index.html

Don Yoder · Dec 29, 2010

I have spark at tdc on #1 cylinder - timing is on mark after I created my own timing mark. I still fail to even get a firing with starter fluid in number 1 cylinder.

I am interested to know if you still have the magneto front end and what price you want for it ?

Magneto timing and play in rotor drive gear

Don Yoder

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Zachary Hoyt

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gene bender

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Anonymous-0

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Bob

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Don Yoder

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