Farmall A and B magnito install

dtaylor · Nov 1, 2012

I have installed 3 magnitos on Farmall cubs following the book.
They timed out perfect and ran well. Now I am trying to put rebuilt mags on a Farmall A and B. I follow the book every step. When I pull the mag away from the block to hear it trip it will not trip. With #1 on TDC the second revolution the mag hits the stop before it trips. What am I missing?

gene bender · Nov 1, 2012

You mite be turning the engine over a little too much on TDC. I instal them just before the piston is on TDC that way is easier for me. I even most times install just after it would trip done it too many times to remember. Are you sure the rotor is pointed at the tower when it trips cause if it isnt makes installing troublesome.I use the gear that turns them with wts removed and can be driven with a batt drill. Then in a vice with all wires and plugs hooked up i can turn it over and watch the fire at implule and running at higher rpms. Then when i instal i know all wires plugs and firing pattern is good. Another test is having all plugs removed and #1 wire hooked to a grounded plug you can turn the engine over with the starter and having your finger in the hole you can see it fire when on compression stroke thqt way you are sure it will work and you dont install 180 out of time.

PretendFarmer · Nov 1, 2012

Not sure what your book says, but I did mine according to the book I have.
#1. take out the plug on #1 and turn engine over(with hand crank) until top dead center on compression stroke. Compression stroke is obvious when you put your thumb over the spark plug hole you will feel the pressure push your thumb off. I'm sure you already knew that.
#2. Look under the bell housing. There is a small round hole up at the front, again on the bottom. Find the mark on the flywheel that reads "D-c 1/4". When that mark is in the center of the window, you can continue to step #3. Keep in mind that the mark is small and difficult to read.
#3. Turn mag by hand until the rotor is pointing at #1 cylinder. Click it over and then turn it to align with the key slot properly, bolt it up. good to go.

El Toro · Nov 1, 2012

I would redo the No1 piston to TDC on the compression stroke. Once you feel agaist your thumb drop a long plastic straw on top of the piston. Have helper to slowly hand crank the engine as you watch the straw rise. When the straw quits rising your piston is at tdc on the compression stroke. Then rotate the rotor so it points to the No1 plug tower which is about one o'clock. Then reinstall the mag. Hal

dtaylor · Nov 1, 2012

Thank you all for the suggestions. I will try again tonight. I don't post often but I read here very day. You guys have helped me more than you will ever know. Thanks!

PretendFarmer · Nov 1, 2012

(quoted from post at 12:39:05 11/01/12) I would redo the No1 piston to TDC on the compression stroke. Once you feel agaist your thumb drop a long plastic straw on top of the piston. Have helper to slowly hand crank the engine as you watch the straw rise. When the straw quits rising your piston is at tdc on the compression stroke. Then rotate the rotor so it points to the No1 plug tower which is about one o'clock. Then reinstall the mag. Hal

you can flash a light inside and actually see the piston.

Jim Becker · Nov 1, 2012

Since you have done it before with no problem, I imagine you are doing the procedure just fine. I suspect the governor gear was not timed properly when it was installed. With the magneto removed, set the engine at TDC and look at the notches the drive lugs of the magneto engage in. They should be at 35 degrees off horizontal, in the 2 - 8 o'clock position. If the drive is off, the governor isn't timed. If the drive is correct, you may have a problem in the magneto: misassembly or the wrong impulse unit installed.

TDC on an A is marked on the flywheel and harder to see than on the front pulley of a Cub.

El Toro · Nov 1, 2012

When you pull a mag or a distributor doesn't matter if it's a 4 cylinder or 8 cylinder gas engines note where the rotor is pointing and either make a sketch or use a magic marker and make a mark inside the housing. THEN DON"T MOVE THE ENGINE while the mag or distributor is off. When you're ready to to install just rotate the rotor to the mark in the housing or by your sketch and reinstall the mag or distributor. Hal

Tom Fleming · Nov 2, 2012

Let me relay something that happened to me. I bought a Farmall A from a guy and it had the engine rebuilt previously. Tractor ran fine, and it had a battery ignition distributor on it. I wanted a mag, so I swapped it out. I could NOT get the mag timed right.

so, I pulled the govenor and discovered that the mag/distributor gear was out of time by 3 teeth. With the battery ignition distributor on, there was enough adjustment to make it run right. with the mag, you don't have that luxury.

so, if all else is failing, it is something to check and is not that difficult to do. took me about 1 hour to disassmble and resassemble.

Just a thought depending upon the history of your tractor.

Farmall A and B magnito install

dtaylor

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gene bender

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PretendFarmer

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El Toro

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dtaylor

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PretendFarmer

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Jim Becker

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El Toro

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Tom Fleming

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