You both seem very knowledgeable about F sries tractors. Instructions for timing F-20's and F-12's say to put #1 on TDC compression and with the impulse locked out have the points just beginning to open.

Will this ensure that the impulse will also "snap" at TDC? What if it doesn't? How much advance is given by the manual spark advance. Will an F-20 idle as smoothly as an H or M? I have never been able to achieve this. Thanks in advance.

Not Athol or Dave, and they would likely have much better details, but I think I saw you posting this in Deere [yes, I've had to sneak around there just a bit lately :lol: ] and somehow didn't get around to a replying.

A lot of mags will fit an F20, so it's a bit hard to say what exactly, but it sounds like you have a manual advance yet, so let's rule out after market and say F-4.

I've always found it easiest to just put the audible click right at to slightly after TDC. Of course, still worth double checking that the points are just opening at TDC with the impulse off after you bolt it up [and minimum advance, though more if you want to try it a little late first].

Not sure about an F4, but when I had the timing light on an E4A on a 22-36 recently, I don't think I had over 15 degrees of travel (though a tempermental grounding point took up more than it should have). I think the impulse on that is an additional 17, but don't quote me there either!

Just noticed you wrote this:

Unlike the Wico, the magneto is bol...napping at TDC? What do you do if it doesn't?

If it's right-on without the impulse, it will be slightly latter with it, so you should be good to go.

During plowing and discing season we always moved the timing 2 holes ahead of TDC. Always pulled them to start them.

The F-4 magneto was the standard on F-12 and F-series tractors 1934 onward and it replaced the E-4A magneto that was used on earlier tractors ( parts books also show a Bosch magneto as an option).
Timing is discussed in the Blue Ribbon Service Training Course "Magneto" and in the Blue Ribbon "Service Shop Service Manual, F-12, O-12, W-12, F-20 etc" It states that the engine should be cranked so that number 1 cylinder is at top dead center, at this point the mark DC on the flywheel is in line with the index on the flywheel housing. The impulse coupling should be dis-engaged by inserting a small nail through the impulse oiling cup and lifting the pawl until it engages the pawl latch (it may be necessary to rotate the impulse coupling while lifting the pawl. After the latch is dis-engaged raise the spark lever on the breaker housing as high as it will go (fully retarded (earthed)) and remove the cover from the magneto. Rotate magneto coupling in its normal direction until breaker are rubbing block is on high point of breaker cam and adjust breaker points to 0.013".
Remove the distributor block and rotate magneto coupling clockwise (viewed from coupling end) until distributor disk is in position of Number pin on distributor cover (about 2 o'clock) with the breaker points just about to open.
Without disturbing this position place the magneto on its mount (magneto bracket) and insert base cap-screws loosely. Insert the proper amount of shims between the drive coupling members and install the two coupling cap-screws, The coupling is made that so that two holes line up exactly opposite each other; the cap-screws must not be forced or the setting will not be correct.
Then, to check timing, it suggests that the engine be turned so again the DC mark on the flywheel lines up with the register on the flywheel housing the breaker should be just starting to open and the distributor disk is lined up with the contact for number 1 cylinder.
Tighten up the base cap screws, install breaker housing cover and distributor cover (block). To return impulse pawl to operating position inset nail through impulse oiling cup and lift pawl latch. Then reinstall drive coupling cover with drain hole in lowest position.
I used this approach when reinstalling the magneto on my O-12 after the engine was completely rebuilt. On full advance the F-4 magneto fires 35 degrees ahead of top dead center whereas fully retarded fires on TDC.