Farmall Cub

Oh, boy. I'll try to be brief, as maybe what I'm fighting is well known out here amongst the Cub cognoscenti. When I first looked at timing my new-to-me Cub, I noticed that the magneto was advanced as far as it could be (turned clockwise all the way, from the distributor cap end view). The impulse couple snaps at about 8 degrees after TDC. I can, of course further retard the impulse, up to maybe 30 degrees after TDC, but the manual wants TDC. After carefully going through the procedure in the manual several times I was unable to get any better than the 8 degrees after TDC. I usually expect a rotational adjustment of this sort to be about in the middle of its range of motion when it's "on" the spec... not pegged. Since I can't even get to the spec, I'm wondering if the magneto-governor drive gear is off by a tooth. I suppose that it might be possible to get to this condition due to excessive wear, but that would be one heck of a lot of wear to my thinking. But you see where my best thinking has gotten me... BTW, I'm trying to deal with a no-start situation. Carb/fueling seems to be right on specs, and I do get good spark. The magneto was a bit dirty and I cleaned it with kerosene per instructions. I have seen this tractor run, and I drove it before I bought it. It seemed to have plenty of power then... Any ideas, anyone? Thanks a bunch!

Something is wrong in the way your magneto is aligned. Either the governor was installed a tooth or three off, or the rotor and pinion are off. Since the issue is noticeable in the impulse coupling, it points to the governor being off. When mag is removed, the two drive slots in the governor gear should be at approximately 2:00 and 8:00 positions as viewed from the driver seat. If not, you need to remove and reinstall governor. There are timing dots on the teeth, but hard to see with mag seal installed. Also uses helical gears, so you have to guess where to start in order to end up in the right place. You should be closer to the middle of your range of movement when the impulse snaps.

Also, make sure that your pulley has only one notch that lines up with the pointer at TDC; if two slots, you should be using the second one as TDC.

John Wilkinson wrote:I'm wondering if the magneto-governor drive gear is off by a tooth.

Quite possible since one idler tooth represents 10 crankshaft degrees.

Rotate engine to TDC #1 cylinder and remove the magneto. The magneto drive slot should be around 2 and 8 o'clock.

John, several years ago i bought a cub from a long time owner that had supposedly been overhauled. The shop told him his magneto was bad, among other things, and sold him a distributor. since it was all original other than that and the carb. I decided to put a magneto back on it and ran into the same problem. I fought it over an hour before I realized what the problem had to be. There is an idler gear that runs on the crankshaft and turns the governor which drives the magneto. Both the idler gear and governor gear have to be timed to the crankshaft to get the magneto to trip at the right point, and they had mine a tooth off. You can verify if this is your problem by comparing your engine with this picture. The drive slot should be at this angle anytime the TDC pointer is lined up. If not the gears are not timed right.


If it is not at this angle, you will have to retime the gears. You can with a little work remove only the governor and turn it one tooth and reinstall, but that may take a little trial and error. To time it by the marks the front end assembly has to be removed, the crankshaft pulley removed (requires a special puller), and the cover removed. Due to the differences in the way they are designed, you can replace the magneto with a distributor and not have to retime the gears if you prefer going that route.

Yeah, my magneto drive is not where it should be, much as I suspected--and feared! The notion of going in through the governor has a lot more appeal than pulling the crank pulley, timing cover, &ct. This option was not apparent from looking through my manual. So... I'll start there at the governor and see how it goes. I'll post the results, bloody knuckles or no, and hopefully without have to (delete explicatives). Thanks to all.

Oh, and special thanks for the photo and Patrick Henry!

Changing the governor timing is not that difficult, but can be time-consuming. If you remove the pin that holds the carburetor reach rod, two bolts that hold the rockshaft on the tractors left side, then remove the two bolts that hold the actual governor. This should allow you enough play to be able to pull the governor out from the engine block enough to adjust the position of the drive slot. Your block should have a punch mark on it just like the one in John's picture above (it is at about 2:00 on the round housing). This will allow you to be able to do it without seeing the punch marks on the gears inside. Just make sure that your engine is on TDC #1 cylinder first!! Once you get it all lined up, install all the bolts you removed, then make sure that the carburetor rod is adjusted properly and you should be good to go on the mag timing.

Okay! Fifteen minutes, and I got to position on my fourth try--had to figure out the helical gear "offset" as the governor housing slid in. No bleeding, and no explicatives need be deleted. You guys are great! Thanks.

That is STRONG WORK, John W! Congrats! Does the mag now trip as it should??

John Wilkinson wrote:Okay! Fifteen minutes, and I got to position on my fourth try--had to figure out the helical gear "offset" as the governor housing slid in. No bleeding, and no explicatives need be deleted. You guys are great! Thanks.

John,

You lead a charmed life!

Bill