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1948 cub ignition

4

4936farmall

New User
I have a farmall cub that still has the j-4 mag on it. The pinion gear is worn out and it broke the rotor, the prices on these parts surprised me so I would like to put a distributor on it. The only distributor I have is off of an M farmall which by the book has the wrong timing curve being 40 instead of 16 degrees advance. I looked at the new distributors from various suppliers but they are all advertised as 40 degree units even though they say the still work for the cub model. Has anybody run a 40 degree dist on a cub without issues?
 
Just my opinion. Mags when working correctly are just super. I would fix it or somebody on here should be able to help you.
 
You can limit the advance easier than making it exend. Look at the mechanism and see what it takes to put 2 pop rivets with washers in the slots. (just an idea) with modern gasoline the timing could be at or near 20 degrees. Jim
 
You could keep it simple and buy a used mag. There is no shortage of ih parts around. Should get a mag for 50.00.
 
I have heard about limiting the advance on a distributor, bur have never tried it. I am curious what would happen if one ran the 40 degree distributor right out of the box seeing how it is being sold for these tractors, or if it would cause harm. Maybe back the timing off to where the total timing is at around 20 degrees, which of course would make the timing around 20 degrees late at low rpm not sure if that would cause trouble or not?
 
Not a good idea. The advanced timing will detonate (knock) the rings off the pistons, or burn a hole in one of them. A M has a lot more volume in the chamber. The cub turns so slow, that it needs a limited advance. Retarding the timing to allow 20 when at speed will over heat the engine radically at anything but wide open, even starting will be difficult. Just look at the mechanism and you will see a method to limit the advance. Do they run? yes, do they work? no. Remember the mag has no advance curve, just full advance when running, and TDC when cranking. Jim
 
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