1950 John Deere BN Rectify Timing Issues, Small Generator Issue, and Start/Run/Slow Idle Video

It has been awhile since I mentioned my JD BN. I've put a couple of videos on Youtube about fixing the timing of this tractor.

Regarding the timing issue, there were no less than three separate problems that a previous owner (or owners) had caused.

The first problem I tackled was the magneto drive being one tooth off. With 23 gear teeth, one tooth off is almost 16 degrees of timing. This next photo below shows the drive spline that couples to the magneto. The engine is at Top Dead Center (TDC) and the two slots are supposed to be horizontal.

This next photo shows the drive gear for the other end of the above coupling. This driven gear meshes with a drive gear on the crankshaft. There is a red paint mark on this gear and another red mark on the drive gear. Neither of those two marks match up with how the gears should be timed so I don't know why someone put them there. This driven gear does have a "V" stamp for timing but I was unable to find a corresponding mark on the drive gear, so I just put the V stamp at the 6 o'clock position when the crankshaft was at TDC, and this made it so the magneto drive coupling slots were horizontal. Who needs timing marks anyway.....

The second problem I addressed was the flywheel being off one spline. There are 10 splines, so the flywheel timing mark was 36 degrees off from where it should have been, and out of balance by that amount also. The below photo shows the center access cover popped off. There is a dowel in the center of the crankshaft, and the dowel is vertical. This is correct. As can also be seen, the split line on the flywheel is supposed to be horizontal but it is what looks to be about 36 degrees off. Hmm.

The flywheel cover had to come off, then the large nut on the end of the crankshaft, then two nuts had to be loosened. The flywheel has two threaded holes to use for pulling the thing off the splines, so I found a 5/8" piece of flat steel plate and drilled two holes the right spacing. It took awhile but the flywheel finally came off. It was just a matter of repositioning the thing on the flywheel and sliding it back on.

Anytime the flywheel comes off the crankshaft and reinstalled, crankshaft end play (side to side movement) needs to be checked. It needs to be 0.005-0.010" (5 to 10 thousandths of an inch), so a dial indicator is the best method to check it. End play is measured, then the large crankshaft nut is tightened slightly to move the flywheel further onto the crankshaft. This is done repeatedly until end play is within spec. I got this one to 0.008", right in the middle of the range.

The last part of the timing debacle was to properly set the magneto impulse lug. For a little background, John Deere had a number of different magneto drive cups (see below) that corresponded to the amount of advance they wanted the magneto to have. Early tractors with the distillate/gas engines predominantly ran at 35 degrees advance. The later engines that were gasoline-only predominantly used a drive cup with 25 degrees advance.

This next photo below shows the drive cup on my magneto. Its number is 6274, and my magneto's points will open 25° before the lugs get in the horizontal orientation when the engine is running.

These magnetos are equipped with an impulse and retard mechanism that provides a quick rotation of the magneto rotor (for improved low-RPM spark) and also causes the magneto to provide spark at TDC when the engine is being started. This is so that if the engine is being started by hand (which it could be), the spark occurs at TDC instead of 25° before TDC, which would make hand-starting nearly impossible without the retard. Once the engine starts, the retarding components are retracted by centrifugal force, permitting the advance to kick in.

The retarding lug is adjustable and it must be set to the same degrees as the drive cup, 25° in my case. The below photo shows some timing marks on the magneto case and the retarding lug plate. The purple circle in the photo is of the retarding lug. This lug is attached to a base plate that is held in place with four screws that you can see in the photo. The screws are loosened and the lug is moved to where it needs to be. The degree markings are described next. The orange line is a mark on the retarding lug base plate. The blue line is the top center degree line on the magneto's case and represents 12-1/2 degrees. Each mark is 5 degrees. The green line is 2-1/2 lines from the blue line, so 2.5*5=12.5, and adding the two 12.5's together gives us the proper 25° retarded start timing. The red line (35°) is where the retarding lug was set to when I first took the magneto off the tractor.

Before I started messing with anything, I checked the timing of the magneto with the engine running It was 45° before TDC (!!!!), which is way off from the 25° it should have been. With everything back together and put back to spec, the timing was correct.

Of course, as soon as I get the tractor running great, the generator stops working..... Argh. But that is the nature of these old relics. You fix one thing and another pops up. Fortunately the problem wasn't that difficult to fix. I removed the brush dust cover from the generator and with the engine running, I pressed each of the three brushes onto the armature to see if that would fix the problem, and it did. Apparently a little dirt or oxidation made the generator not work.

Below is a video I made after all the problems were fixed.

If anyone is interested, this next video shows what I did to the tractor to fix the timing.