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Timing J4 Mag to Engine

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Anonymous-0

Well-known Member
1948 Cub. Just recieved newly reconditioned J4 mag. Following instructions in Original Cub Owners manual page 32. I have turned mag impulse coupling until the rotar arm points toward the No.1 terminal on the distributor cap. Then assembled the mag on the engine making sure the lugs on the impulse coupling engaged the slots on the magneto drive coupling and rotated mag as close to the block as possible. I then cranked the engine 1 complete turn until top dead center again. Then as the book instructs, I rotated the mag away from the engine block waiting for the impulse coupling to click. I moved it as far as it will go and NO CLICK. I know it is working because when I rotated the crank it clicks. Just not where the book says it should. Suggestions please!!!! Thanks
 
Sooooooo, has it clicked already or not yet? Sounds like the gear may be off a tooth or two but which direction? Have you had it apart and did it run before?
 
After installation of the new mag it clicked once while I was turing the engine the required one turn back to TDC. Seems like at about 3/4 the way throught the turn. Yes the tractor did run. I just bought it from the widow of my deceased cousin. The reason I am doing this is the original mag had been by passed with a regular coil. It was very hard to get started but ran fine. I checked the old mag at the impulse coupling had failed. I am in the process of installing a new mag, all new wiring, switches, battery and cables, plugs and wires,rebuilt generator and new regulator. I should mention 6V. I love using these old tractors, but want them to be maintained as they were from the factory. Any way, when you say a tooth or two off are you taling about in the mag or the engine? What is the fix?
 
In the Mag, When you replaced the points and you pulled the points cover(and rotor off the unit ,did the little 1/2" gear that runs the rotor happen to fall out and it didn't get put back quite in register????Don't ask how I know that....
 
Did you have the No1 piston at TDC on the compression stroke before you installed the mag?
You need to do this and see where your rotor is pointing. Hal
 
Yes, the engine was at TDC before the mag was installed. That is the piston closest to the radiator,removed spark plug,turned engine until compression on finger was felt,continued turning until notch on pully ligned up with pointer on front cover.
 
I would remove the cap then remove all the plugs then puting finger over the @1 hole turn the engine over and watch to see where the rotor is pointed when TDC happens you mite then need to time the rotor as the other post said doing it this way is fool proof on getting the rotor timed with the cap and the correct cycl as the mag itself can be off 180.Thats the way i do it when they say i cant get it started ect.
 
That wouldn't change the impluse any. Sounds like it was set a little fast. Which would be about right if they were use the mag as a distributor and without the impulse working. The impule naturally retards the mag at start-up. You are going to gave to move the gear a tooth or two. I don't remember how hard a job that is. It's been a year since I've been in a Cub.
 
Are you by chance replacing a distributor with a magneto, or has the tractor just been overhauled? Your description sounds as if the governor gear is out of time. I had the same thing on one I bought that a shop had done an overhaul and sold the owner a new distributor because the mag was bad. problem was they had not timed governor gear. To check governor the drive slot should be in this position when piston is at TDC mark.

drive%20slot.jpg
 
I think you just solved my problem. The photo did it. My slots are way off from what you are showing. How do I make this adjustment?
 
47H, has the engine been overhauled? The reason I ask, is that if you are replacing a mag with a mag, and the engine hasn't been touched AND it was running before, then the govenor pinion shaft is NOT out of time. Your mag is internally out of time, which is actually pretty easy to correct.

If it is the govenor pinion shaft, you have to take the govenor off the front of the block. there will be timing marks on the cam gear and the govenor pinion gear. it is about a 45 minute job if you take your time.

If the mag is out of time, you have to take the cap and rotor off, and then the internal cover off that holds the rotor shaft on. there are timing marks L or R on the rotor shaft. I believe the J4 mag is CW rotation. If so, then line the rotor shaft up to the R mark with the bevel ground tooth on the mag shaft. Should take all of 15 mins.

I am sure that Gene or I will be happy to talk you through this in either case.
 
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