Case SC magneto impulse coupling

It has been sometime since I posted about my 1949 Case SC. I thought I would share this story to help others like me with limited magneto experience and not fully understanding the task of the impulse coupling.

It all started with the SC having to sit for about a year. A lot of that was do to the SC being hard to start, and life's twist and turns. Built a new building to hold the truck and SC so it was time to get the SC running again.

A few years ago I had bought a refurbished case magneto said to have a good spark. It did, but the SC was always hard to start often ending with a dripping zenith carb. The carb had always dripped back when I was a boy and the tractor belonged to my Grandfather so i thought that was normal for a zenith carb.

Could not get the SC started.

Pulled a plug and checked a for a spark. Got none. Carb flooded.

Cleaned the points, hit the starter and got a good spark. Carb flooded again and the the SC would not start.

Rebuilt the carb. Carb flooded but the SC would not start.

Convinced it was still the carb, I drained the tank, blew out the lines, pulled the sediment bowl and cleaned the screen(broke the bowel and had to order a new one) Carb flooded but the SC would not start.

Removed plugs, hand cranked to check timing. No spark.

Cleaned the points again. Hit the electric starter and got a good spark. Carb flooded again and the the SC would not start.

Bound and determined to fix a flooding carb that must be the SC problem I pull the carb clean again and doubled checked the bowl floats and settings. Carb flooded again and the the SC would not start.

Pulled the plugs, hand cranked again, to check timing. No spark.

Cleaned the points again. Hit electric starter and got a spark.

Finally the SC starts but will not run below half throttle. Engine surges constantly. Carb drips again when engine stops. Pulled the carb again.

Engine very hard to start, but when it did it surged.

Pulled the plugs again. Hand cranked to check timing but no spark. Pulled mag went through the mag. With cap off, I got a hit from the wire going to the cap. Life is good.

The SC starts but will not run below half throttle. Engine surges constantly.

Pulled plugs, hand cranked to check timing but no spark.

Convinced the sc will run with a stronger battery I replace the battery(it was getting old). Hit the electric starter and I get a spark.

The SC starts but will not run below half throttle. Engine surges constantly. carb floods.

Keep in mind this rebuilt magneto I bought from a fairly well know parts supplier is the only mag I have ever worked with.

I pulled the cap thinking this mag was just weak and somehow I had the timing wrong. I hand crank letting the wire going to the cap hit me in order to verify timing. It was not. pulled and checked the mad and checked the slot alignment on the governor. It was right but the timing checked using the coil hit on my finger said the timing was off.

My Son comes over. We changed the timing by going a tooth shift on the governor. SC started, had a lot of power but would not run below half throttle. Engine surges carb floods.

Pulled carb, cleaned rechecked floats.

Engine starts, surges and carb floods often.

Pulled the plugs, hand cranked, no spark.

Pulled mag. Found google page that said we should be hearing a click when hand turning. Got a click. Then I found a manual passage that said you might need a wrench to turn the mag over. This mag was easy to hand turn. Very easy.

Then I remembered I had the original mag that had been gutted and converted to use an automotive coil and points. I got it out and sure enough I had to put a wrench on it to snap the impulse.

Removed the Pal nut and securing nut on impulse cover on both mags. The mag I had bought had spring tension but very little.

I put the impulse spring and cover from my old mag on the mag I had bought. Used a wrench to turn the impulse coupling, not knowing my hand was touching the coil wire. Pow, I got a hard hit.

Put everything back like it should be including the timing.

Hit the starter and the SC came to life as if it was brand new. Ran like a top and no carb flooding.

I sure hope this story helps someone else. A weak impulse coupling can really give you all sorts of symptoms.

Thanks for the story the information may come in handy someday. Steve

Just knowing the impulse cover and spring can be removed without a gear like puller may help as well Steve. The SC starts like a brand new tractor now.

Good to know as well to check the strength of the impulse if you buy a rebuilt mag.

Thanks for the reply.

The purpose of the impulse coupling is to spin the magnet faster while the engine is cranking so the magneto will produce a hot spark. After the engine starts centrifugal force causes the impulse paws to retract. The impulse is not used while the engine is running. If the spring is broken or very weak. It can't hold the drive cup in the correct position to the magnet. Makes the spark out of time.

A lot of the time you can't tell if the spring is broken unless you remove the spring from the drive cup.

Back in 08. I had just finished sewing 14 acres of wheat with my 48 SC. I parked it in the yard everything was fine. I went to start it the next morning and all I would get was a few "pop,pop,pop" clearly out of time. I also didn't hear the click of the impulse. Took my mag a part and found the spring was broken. Put my spare mag on. I put a used spring from a Wico XH mag in my JMA Case. The spring was slightly smaller. Worked when I tested it on the bench. never put it on the tractor.

Started watching Ebay and found several listings for NOS Case magneto parts. I still watch today but rarely do I see any parts on Ebay now. I have a NOS complete impulse coupling, NOS points with new fiber cam, NOS distributor cap,two NOS new style springs and one old style spring. Been watching for NOS rotors and condensers. Have never seen any.

A condenser for an IHC H4 magneto can be made to work in a JMA or model 41 magneto.

I have also used coils for Fairbanks Morse type J or X magnetos in a Case model 41. You have to modify the coil to make it fit. Will throw spark like crazy.

You don't need any special tools to remove the drive cup. You can remove the paw plate with a small two jaw puller and a little heat.

You can also press the paw plat off. You must remove all the screws in the front bearing plate or breaker plate. Then make sure the plate is loose from the main housing. Set the magneto in the press facing down. Press on the end of the threaded shaft with the nut on the end of the shaft. Press slowing and you will hear a "pop" and the paw plate is loose. The magnet and front plate will come out the front. I've done this several times and have never damaged a magneto.

tractorguy2, I printed your reply and stuck it in my manual for future reference. Good stuff.