H4 magneto weak spark

[Lee_K]

I am trying to bring an H4 magneto back to life. The tractor was not running when I bought it and was told the mag was bad. I determined the coil was the culprit and bought a new one. I can now get a tiny blue spark when I get a ground very close to the coil output (less than 1/16"). I am skipping the cap, wires, rotor ect here so those are not the problem. I will add that the impulse was also messed up and I have been spinning the mag with a drill. The magnets will attract a medium screw driver but will not hold it up when I turn the mag upside down. Do I need the impulse to get an appropriate spark? I assume the drill rated for 1300 rpm should be close to the rpm of a running tractor. Any advice is appreciated.

Thanks,
Lee

 

[rustred]

you are turning the mag way too fast. if engine rpm is 1600 then 800 is max and no way you need to turn it that fast. even 100 rpm is good to test it. the impulse will simulate a fast rpm in order to give it spark to start the tractor. i think those magnets are too weak. that is one advantage to having a magneto you can hand crank it very slowly and these engine start easy. stick the mag in a vise and make it snap with a pair of vise grips and you should have a long blue spark.

 

[teddy52food]

The mag turns at crank speed.

 

[rustred]

the mag turns at camshaft speed , on my tractors anyhow... and that is half of crankshaft speed. so i dont know what tractor you have turning the distributor at crank speed.

 

[Dave41A]

Rustred is right. Your should be able to turn the mag by hand with it held in a vise and get a hot spark. A crescent wrench is a good tool to grab those flat bars on the impulse coupling. You should be able to turn it and feel the impulse spring wind up, then suddenly release. A wire from the vise/ground and held about 1/4 inch above the nub on top of the coil should throw a bright blue spark.

There are a few options. A weak magnet is one, but magnets do not "wear out" unless the magneto was stored for a long time somewhere it was subject to some sort of electric field or excessive heat. I would start by looking at the wire that goes from the coil down to the condenser and then down to the points--it might have frayed or bad insulation. Is the points cover on? Try turning it by hand with that cover removed--I have had a situation where the stationary point (new) was just a hair too tall and hit the points cover, grounding out. I had to file it down just a touch. You also have to make sure the little ground clamp that holds the condenser in place is making good contact. Mine was missing entirely but making a new one from a scrap piece of aluminum is not too hard to do.

Post back here with what you find. You should be able to get a lot more spark than you are. Magnetos are great but a lot of little things can mess then up. Good luck, Dave

 

[teddy52food]

Crank gear drives the cam gear which is twice as big as the crank gear so the cam turns at half speed of the crank. Then the cam gear drives the governor and magneto gear which is the same as the crank gear, so it is driven by the big cam gear at the same speed as the crank. This is on IHC and Allis for sure.2n,9n, and some 8N fords were driven by the cam, but that was a battery ign. not an H4 magneto.

 

[rustred]

It takes 2 crank revolutions to make one rotor revolution .so it’s 1/2 of crank speed. Don’t matter if mag or dist.

 

[teddy52food]

Yes, but the magneto still turns at crank speed as there are 2 gears in the mag that make the rotor turn at half speed.

 

[Dave41A]

Teddy is right--the magneto/distributor input shaft is "stepped up" from the camshaft by a ratio of 1:2, so it runs at crankshaft speed. The camshaft itself is "stepped down" 2:1 from the crankshaft of course. The internal timing gears in the magneto drop the speed 2:1, so the cap only turns once for every 2 revolutions of the magneto input shaft. Distributor works the same.

Photo of H timing gears is from another post here by "SW EM" on YT and shows the ignition gear is the same size as the crank gear, with the larger, slower camshaft gear in-between.

original link to photo

 

[rustred]

you are correct.

 

[Lee_K]

I used an ohm meter and could not find any indication of a short between on the wire that goes down to the points. The coil is new. This mag is actually off a case tractor. The mag drive on Case letter series and Farmall letter series is nearly identical so this IH mag bolts right up. Someone put a roll pin through the impulse coupling so it is solid. That is why I was using a drill to turn it. What value should the condenser read? If I remember right I was getting around 250 nF. I am thinking it might be a weak magnet. I am wondering if I should just convert it to battery ignition to get the tractor running then make a decision from there on a full rebuild on this mag, a refurb case mag, or distributor. Any further advice is appreciated.

Thanks,
Lee

 

[Dave41A]

Ok...just to be clear, this is a "Case" magneto, not an IH-made magneto? It is not really an "H4" then, unless IH made magnetos for Case, or someone put an IH magneto on a Case at some point.

Anyway, as luck would have it, I have a new capacitor for an IH H4, and a RLC meter. My new capacitor tests out at about 0.25 uF, so yours is probably good.

Assuming the construction of this magneto is the same as on the H4, I would try the magneto with the points cover removed to see if that improves things. There is also an adjustment for the points--they should be set to 0.013" when open, so you have to remove the points cover anyway to check them. Also, have you verified that the condenser is properly grounded? There should be a little aluminum clamp holding it down, held in place by what I think is a very long 8-32 screw (I could be wrong on the size). Mine was missing this clamp, and would not spark. Installing the clamp gave me big, bright sparks. The condenser itself is held in place at its top end by special threaded nut that sits in an insulated bushing. This has to be clean of oil and dirt to ensure there is no spark "leakage."

On the H4 the armature is what is magnetized, not the housing, so it may seem to have less magnetism than it really does. Honestly, I would keep looking for the real cause--I do not think "weak magnet" is your problem. You should be able to flip the magneto shaft a half turn at a time by hand--either direction is fine--and just spinning it like this with your fingers should throw big, hot sparks.

Post back what you find on how the magneto works with the points cover removed, points set properly, and the condenser clamped in place. Clamp the magneto in a vise and run a wire (clamped in the vise with the magneto for good contact) to 1/4 inch just above coil "nub." Spin the magneto shaft by hand and let me know what you see. I have a service manual and can help you troubleshoot from there. Good luck, Dave

 

[Dave41A]

Lee: I re-read your post more carefully--this is an IH H4 Mag on a Case tractor (not the other way around). I have the "IH" H4 service manual. Dave

 

[Lee_K]

Ok guys, I feel like an idiot for this one but... I had check for a short to ground with the points open but never for a connection to ground with the points closed. I had lightly sanded the points when I first took it apart, but had not done any thing else but set the gap. It turns out they were very dirty and not connecting properly. I now have a very nice spark.

Thank you for all your help!!!!
Lee