Magneto only fires one side

If points open and close properly at all times, both lobes of the cam (I believe it will have 2 lobes?) most likely culprit is the cap (or CAT calls it a distributer plate or assembly)

I expect you could check it with an ohmeter, but a megger would more likely tell the story on it. It may be shorting to the other cylinder wire connection or to the casing when it's under load. Or one of the brushes could be damaged or worn out completely so it no longer makes good contact.

maybe condenser

Thanks Guys,
I got stuck into those cap connections and cleaned it all out. I found that one old spring to carbon rod had poor connectivity. I found some auto distributor cap carbon brushes that fitted perfectly. I used the old springs though as they were longer. Re-tested connectivity through the cap at all points . I drill spun the magneto , but I still get poor and sporadic results. 2- 15 volts on the good side , not that high on the other.

Thanks , David

Thanks Guys,
I got stuck into those cap connections and cleaned it all out. I found that one old spring to carbon rod had poor connectivity. I found some auto distributor cap carbon brushes that fitted perfectly. I used the old springs though as they were longer. Re-tested connectivity through the cap at all points . I drill spun the magneto , but I still get poor and sporadic results. 2- 15 volts on the good side , not that high on the other.

Thanks , David

I tend to discount any problems with the generator section of the magneto since it makes the "juice" for each cylinder in turn because of the gear reduction between it and the rotor. You should probably be looking at things that only affect the one cylinder, not both of them. It wouldn't hurt to replace the condenser, but if it makes a good spark on one cylinder, I doubt it will help things much on the other unless it can increase the strength of the spark enough to overcome some other problem.

Are there two lobes on the cam that opens the points? (I need to look at one of these again, been a long time) If two lobes, do both cam lobes on the rotor shaft create the same point gap? You may have to hit the middle of the range between the two gaps if one lobe is worn more than the other.

Not sure how you are checking the spark? voltage. Most folks use some kind of spark gap or just hold the lead near a ground to check.

I still think it's likely that the cap is bad and is tracking the voltage from that spark plug connector to a ground somewhere. To completely take the cap and rotor out of the picture, you should be able to remove both cap and rotor and clip a lead somehow to the coil output and spin the magneto over to create sparks whenever the breaker points open. I'm not sure how you would tell if every other spark was dropping out, but perhaps you can hear the difference if it is intermittently failing to make a good spark on one side. Well that doesn't make sense, there won't be any "sides" without a rotor and cap on it.

Are you sure the one plug wire isn't bad somehow? Are you using two different wires to test the spark, can you swap them?

Hi auscuscus,
I would suggest testing the mag. at the coil high tension spark outlet button-spring tag. You could Blu-tak a plug wire onto it and then be able to check the high tension section of the mag. with one wire and spark plug. If you get two good sparks here then you can move on to the rotor button and cap. I see you have gouged out the carbon track on the rotor, I have succesfully filled the gouged out grouve with slow set araldite and smoothed it off. I also spray the rotor button- not on the brush track surface- and inside and ouside of the cap- again not in the brush holder bores- with :- "CRC Clear Urethane Seal Coat" - motor and generator winding seal coating insulator. It comes in a blue and white spray can, I got mine from the local Cat Dealer but Repco , Bursons, auto elec's, or such places should have it.
Hope this helps.
Cheers,
Eddie B.

My D4 7U's mag is now only firing on one side too. When I first start it, it fires on both sides and soon the right side starts to stumble and quits firing. I tried sparking the plug wire to ground and there just a very weak spark left. I put in new plugs, wires, filed and set the points, checked for cracks and carbon traces on the cap. I haven't changed out the condensor yet. We had a missing 1912 Buick last week, as it was dead on number one cylinder most of the time with a very weak spark. After changing the condensor in it's magneto, it now runs perfect on all four cylinders now. Maybe that will help with the D4. I hope. I have been able to start the D4 with only one cylinder on the pony motor firing. It takes plenty of one cylinder warm up time and ether to fire the engine off. Two cylinders on the pony are greatly needed.

I believe he maybe onto something valid. I have the heads off from my pony right now and while rotating the crankshaft noticed that both pistons move out at the same time (Ithink?). Well what I am driving at is on my John Deere, it doesn't fire every 360 or 720. Instead it fires both cylinders at some other oddball point, then fires in a boom, boom, coast, coast fashion.
Anyways the mag would need to fire two pulses in rapid fashion, this means that the condensor would need to supply a large portion of the voltage for the second cylinder!
Well is a CAT mag the same? In that the 2 cylinders fire 180 degrees from each other BUT there are 720 degrees total in a 2 cylinder four stroke engine.
Food for thought.........😕

No, the horizontal opposed cylinder pony motors are even fire. Sparks are delivered at 360 degree intervals. You can look at the cap on the magneto and the two wires are opposite each other. Magneto generator rotates at crankshaft rpm and has a 2:1 reduction gear to the rotor/points cam shaft so it rotates at half crank rpm. rotor makes 1/2 turn to get from wire 1 to wire 2 while the engine has gone around a whole turn.

The side mount pony motors on the D7 and D8 were odd fire and even fire at different times. It depends on the crankshaft design and the cylinder layout, but generally the designer tries to keep the engine from jumping around by having one piston balance what another is doing by moving in the opposite direction at the same time. With small, simple engines, that's usually about the extent of the balancing. The even fire side mount pony engine is an exception to the opposite piston movements "rule", it has both pistons moving together and counterweighted by a large weight on the crankshaft.

I expect the condenser change, points gap adjustment, etc can only bring back a weak cylinder by raising the spark voltage on all the cylinders. It was already good enough to spark the good ones, so no real change there. The difference is on the ones that have some problem, a hotter spark can overcome some things.