JD 420c spark

Wyattrowe said:

My confusion with that is that the cylinders both fire when plugged into the same spot in the cap if I rotate them to where they’re supposed to be. The video shows that both plugs and wires are good. It also shows that the distributor is clocked to the correct position. It’s like it uses up all the energy on the first spark and can’t spark a second time. Is it possible it’s a bad coil? Condenser? Still a faulty cap? To reply to the other question i’m unsure when it started happening. Maybe 4 years ago? I haven’t had to use it too much so it hasn’t been a massive issue it’s just something I would like to figure out.

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To make good spark, the points must be closed long enough for current to ramp up near maximum, then when the points open the collapsing magnetic field produces spark. So if the points are badly misadjusted, right on the edge of working, it is possible for the distributor cam to either not be closing or not opening the points for one of the cylinders. This could also happen if the distributor bushings are worn out, letting the shaft slop around. Since you changed the cap, I'd think it unlikely that both old and new are bad. And the same coil and condenser fire both cylinders, so those also seem unlikely. I still like my earlier guess, but check the points condition and gap and shaft slop and make sure the points are opening and closing twice. And here's a quick check you might be able to do without special equipment - pull the coil wire out of the center of the distributor and plug it onto a spark plug lying on the frame or somewhere grounded (not the fuel tank), watch it while cranking the engine and see if you get two sparks. You can also do similar with the plug wires, expecting one spark at each cylinder.