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Discussion Starter #1
A few weeks ago I bought a Dyna S ignition for my 75 Cb750 and installed it. After installation and timing I could tell the bike wasn't running right. I used the timing light to check the wires for spark. Cylinders 1-4 are fine. Cylinders 2-3 are both running at idle, but when I rev the bike cylinder 3 cuts out for a second and then comes back, and cylinder 2 completely quits and doesn't start sparking again until idle.
When I first got the bike running (a year ago) cylinder 2 wouldn't run, and then it just started working. A couple months ago I started the bike for a ride and cylinder 2 wouldn't work for about 5 minutes, then it kicked in and ran fine. There has been an off-idle stumble since I got the bike that I couldn't tune out with the carbs, maybe it's been from the 3 cylinder cutting out for that second?
The coil tests fine, and I've checked all connections. Any ideas? I was just going to buy some Dyna coils, but I want to be sure the problem is the coil. Thanks, Jimmy
 

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A couple of things to try before you start throwing parts at it. It could be the hot lead going to the coils. I seem to recall that if there is low voltage going to the coil it will fire the side with least internal resistance and not the higher one. That could give the symptoms you have. Each coil has 2 wires going to it a yellow or blue from the points(pickups now)and a hot lead (Black? or Black w/ white tracer). Check that the hot lead is firmly in its connector and that it is not loose. The hot leads are identicle so you can switch them from coil to coil and see if your symptoms move. The other thing is that the coils ground through there bracket so make sure that there isn't rust insulationg the bracket from the frame and that the coils are bolted on securely. If that isn't the problem then it might be coils that staticlly test good but break down under load and heat.
Good luck,
Ken

AHRMA 412
Vintage racing - old guys on old bikes
 

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Discussion Starter #3
I took the known good coil off my cl175 and put it on the cb750. It looked the same except for a rotated bracket and longer leads. The bike fired right up and definately feels like it pulls harder (I replaced the very old points with a Dyna S). I looked for a stock coil on the net and found one for $30, but they were out of stock. Partsnmore has them, but they cost $20 more ($50). I'm checking Ebay, but no luck yet. Thanks for the help, Jimmy.
 
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