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No spark cylinder 1 & 4

rodsurbina

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Hi there. New here and very excited to get my CB back up and running! I have a ‘81 CB750F, all original excluding aftermarket exhaust (4 into 1).

Before any work, it ran...not amazing...but it ran. Carbs needed to be cleaned, boots were cracked and were leaking air badly, exhaust was all dented up underneath.

I rebuilt the carbs, replaced the boots, replaced the exhaust with the same model (4 into 1), replaced the regulator rectifier with a Ricks model, and popped in a new battery. Went to start....only got cylinders 2 & 3 to fire.

Replaced all spark plugs, wires, and both ignition coils. Same issue. Followed the ignition coil harness to the spark units/CDI boxes. They looked ok but I switched them to hopefully get spark on the outer cylinders. No luck, same issue. Getting spark out of one coil (cylinders 2 & 3) but nothing out of the other (cylinders 1 & 4).

I’m at a loss and I’ve come for guidance! Can anyone point me in the right direction?

Thanks in advance!
 
Check for key on 12 volts at the black wire on bad coil not firing. Kill switch on 'run' of course.

Has anybody ever set valves on the bike? A whopping problem, nobody does it then you lose cylinders and why so many go after the carbs although the carbs DO present issues themselves. First thing I do on one before I touch anything else is a compression reading. It has saved oh so much grief.

You DID charge that new battery right?
 
Check for key on 12 volts at the black wire on bad coil not firing. Kill switch on 'run' of course.

Has anybody ever set valves on the bike? A whopping problem, nobody does it then you lose cylinders and why so many go after the carbs although the carbs DO present issues themselves. First thing I do on one before I touch anything else is a compression reading. It has saved oh so much grief.

You DID charge that new battery right?

Ok I’ll check that out.

I doubt that anyone has set the valves. Has a new charged battery and yea, I should get the compression tested.

These are good points but they still don’t quite tell my why that coil is not getting spark. I read in a post you wrote a while ago saying the blue and yellow wires going to the ignition coils are grounds and the black wires with white stripes are the hot. Is that correct?
 
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Yes. The 'grounds' are actually grounded by the two spark units under the seat. If suddenly you lost one of the pulsers under the left side engine cover, well, they signal the spark units to fire by opening the ground up, no signal means no opening up ergo no spark from that coil.

Quick test there if my first sentence in post #2 already done. Kill switch to run again and key on with your suspect coil wired to the black and the blue or yellow wire off hanging in space. Take that blue or yellow and touch it to a good ground for a second (no longer to preserve coil) and then break the ground, the coil should spark a plug properly grounded. You can do it over and over. That tells you coil, wire, plug are firing and points at pulser issue under the left side cover, or a spark unit.
 
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