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Half Spark Only On an 1980 CB750

utfrider

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Im only getting spark outta one coil. Specifically, the one powering #2,3 cylinders with the yellow wire. The other coil powered by the blue wire is dead. Both new coils and wires. Thought maybe I got a lemon of a new coil, so I swapped them and they both checked out fine. I cleaned all electrical connections. Traced the blue and yellow wires back to the so called "spark units". The 2 small metal boxes with heat sinks on them near the battery. Do these go bad? Can they be tested? Any other suggestions would be appreciated. Im at a loss.

Oh, and its a 1980 CB750c DOHC. All stock.
 
Oh, Thats a bummer. Guess I should replace em then. Im sure they aren't cheap. Any other potential problems it could be?
 
I tested the spark units. ( by swapping them with eachother. ) They both seem to be working fine. Also checked the pulse generator (under the left side cover that says HONDA) It looks clean, screws are tight, and there are equal gaps between the 2 pick ups. Any way of testing these?
 
Hey utfrider-
Sounds like everything checks out if you've replaced and inspected everything. Did you try cranking the engine over in the dark to see if there is an arc elsewhere?Break out the ohmmeter, and test the coil resistance.
Should read 530ohms, +/- 50 @68 degrees F. Cylinders 1 & 4 blue lead, 2 & 3 yellow lead. If this all checks out, you may dig into the Ignition Advance.
 
Hey utfrider-
Sounds like everything checks out if you've replaced and inspected everything. Did you try cranking the engine over in the dark to see if there is an arc elsewhere?Break out the ohmmeter, and test the coil resistance.
Should read 530ohms, +/- 50 @68 degrees F. Cylinders 1 & 4 blue lead, 2 & 3 yellow lead. If this all checks out, you may dig into the Ignition Advance.

I'm at the same place accept my problem is the yellow wire
 
With key on and kill switch to off (ignition on) the (black white?) wire(s) must have power to it(them) at the coils. The yellow or blue are GROUNDS.

You can also check the pulsers for action by using analog voltmeter on them as the engine is spun with starter. Use the lowest DC range you can to read the output. You'll see a needle blip every time they send a pulse down the leads.

Ignition advance has nothing to do with no spark.

The ignition system is waste spark meaning out of each pair of plugs one fires forward like normal but the other fires in reverse, or from the wire electrode to the center. Yeah. Backwards. That means that each plug pair makes a circuit, the spark flows out one plug, to ground in head, through head to other plug then backwards through it to complete the circuit. Why often losing part of the system results in BOTH plugs going down.
 
I'm having a similar issue with the blue wire?

I've got new coils and have tested ignition, spark units, pulse generator and numerous wires with a voltmeter. Everything looks to spec and I've swapped them with a working CB and all seems to be working fine.

Did you ever figure this out?

I rebuilt this 1980 CB750F it ran but poorly...my cam chain tensioner broke. After replacing tensioner and ensured that everything torqued to spec this lack of spark occurred. Did I miss something?

...this is killing me.

Any ideas would be appreciated.

Thanks for taking the time to read.
S
 
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