Topic: 1971 CB750: No Spark

[Encierros]

Hi SOHC/4,

I'm rebuilding a 1971 cb750 with a Motogadget M-Unit Blue. Starter works. Engine cranks over. Won't start.

I've replaced the two ignition coils with 3.0 Ohms Dyna Coils, as well as new plugs and wires. I have a line from the Motogadget M-Unit "Ignition" out to a connector, which attaches to the two coils. I'm getting proper voltage at the coils from the 12v M-Unit source and getting a resistance reading of 3.5 Ohms from the coils. Stator also seems to be wired correctly.

I pulled the plugs and am not getting a spark at any of the four. There's a good ground at the points but unsure which points lead goes to which coil. I know that doesn't matter regarding the no spark issue, but not sure what else to try. There's compression in the cylinders. I can feel the air being pushed out with the plugs removed.

I'm beginning to suspect a poor ground. The frame was powder coated and perhaps it's possible the engine is no longer grounded to the frame appropriately?

I'm also considering that I've used the wrong coils. I purchased the XS Performance Dual Output Ignition Coils for all Honda CB750's (1970-78) from https://www.vintagecb750.com/products/5/electrical/36/ignition-systems-coils-plugs-ignition-switches. Perhaps this set from 4ino1 would have been better? https://4into1.com/ignition-coils-and-caps-honda-cb750-sohc/

Thanks much for any help.

[calj737]

If you have 12v present at the wire to the coils from the M-Unit, then your issue is the wire from the ignition plate. You need the ground signal to release the spark.

Yes, powder coating a frame creates all kinds of havoc in certain locations. The motor needs a bare metal contact from the battery NEG to the frame, to the motor. Grind that internal frame boss and the engine case and any spacer used to insure 100% bare metal contact. Same for the coils (depending upon year/model).

Maybe a picture of how you have the coils wired (including the powered wire) would help.

[Encierros]

Thanks Calj,

Are the coils ground through the hanger attached to the frame? One of the OE wiring diagrams I was using didn't show the coils ground. (Attached)



Does it matter which of the two terminals on the coil the leads from the ignition plate and power connect to? I vaguely remember reading that it doesn't. I want to eliminate that as a potential issue as well.


-Enc

[calj737]

It doesn’t matter which of the two screws on the coil you use, but you do need to use the same screw on both for power and ignition. And don’t use the coil mounting screws, use the signal screws on the end.

I don’t believe your coil mount is grounded on that model. Others may know definitively.