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.

[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