CB750 - Only Cylinders 2 and 3 firing

S

syd_cb750

New Member
Hello people,

I have spent the last few months painting/cleaning up an old 1981 CB750F DOHC. I rebuilt the top end and re-installed the engine. Today I tried starting it, and only cylinders 2 and 3 are firing. I have gone over all the electrics and everything seems fine:
- Swapped over the spark units.
- Swapped over the coils
- Swapped over spark plugs
- Swapped the pulse generators 180 degrees and swapped blue and yellow wires to the coils

In all cases listed above, the bike starts, but only cylinders 2 and 3 are firing.

Does anyone have any other ideas? I'm kinda stumped on this one.
 
One coil is for 2-3 and the other is for 1-4 as you know. If you have swapped everything form one "side" to the other it suggests a bad connection/broken wire/missing ground on the 1-4 circuit.
 
I forgot to mention that I get spark on both 2-3 and 1-4. I have swapped everything from 2-3 to 1-4.

As both 'sides' have been swapped out completely, and still only 2-3 fire do you think it's a wiring issue?
 
Just took the carbs completely apart, there was a blockage in one of the jets on cylinder 4, cleaned everything in the ultrasonic cleaner. Now cylinder number 4 is firing but number 1 is not. Any other ideas?
 
spray a little carb cleaner in #1 when the bike is running of teh cylinder picks up it a carb issue
 
cxman, thanks for that :) fired when I sprayed carb cleaner in the air intake. Turns out I didn't clean the #1 carb properly, fires on all 4 cylinders now.
 
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