Only 2 carbs firing

[Jenna]

Hi there
Recently bought a 1976 cb750 DOHC. Carb 1 and 2 are not firing and can't understand why.
Can anybody help? Any kind of insight to this would be muchly appreciated!
Thank you.

 

[dirtdigger]

first we will have to clarify. 76 is not dohc. dohc didnt come out till 79. What makes you think it is the carbs? Have you verified compression and spark?

 

[Jenna]

It's registered as a 1976 and thats what the guy who sold it advertised as, but we too got confused because i know they didn't come out till 79, maybe it's a different frame because it has been converted to a bobber?
Well tbh just guessing really but two of them are not heating up at all when being run. The compression is low reading about 80. I've just ordered some new spark plugs so hopefully that will help. Could it be related to the pistons or any of the seals?

 

[dirtdigger]

Engines wont run on 80 psi compression no matter what you do with the carbs or ignition. If it is a dohc motor then you need to check valve clearances as the valves may be held open.

 

[amc49]

X2, anything under 100 psi is not worth messing with, the engine itself is defective to not run. Always suspect the compression test though many have trouble getting good readings doing it, the nature of the procedure and the particular tool used there. Skills figure in there bigtime.

Pic the motor so we know what's up there.