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Cylinder 4 not firing after carb rebuild


CB750 Member
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SE Michigan
Hello everyone,

After a few evenings of troubleshooting I turn to you, almighty internet, for a bit of help. I have a 1979 CB750K (the rare and highly sought after 10th anniversary edition, featuring such additions as plastic sidecover badges and... okay that's it). I purchased a Randakk's rebuild set and went through the carbs. JIS screwdrivers and a handful of JIS screw from McMaster make this much more enjoyable.

Put the carbs back in and it fired right up... on 3 cylinders. The #4 header doesn't get hot and pulling the spark plug wire doesn't cause an RPM drop. Troubleshooting thus far:

1. All four cylinders are at just under 150 psi compression.
2. Pulling the spark plugs and cranking over with the plug held against the block yields approximately the same brightness of spark on all cylinders.
3. Swapping spark plugs doesn't seem to work.
4. There is fuel in the #4 carb bowl.
5. To ascertain that the problem is fuel I removed #4 plug and poured a bit of gas directly into the cylinder, started the bike and had all 4 firing until the gas ran out.
6. After I determined all this I pulled the carbs back out and pulled apart #4 again.
7. All passageways seem to be clear, probed with either thin wire or aerosol carb cleaner.
8. Jets are in the right place and clear.
9. Throttle plate and choke plate are synced with the rest of the carbs.
10. Vacuum puck moves smoothly.

There is fuel in the bowl and the jets are clear up to the venturi. Other things could be wrong that would prevent it from running well but given this, wouldn't it at least fire a bit?

Any other tips?
I'll add my 2 cents, but not sure how far you want to run with it...

1. I wonder about float height in case you aren't getting any gas through the #4 carb...
2. I had a 900 which I think would have been the same carbs with different needles and jets... I had to clean the carbs multiple times to get it to run right. Maybe just tells that I don't clean carbs well the first time.
3. Any chance the #4 insulator (intake boot) is sucking air? Spray some carb cleaner or use a propane torch around the insulator to see if RPM changes due to #4 kicking in
4. I think you could remove the top from the #4 carb and see in side the throat. Maybe you could twist the throttle to see if the fuel pump is squirting gas, which might at least tell you something like maybe gas is getting past the float needle. You'd have to look toward the rear of the throat... might eliminate the float height as an issue.

Probably not much help, but what I'd consider looking at.
Hey Tommycarl, thanks for the tips.

I actually tried swapping the coils tonight and I'm pretty sure that it runs on all 4 off idle. I should hook up a tach and go through the procedure for properly adjusting all the idle jets. If that doesn't work then I'll go through a few of your steps, the float height looks promising.

Thanks again and happy riding.
A brief update:

The problem turned out to be the slow speed jet, the ones under the black plug between the jets. Page 18 of MacGregor's manual outlines a cleaning procedure. Lo and behold, cylinder four was plugged up. Some serious compressed air cleared them and the bike's back together and running.