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Motorcycle - 1981 honda cb750c dohc 4 carbs keihin.

Cylinder 3 is not firing, motorcycle starts up fine with 1,2 and 4 firing. Just cleaned carbs by hand, did not use ultrasonic cleaner.

Spark plug on 3 has gas on sparkplug. Not sooty or oily.

Cylinders 1 and 2 are my best ones with lightly brown spark plugs, issue is not ignition coils. Have not checked spark plug 4 yet.

Alll valves are within tolerance.

Just put a brand new battery in.

I'm taking carbs tonight to be professionally cleaned and buying a compression tester.

Anyvother ideas on what to do?

UPDATE: 8/13/2018 Sorry for the extreme delay in response. Been hectic with getting married, finally had time to work on the bike.

What I did to try and solve the problem within the last week, after my initial post.

  1. I replaced the ignition coils , spark plug wires and boots. With this I checked to make sure the coils were good and measured both primary and secondary windings before putting onto the bike.
  2. rechecked valve clearance and all were within tolerance
  3. did (cold) compression test. cylinder 1 150 psi, 2 150 psi, 3 145 psi, 4 150 psi.
  4. spark plug 3 does get spark after putting the new coils on. its a solid white color.

I do not think there is a significant lack of compression in 3 there is more of a probability I was not holding the tool on the spark plug hole well enough. I had to use the push down tool, I could not get the compression tool to screw in properly.

After all this I still could not get cylinder 3 to fire. I'm left thinking I need to get the carburetors professionally cleaned.

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  • Is the lead to plug 3 in good condition? Why do you say the issue is not the ignition coils - as you have 1 plug not firing - what have you done to test the coil output to cyl 3?
    – Solar Mike
    Jul 25, 2018 at 14:27
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    This is extremely unlike to be a carburetor issue. Instead, there's something wrong with the HT system on Cylinder 3. Fault could be in the plug, the connector, the HT line, the coil, or the wire connecting to the coil. Swap the pieces one by one with another cylinder, and see if the non-fire stays with #3 or moves to another. When you move part x to another cylinder, and the non-fire moves to that other cylinder, the part you moved is the problem.
    – DavidRecallsMonica
    Jul 25, 2018 at 14:31
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    Have you actually checked for spark on plug 3?
    – GdD
    Jul 25, 2018 at 14:56

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Checking compression is good, as is swapping plugs. Be sure to dry off the wet plug with a propane torch before each try. If this '81 CB750 is like my '82 VF750, you have 2 coils where each fires 2 cylinders on alternate revolutions. It seems likely that the coils are OK but swapping the plug wires on those 2 cylinders (same coil) would be a quick way to test it (feel exh pipe heat). If your carbs have a drain screw you can try draining each one and comparing how much you get. Open the tank petcock (need vacuum if you have auto-shutoff) and apply fuel pressure for a minute to check for a stuck or leaky float valve. One old-timer trick is to shut off the gas, drain all carbs, squirt some aerosol starting fluid (diethyl ether) in the air box, and hit the starter. The engine will only run for 2 seconds but this should tell you whether that cylinder has compression and ignition. I feel your pain; good luck!

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