Sputters when engine warms up - 79 CB750

[tyler88]

Hi all,
I have owned my 1979 Honda CB750 since it had zero miles on it. It is in mint condition and gets used every week (except for in the winter). It took 37 years for me to have a problem with it. When the bike is cold it starts and idles fine, but when the engine warms up (after a few minutes of riding) it feels like the bike is running on 3 cylinders or less. I replaced the spark plugs and wires, but that didn't fix it.. I also took it to a mechanic who put a scope into each cylinder and found everything to look okay.. I don't know how to troubleshoot this or what step to take next. The mechanic quoted me $400 to overhaul the carburetors, but he is not positive that will fix my issue. Any advice would be greatly appreciated.
Thank you!

 

[amc49]

Compression check, how many times have you set valve clearances?

Mileage?

If motor has no squeeze to it then all other effort is a waste of time.

 

[Dave]

Aye, check compression 1st.

Then if that's OK, warm it up until it "feels like it's running on 3 cylinders" check the temp of the pipes a few inches off the head, check to see if one is colder then the others, maybe it is only running on three cylinders. I do this by putting spit on my finger and touching it very quickly for a very tiny fraction of a second. An IR temp gun would be much safer though and won't burn your fingers if your too slow or don't use enough spit. LOL

Visually inspect the sparks plugs, after running it while sputtering for awhile, are they all the same color. Are any black or wet?

Visually check the spark across the plug gap after it's warmed up and sputtering. You can either get a nice spark checker or use a spare spark plug plugged into the plug wire and grounded to the head. (don't leave a plug out while doing this, you can ignite the air/fuel mix shooting out the open hole) (and make sure you keep it grounded, the voltage doesn't feel good going through your fingers and up your arm).

Carbs are usually the last suspect, besides you must have good compression and a properly working ignition to be able to correctly make any carb adjustments anyways, so check those first.

Most parts of the ignition system can work fine when cold only to fail as they get hotter.