1981 cb750c cold cylinders 2 and 3

[brugmand]

Hi guys, having some issues with my current cb750 project

My setup. 81 cb750c with delkevic 4 into 1 exhaust, Murray intakes with mikuni vm34 carbs. Ngk D8EA. Stock harness/ ignition.

Problem cylinders 2 and 3 not running or getting like warm.

So far checked spark timing with strobe light on each lead, timing perfect for all cylinders. Swapped spark units, verified valve clearances, compression test came back 150,145,145,140 from cylinders 1 to 4. Checked spark is bright and consistent. No vacuum leaks. Vacuum consistent in each intake. Swapped all plugs around, swapped coils around. Tested both doohickies on the crank which came back within spec.

Left coil wired blue to negative, B/W to positive
Right coil yellow to negative and B/W to positive.

Left coil to 1,4
Right coil to 2,3

I'm at a loss now as to what else to check/try. I'm based in the Republic of Ireland so replacement parts are few and far between.

 

[SenseAmidMadness]

Very odd that it's 2 and 3 by themselves, and you haven't found anything with all the electrical testing.

This is the Murray's setup with just the two carburetors, right?

Make extra sure that the right side coil has a solid connection to the frame ground and that its yellow wire has good continuity to the spark unit and timing rotor.

 

[RogerSnell]

Hi guys, having some issues with my current cb750 project

My setup. 81 cb750c with delkevic 4 into 1 exhaust, Murray intakes with mikuni vm34 carbs. Ngk D8EA. Stock harness/ ignition.

Problem cylinders 2 and 3 not running or getting like warm.

So far checked spark timing with strobe light on each lead, timing perfect for all cylinders. Swapped spark units, verified valve clearances, compression test came back 150,145,145,140 from cylinders 1 to 4. Checked spark is bright and consistent. No vacuum leaks. Vacuum consistent in each intake. Swapped all plugs around, swapped coils around. Tested both doohickies on the crank which came back within spec.

Left coil wired blue to negative, B/W to positive
Right coil yellow to negative and B/W to positive.

Left coil to 1,4
Right coil to 2,3

I'm at a loss now as to what else to check/try. I'm based in the Republic of Ireland so replacement parts are few and far

 

[RogerSnell]

I’m having the same issue with my 1976 CB 750F cold cylinders 2&4.
I’ve gone through the bike compression, adjusted valves, spark is good with all cylinders. Emptied out the bowel chamber 2 was dry. 4 had less fuel then bowels 1&3

 

[barney905]

I've been reading these posts for a couple years now about the spark plugs fouling on these bikes. I have a k5 with 96000 miles on it ( original owner) and never had a problem until a few years ago when they started with the corn syrup (ethanol). Now I'm having the same issues. Mine is completely stock and well maintained. I just replaced my guide seals and lapped the valves and new gaskets just to show it some love. Doesn't burn or leak any oil. Compression is excellent. Rebuilt the carbs and re-synced just in case. Jets and needles are at original settings. I'm thinking fuel. It ain't the same stuff as it was. I guess nothing is anymore. I'll figure it out eventually.