81 CB750c cylinders 2 & 4 won't fire

[tmilio]

I've got a 1981 cb750c. About a month ago I fired it up for the first time after sitting in my garage for 4 months over winter, had just done an oil change, and bought new plugs, it ran like a top. After that I pulled the carbs off to clean the floats, put the carbs back on today to run it and both cylinders 2 and 4 were not firing (cold headers). I know what you're thinking, secondary coil must be junk. It's not. I replaced it with a perfectly working one from my cb900 and it did the same exact thing. Pulled all the wires they all had spark for cylinders 1-4. Racking my brain here, any suggestions would help.

 

[tommycarl]

The first thing I'd suspect would be float related... sounds like that's the last thing you did since it ran well. Float height off or sticking maybe?

 

[tmilio]

Could be sticking. I'm gonna pull the carbs off tomorrow and check further into it to see. But when I pulled off the carbs last time all I really cleaned was the floats because my cylinder 1 carb was dumping gas. I have pods and bypassed the whole vacuum valve on the carbs and ran the fuel line straight to the lower rail

 

[tommycarl]

Before taking off the carbs I'd try tapping 2 and 4 with a screwdriver handle in case they're sticking and might break free... easy to try anyway.

I'm actually surprised to hear that you have pods (on stock carbs?) and it ran like a top. However, since you do have pods it would also be easy to squirt in some gas or carb cleaner while it's running to see if 2 and 4 pick up. Maybe "shade tree", but I'd try it.

Also, might be worth checking the insulators (intake boots) to be sure they aren't leaking and sucking air.

 

[tmilio]

I've had pods on my stock carbs for a while now without problems.

I will try that with the gas / carb cleaner but it very well could be my boots. They've been through a lot of abuse with taking the carbs on and off. With that being said I'm also gonna do a compression test to see where I'm at just in case.

 

[tommycarl]

If your insulators are solid but hard, you can improve that by boiling in wintergreen oil. I think I use about 4 ounces with a gallon to gallon and a half of water, and bowl for 45 minutes to an hour or so. They'll soften up and smell good. But DON'T do it in the house and be sure it's a very well ventilated area if you boil them... it smells good but not at all good for you. Pretty expensive oil too... I've bought it as go-cart tire prep in a quart can for like $20 plus shipping which drives it pretty high.

There's another mixture that you don't boil, but I don't remember what it is. Maybe Toluene and something else... maybe you could google it if you're interested.

 

[tmilio]

Cool I'll look into that, thanks!

 

[Dibs]

I've got a 1981 cb750c. About a month ago I fired it up for the first time after sitting in my garage for 4 months over winter, had just done an oil change, and bought new plugs, it ran like a top. After that I pulled the carbs off to clean the floats, put the carbs back on today to run it and both cylinders 2 and 4 were not firing (cold headers). I know what you're thinking, secondary coil must be junk. It's not. I replaced it with a perfectly working one from my cb900 and it did the same exact thing. Pulled all the wires they all had spark for cylinders 1-4. Racking my brain here, any suggestions would help.

Did you ever figure out whats wrong with the bike. Mine is doing the exact same thing. New plugs, wires, coils and new mikuni carbs.