footshooter5
CB750 New member
Hi all,
First, let me take a minute to introduce myself. I've only recently come into the motorcycle world, but have in my garage a 1992 Honda Nighthawk 750 and recently acquired a 1981 CB750C as a project. By day, night, and all hours in between, I'm a surgery resident in St. Louis, MO, but some of us have hobbies too.
I bought a non-running cb750...and got it running (yes!), but for some reason cylinder 4 sounds like it's misfiring. I say this based on what I can hear, but also the exhaust pipes for 1, 2, 3 are mad hot (I can barely touch them), whereas 4 is just warm/hot.
I checked the compression across the 4 cylinders - all of them are between 120-125; not great numbers, but they're consistent. I checked the valve clearances, and they're all between 0.003 and 0.005 inches (cylinder 4 is 0.004 and 0.005 for intake and exhaust, respectively).
I've checked the plugs (brand new NGK D8EA, all gapped to 0.26), and I think the issue is fuel delivery from the carburetor but I wanted to get some input from people who really know what they're doing. #1 is actually a touch foul (maybe too rich of a mixture), but 2 and 3 look identical with a little bit of yellowing on the rim of the threads, whereas 4 looks like I just pulled it out of the box. I've stuck different coils on it, and it looks like it sparks consistently and doesn't look like there's an arcing (but I can't be 100% certain).
I also wanted to rule out a leak, so I sprayed some carb cleaner around the insulators on both sides of the carbs, and no changes in the RPM.
Currently waiting on a new set of gaskets, jets, and rings for the carbs, and planning on cleaning them up again (did once, but not a total dismantle) and seeing if that gets things going in the right direction.
Thanks again in advance!
First, let me take a minute to introduce myself. I've only recently come into the motorcycle world, but have in my garage a 1992 Honda Nighthawk 750 and recently acquired a 1981 CB750C as a project. By day, night, and all hours in between, I'm a surgery resident in St. Louis, MO, but some of us have hobbies too.
I bought a non-running cb750...and got it running (yes!), but for some reason cylinder 4 sounds like it's misfiring. I say this based on what I can hear, but also the exhaust pipes for 1, 2, 3 are mad hot (I can barely touch them), whereas 4 is just warm/hot.
I checked the compression across the 4 cylinders - all of them are between 120-125; not great numbers, but they're consistent. I checked the valve clearances, and they're all between 0.003 and 0.005 inches (cylinder 4 is 0.004 and 0.005 for intake and exhaust, respectively).
I've checked the plugs (brand new NGK D8EA, all gapped to 0.26), and I think the issue is fuel delivery from the carburetor but I wanted to get some input from people who really know what they're doing. #1 is actually a touch foul (maybe too rich of a mixture), but 2 and 3 look identical with a little bit of yellowing on the rim of the threads, whereas 4 looks like I just pulled it out of the box. I've stuck different coils on it, and it looks like it sparks consistently and doesn't look like there's an arcing (but I can't be 100% certain).
I also wanted to rule out a leak, so I sprayed some carb cleaner around the insulators on both sides of the carbs, and no changes in the RPM.
Currently waiting on a new set of gaskets, jets, and rings for the carbs, and planning on cleaning them up again (did once, but not a total dismantle) and seeing if that gets things going in the right direction.
Thanks again in advance!
