Markuli
CB750 Member
Hey everyone,
New 1979 CB750k owner here, and the bike is great but it definitely isn't perfect. The main issues I have noticed, after going through it to the best of my ability, are:
-Three cylinders run lean based on looking at the plugs and seeing a white tip and no fuel residue on the faces, but cylinder 2 has (what looks to be) dry fouling, we're talking solid black color. I don't have pics at the moment because I'm on vacation. Same results from replacing spark plugs twice.
-The bike sputters upon acceleration, and when holding at around 5000 RPM there seems to be some sputtering as well.
What I've done so far, in chronological order:
-Checked compression with crappy harbor freight tester. With all plugs out and WOT throttle, I got 90psi across the board. As I said it's a crappy gauge, and my bike starts easy and runs so it's probably reading low.
-Cleaned the carbs, blew brake cleaner and compressed air through every orifice I could find (and shot an idle jet plug into low orbit in the process), replaced two of those idle jet plugs which didn't seat snugly.
-Installed a new battery as the old one didn't hold a charge for more than two days. Now, it's smaller than the old battery, but it's a 12v and reliably starts my bike. "maintenance free" lead acid battery.
-Replaced all coils, wires, boots and plugs with new production replacement parts. Originally the bike didn't rev past 5000rpm, it now can get to 9000 after replacing everything.
-Synced the carbs with that 60 dollar quad gauge off Amazon.
-Checked my valve clearances, all my intakes were at .003/.004, and all my exhausts were at .005" with one at .006" on cylinder 2. I got all my intakes to .005" with the exception of 2 at .004, I can't currently remember which ones stayed at .004 right now.
-Checked on my pulse generator, the clearance between the lobe and pickups was out of spec at around .040", brought both down to .016" I didn't check the resistance on it though.
And that's where we are today. I had just done the pulse generator Thursday, and now I'm on vacation so I have some time to write up this post!
To see if it was a carb issue, I pulled the choke out during acceleration and didn't notice any change in the bike's behavior. At idle the choke does its job, so I know it's working.
To see if I had a dead cylinder or something even with the uniform compression, I pulled each boot off its plug at idle and noticed a difference in idle sound with every cylinder.
I really don't know how much power I should be expecting from the bike, but from what I've read on the forum it seems like the bike isn't delivering all it's got. I've pretty much exhausted my mechanical capabilities up to this point, which is why I am asking for recommendations for a guy in NJ who really knows DOHC's to put a fresh set of eyes on my project. But I'm willing to try some more stuff if you guys think I can do it myself, based on what I've done already.
Thanks in advance for anyone trying to help me figure out this bike's issues. I really do enjoy riding it, but I know it has more to offer and I want to get this bike running how it did off the factory line in 1979 (or at least kinda close!)
New 1979 CB750k owner here, and the bike is great but it definitely isn't perfect. The main issues I have noticed, after going through it to the best of my ability, are:
-Three cylinders run lean based on looking at the plugs and seeing a white tip and no fuel residue on the faces, but cylinder 2 has (what looks to be) dry fouling, we're talking solid black color. I don't have pics at the moment because I'm on vacation. Same results from replacing spark plugs twice.
-The bike sputters upon acceleration, and when holding at around 5000 RPM there seems to be some sputtering as well.
What I've done so far, in chronological order:
-Checked compression with crappy harbor freight tester. With all plugs out and WOT throttle, I got 90psi across the board. As I said it's a crappy gauge, and my bike starts easy and runs so it's probably reading low.
-Cleaned the carbs, blew brake cleaner and compressed air through every orifice I could find (and shot an idle jet plug into low orbit in the process), replaced two of those idle jet plugs which didn't seat snugly.
-Installed a new battery as the old one didn't hold a charge for more than two days. Now, it's smaller than the old battery, but it's a 12v and reliably starts my bike. "maintenance free" lead acid battery.
-Replaced all coils, wires, boots and plugs with new production replacement parts. Originally the bike didn't rev past 5000rpm, it now can get to 9000 after replacing everything.
-Synced the carbs with that 60 dollar quad gauge off Amazon.
-Checked my valve clearances, all my intakes were at .003/.004, and all my exhausts were at .005" with one at .006" on cylinder 2. I got all my intakes to .005" with the exception of 2 at .004, I can't currently remember which ones stayed at .004 right now.
-Checked on my pulse generator, the clearance between the lobe and pickups was out of spec at around .040", brought both down to .016" I didn't check the resistance on it though.
And that's where we are today. I had just done the pulse generator Thursday, and now I'm on vacation so I have some time to write up this post!
To see if it was a carb issue, I pulled the choke out during acceleration and didn't notice any change in the bike's behavior. At idle the choke does its job, so I know it's working.
To see if I had a dead cylinder or something even with the uniform compression, I pulled each boot off its plug at idle and noticed a difference in idle sound with every cylinder.
I really don't know how much power I should be expecting from the bike, but from what I've read on the forum it seems like the bike isn't delivering all it's got. I've pretty much exhausted my mechanical capabilities up to this point, which is why I am asking for recommendations for a guy in NJ who really knows DOHC's to put a fresh set of eyes on my project. But I'm willing to try some more stuff if you guys think I can do it myself, based on what I've done already.
Thanks in advance for anyone trying to help me figure out this bike's issues. I really do enjoy riding it, but I know it has more to offer and I want to get this bike running how it did off the factory line in 1979 (or at least kinda close!)