Hey folks of the CB750 forums. Longtime lurker, first time poster here.
I've got a 1980 CB750K with a very annoying ride quality problem. The bike has been serviced in pretty much every aspect; it was taken down to the frame and back up, had a ton of parts replaced including the engine head, upgraded front brake, forks rebuilt, new Ikon shocks, new front wheel bearings, etc. It runs great and rides alright...for the most part.
My issue is that the ride is uncomfortable. It feels like the forks are "hopping" over smaller bumps, and not compressing at all, rather than floating over them. This results in the whole front half of the bike bopping up and down at pretty much every speed from 10-55mph. Faster than that and the bumps are less noticeable. This is even visible in the headlight pattern at night, which jumps up and down all the time.
This isn't like an imbalanced wheel, as it is totally irregular and doesn't follow any kind of rotational or speed pattern. It'll hop a few times, stop, hop again, etc. I can see while riding that the forks are not compressing most of the time, but do compress when a more substantial bump hits them. I can get the forks to compress with just a bit of front brake, and they feel like they slide smoothly testing them while stopped with the front brake on.
The steering head bearings aren't loose and the bike tracks perfectly straight.
The forks were taken apart when the bike was all apart and got new seals and wipers, Progressive dual-rate springs, and 15 weight fork oil. One of the original forks was totally unserviceable -- the damper rod bolt was completely rusted in place and I could not get it out. A used right fork from Ebay (from another 1980 CB750K) was sourced to replace it. Both chrome tubes were polished by hand. Internally I didn't notice any broken parts or severe wear, and at the time I didn't have the measurement equipment to check dimensions. All the internal parts were very dirty with ancient gross fork oil and got cleaned thoroughly before reassembling. The bike then wasn't ridden for about a year due to engine problems.
Since I'm now riding this bike regularly, I've been trying to chase this down and cannot figure it out. I went through a loosening and tightening procedure to remove "stiction" from the forks and lubricated the fork seals with quality non-petroleum grease -- no change (though I didn't loosen and tighten every single part I was supposed to). Played around with the preload spacers for the Progressive forks, including removing them totally at one point -- didn't fix it. Today I even changed out the 15 weight oil for proper Honda 10 weight fork oil, filling both legs with 7.5 oz (which is about 6 inches of oil level), and that did nothing.
The only alteration that's been made to the stock fork geometry is a custom spacer and bracket mounted in the stack of the front axle/wheel bearings -- we changed to a digital Koso speedometer, and I replaced the speedometer drive gear with a custom-machined cone part and bracket for mounting the new speedo sensor. I tried my best to make sure that these two parts perfectly matched the width of the speedo drive gear, but it's possible that the axle when tightened is ending up slightly wider or narrower than it was with the drive gear installed.
I'm out of ideas. What could be wrong? Is something busted inside the forks, or do I have a bad fork misalignment problem?
I've got a 1980 CB750K with a very annoying ride quality problem. The bike has been serviced in pretty much every aspect; it was taken down to the frame and back up, had a ton of parts replaced including the engine head, upgraded front brake, forks rebuilt, new Ikon shocks, new front wheel bearings, etc. It runs great and rides alright...for the most part.
My issue is that the ride is uncomfortable. It feels like the forks are "hopping" over smaller bumps, and not compressing at all, rather than floating over them. This results in the whole front half of the bike bopping up and down at pretty much every speed from 10-55mph. Faster than that and the bumps are less noticeable. This is even visible in the headlight pattern at night, which jumps up and down all the time.
This isn't like an imbalanced wheel, as it is totally irregular and doesn't follow any kind of rotational or speed pattern. It'll hop a few times, stop, hop again, etc. I can see while riding that the forks are not compressing most of the time, but do compress when a more substantial bump hits them. I can get the forks to compress with just a bit of front brake, and they feel like they slide smoothly testing them while stopped with the front brake on.
The steering head bearings aren't loose and the bike tracks perfectly straight.
The forks were taken apart when the bike was all apart and got new seals and wipers, Progressive dual-rate springs, and 15 weight fork oil. One of the original forks was totally unserviceable -- the damper rod bolt was completely rusted in place and I could not get it out. A used right fork from Ebay (from another 1980 CB750K) was sourced to replace it. Both chrome tubes were polished by hand. Internally I didn't notice any broken parts or severe wear, and at the time I didn't have the measurement equipment to check dimensions. All the internal parts were very dirty with ancient gross fork oil and got cleaned thoroughly before reassembling. The bike then wasn't ridden for about a year due to engine problems.
Since I'm now riding this bike regularly, I've been trying to chase this down and cannot figure it out. I went through a loosening and tightening procedure to remove "stiction" from the forks and lubricated the fork seals with quality non-petroleum grease -- no change (though I didn't loosen and tighten every single part I was supposed to). Played around with the preload spacers for the Progressive forks, including removing them totally at one point -- didn't fix it. Today I even changed out the 15 weight oil for proper Honda 10 weight fork oil, filling both legs with 7.5 oz (which is about 6 inches of oil level), and that did nothing.
The only alteration that's been made to the stock fork geometry is a custom spacer and bracket mounted in the stack of the front axle/wheel bearings -- we changed to a digital Koso speedometer, and I replaced the speedometer drive gear with a custom-machined cone part and bracket for mounting the new speedo sensor. I tried my best to make sure that these two parts perfectly matched the width of the speedo drive gear, but it's possible that the axle when tightened is ending up slightly wider or narrower than it was with the drive gear installed.
I'm out of ideas. What could be wrong? Is something busted inside the forks, or do I have a bad fork misalignment problem?
