Buzzy vibes on '81 Custon

[DicktheNick]

Hey all,
New to this forum but been doing bikes since the 70s. New to the Honda world also, been a Yammy guy with two Kawasakis thrown in to mix it up. All of it has been two-stroke except for a KLR650 and an XVZ1200 Venture.

I picked up this 1981 CB750C last March and did a 30-miler threw the mountains where I live. Noticed a buzzy high-freq vibration around 3500 rpm, and the bike didn't much want to rev above 4500 without getting really buzzy. Brought it home and started investigating.

First thing I noticed was a leak at the front sprocket. Didn't know it was locked in a groove until I'd already boogered it up pretty good. As good a time as any to dive into the motor, says I. So in we went.

All together again after replacing the seal and some other stuff. Fired it up today and on rev-up got the vibration is very strong between 4500-5000 rpm. If I push through to higher revs it smooths out fairly nicely.

Bike has 29k on it. When I split the cases I didn't find anything horrible. The main bearings were mostly gray but some copper showing through. Was gonna order some till I saw all the different sizes.Since I don't have any mics I put it back together as-is.

One thing that got me when I was removing the engine happened when I pulled the drain hose off the breather bottle. I got a huge amount of water and oil mixed together that gushed out. Wasn't expecting that and don't know if water's been draining back into the case, but saw no evidence of that.

So for the vibration. Does this sound like bearings? Loose chains? Something else? Do the bearings normally have copper showing through after 30k miles?

I have a spare motor to put together if I need to tear this one back down, and another on the way from Ohio. Thanks for your advice; been lurking here since actully buying the bike in December ( didn't pick it up till end of March). Lots of knowledgeable folks here.

Richard in NM

 

[DicktheNick]

Meant to post this in the engine section but was misled where it said "engine". Anyway, looks like I got it sorted. The other CB forum had vibration posts suggesting to try tightening the A cam chain. I did that last night and it seems much better. Will take it out for a road test today.

Richard

 

[DicktheNick]

Well, the vibration is still there but not like it was. It picks up between 4800-5100 rpm and then fades as the rpm increases. I've read that this year and others tend to pass on any engine vibes due to the solid mounting in the frame. Does that sound about right? My first store-bought bike was a 1975 Kawasaki 175 and that thing would buzz your fillings out of your teeth. When I got the '77 Yamaha DT250 I was amazed at the difference, hardly any vibes and none in the high-freq range. Evereything I'm feeling on the CB is high-frequency.

 

[dirtdigger]

Ive never ridden a bike that doesnt have a vibration at some rpm range. wider engine bearing clearances can increase vibration. Copper showing is pretty far into the bearing material, they are at the point they should have been changed.

 

[DicktheNick]

My only street bike experience was with a Yamaha Venture and it had some low frequency vibration in 4th and 5th gear but no high frequency buzzing. Going to see if Autozone has loaner micrometers so I can get the crank measured for bearings.