newb81c
CB750 New member
Hello all- First post here.
I have an '81 CB750C with about 17k known miles on it - garage kept, almost no rust, PO kept very good care of it. I picked it up a few weeks ago, changed the oil, new air filter, new gas, and have put about 400 miles on it. As far as I could tell, the engine was running strong... Some clutch noise, a bit of valve tap. Nothing that I shouldn't expect on a 30+ year old bike.
I went for a ride yesterday and when I got home heard a noise that sounded like a rattling tail pipe coming from the no. 3/4 side of the engine. I checked the exhaust mounts and everything is sound. I cannot find anything loose.
Initially thought it was low end noise... Bearing/ connecting rod or something, but I put an aluminum rod up to the engine and the pinging sounds the strongest to me toward the top end near the no. 4 exhaust clamp. The sound resonates through the no. 3 and 4 pipes. The sound is restricted to what I believe is a single stroke of the no 3 or 4 piston. it's not entirely consistent but increases in frequency as engine speed increases. I'm also led to believe that it has something to do with oil pressure, as it does not sound for about 5-10 seconds after the engine starts.
As far as I can tell, no loss of compression, No detonation, the engine runs pretty darn smooth except for this one sound.
All just in time for a week long trip that I am no longer taking. I am not riding the bike. Only running it enough to locate the sound. It seriously sounds like someones taking a wrench to one of the pipes. If it helps, I might be able to record the bike running and upload it.
I'm pretty sure the engine needs to come out- just would like an idea of where to start looking. I've had experience with rebuilding auto engines, never even looked inside a bike engine though.
Any ideas would be greatly appreciated. Thank you in advance.
I have an '81 CB750C with about 17k known miles on it - garage kept, almost no rust, PO kept very good care of it. I picked it up a few weeks ago, changed the oil, new air filter, new gas, and have put about 400 miles on it. As far as I could tell, the engine was running strong... Some clutch noise, a bit of valve tap. Nothing that I shouldn't expect on a 30+ year old bike.
I went for a ride yesterday and when I got home heard a noise that sounded like a rattling tail pipe coming from the no. 3/4 side of the engine. I checked the exhaust mounts and everything is sound. I cannot find anything loose.
Initially thought it was low end noise... Bearing/ connecting rod or something, but I put an aluminum rod up to the engine and the pinging sounds the strongest to me toward the top end near the no. 4 exhaust clamp. The sound resonates through the no. 3 and 4 pipes. The sound is restricted to what I believe is a single stroke of the no 3 or 4 piston. it's not entirely consistent but increases in frequency as engine speed increases. I'm also led to believe that it has something to do with oil pressure, as it does not sound for about 5-10 seconds after the engine starts.
As far as I can tell, no loss of compression, No detonation, the engine runs pretty darn smooth except for this one sound.
All just in time for a week long trip that I am no longer taking. I am not riding the bike. Only running it enough to locate the sound. It seriously sounds like someones taking a wrench to one of the pipes. If it helps, I might be able to record the bike running and upload it.
I'm pretty sure the engine needs to come out- just would like an idea of where to start looking. I've had experience with rebuilding auto engines, never even looked inside a bike engine though.
Any ideas would be greatly appreciated. Thank you in advance.
