MilfordCubicle
CB750 Member
Hello everyone,
I am new here, so first up something about my bike. I bought a CB 750 KZ '79 DOHC with 80.000 KM on it last winter. It's my first 4-Stroke motorcycle, but I was repairing/restoring some cars before that and wouldn't call myself unhandy with vehicles in general. The bike was sitting around for about 3 years and not in the best shape, neither the worst. But it was clearly lacking performance and hard to start. The PO didn't know much about engines, but gave it frequently to a mechanic to get little issues sorted out once in a while.
Since it was burning oil and the compression wasn't that well I decided to overhaul the top end, as well as getting the whole bike in a good shape. I also wanted to get rid of that clutch rattle.
Long story short, with the help of a Clymers manual I did the following things to the engine:
- New gaskets for the top end (headgasket, oilshaftseals, etc.)
- New piston rings
- Cleaned, lapped and shimed all the valves to factory recommendations
- Ultrasoniccleaned the carbs, new gaskets, new float valve, even the little filtgaskets on the chokebutterflys
- New clutch and clutch rubber dampers, aswell as new harder springs
- New starter clutch gear springs. Bolts were torqued with loctite on it
- Exhaustmanifold has new coppergaskets is tightened properly, so no leak there
- Timing spot on, Advancer works as it should
- Carbs synced
It's also to mention that the exhaust is a Sebring 4-1, probably fitted in the 80s. Otherwise it's pretty much stock.
I didn't go into the lower end of the engine, since everything in the topend looked fine and messured well in specs.
I also tried to keep the costs down, which I didn't turn out to be successful with anyways..
Just wait, we get closer to my problem:
The bike runs good now, steady idle, revs and accelerates great without any issues and trough out the whole RPM range. The spark plugs have all a nice and even color and compression is all the same over the 4 cylinders. The only thing is that its a little "buzzy" after around 5k, but thats not what I am concerned about.
My problem is, that it makes this unround noise, which first appears a few minutes after cold start and gets worse when warmed up. It seemes mostly on the right side of the engine. If I hold a hammer to the engine, and use the handle as a stethoscope, I can mostly hear it when I put the hammer to the exhaust, right at the manifoldclamps. It gets quieter if I move beyond the 4-1 collector and is not that loud if I put it on the valvecover, cylinders or bottomend. It appears to be evenly loud over all 4 exhaust pipes. Seems to come from the inside of the combustion chamber, but I don't know for sure. You can hear the sound on the video. It increases with RPM, but stays "unround" like that. Can't hear it while driving, but thats most likely due to exhaust noise.
The bike is 600km into the break-in periode for the pistonrings. I will change the oil once its at around 850km. Then I will also take the oilpan off and take a look what I'll find in there. The reason I didn't do that before is, that the pan was kind of stuck on the block, so I though "No need to change the gasket there, since it's not gonna get more sealed than that"
Hope this contains all the usefull information to help me with this. I posted a video of it aswell. Hope to hear from you guys soon!
I am new here, so first up something about my bike. I bought a CB 750 KZ '79 DOHC with 80.000 KM on it last winter. It's my first 4-Stroke motorcycle, but I was repairing/restoring some cars before that and wouldn't call myself unhandy with vehicles in general. The bike was sitting around for about 3 years and not in the best shape, neither the worst. But it was clearly lacking performance and hard to start. The PO didn't know much about engines, but gave it frequently to a mechanic to get little issues sorted out once in a while.
Since it was burning oil and the compression wasn't that well I decided to overhaul the top end, as well as getting the whole bike in a good shape. I also wanted to get rid of that clutch rattle.
Long story short, with the help of a Clymers manual I did the following things to the engine:
- New gaskets for the top end (headgasket, oilshaftseals, etc.)
- New piston rings
- Cleaned, lapped and shimed all the valves to factory recommendations
- Ultrasoniccleaned the carbs, new gaskets, new float valve, even the little filtgaskets on the chokebutterflys
- New clutch and clutch rubber dampers, aswell as new harder springs
- New starter clutch gear springs. Bolts were torqued with loctite on it
- Exhaustmanifold has new coppergaskets is tightened properly, so no leak there
- Timing spot on, Advancer works as it should
- Carbs synced
It's also to mention that the exhaust is a Sebring 4-1, probably fitted in the 80s. Otherwise it's pretty much stock.
I didn't go into the lower end of the engine, since everything in the topend looked fine and messured well in specs.
I also tried to keep the costs down, which I didn't turn out to be successful with anyways..
Just wait, we get closer to my problem:
The bike runs good now, steady idle, revs and accelerates great without any issues and trough out the whole RPM range. The spark plugs have all a nice and even color and compression is all the same over the 4 cylinders. The only thing is that its a little "buzzy" after around 5k, but thats not what I am concerned about.
My problem is, that it makes this unround noise, which first appears a few minutes after cold start and gets worse when warmed up. It seemes mostly on the right side of the engine. If I hold a hammer to the engine, and use the handle as a stethoscope, I can mostly hear it when I put the hammer to the exhaust, right at the manifoldclamps. It gets quieter if I move beyond the 4-1 collector and is not that loud if I put it on the valvecover, cylinders or bottomend. It appears to be evenly loud over all 4 exhaust pipes. Seems to come from the inside of the combustion chamber, but I don't know for sure. You can hear the sound on the video. It increases with RPM, but stays "unround" like that. Can't hear it while driving, but thats most likely due to exhaust noise.
The bike is 600km into the break-in periode for the pistonrings. I will change the oil once its at around 850km. Then I will also take the oilpan off and take a look what I'll find in there. The reason I didn't do that before is, that the pan was kind of stuck on the block, so I though "No need to change the gasket there, since it's not gonna get more sealed than that"
Hope this contains all the usefull information to help me with this. I posted a video of it aswell. Hope to hear from you guys soon!
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