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CB 750 KZ DOHC - Knocking noise

MilfordCubicle

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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!

 
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Internal engine problem, that sounds like rod knock to me. Possibly piston striking head..........hoping I'm wrong of course.
 
Thanks for the answer, even though I would like to have heard another one instead... Is there a way to validate the rod knock, without pulling the engine? I don't understand though, how it should be striking the head, without touching the valves first. A new headgasket has been installed, aswell as the bottom gasket. The head was straight, so I didn't resurfaced it, so no material missing. Is the clearence on TDC so little on these engines, that a little worn bearing would cause the piston to strike the head?

The rod knock/Piston knock i heard before has been of much more "round" nature. Would you say, that this "unroundness" could be typical for rod knock, though?
 
You use the word 'round' and a very uncommon use of it, although thinking I grasp what you are trying to say. The noise is actually regularly cyclic in nature and somewhat unusual although not impossible. Usually they do it all the time or it is irregular, coming or going with no regularity to it other than it happens again. If you have a spun bearing and the bearing is actually rotating in rod bore then that could cause the type of cycling there. Usually they jam though to not rotate. Other than the regular cycling it is rod knock to me.

'Is the clearence on TDC so little on these engines, that a little worn bearing would cause the piston to strike the head?'

Yes, almost all engines at least at the outer piston periphery have a very close clearance for squish or quench to stir up turbulence to burn mixture faster. That clearance is .030" or smaller whereas proper valve distance is .100" or greater if the valves are correctly in time. Machining a head does not change that quench clearance number (it DOES change the valve clearance though) as it is the absolute bottom flat of the head, machining block DOES change it. Losing a rod bearing lets piston move up slightly further to hit head immediately or shortly thereafter as the bearing pounds out thinner and thinner. On these DOHC likely to hit piston to head at front or rear of it as piston rocks going over TDC.

Drain the oil looking for bits of bearing material, that can often tell you. You have a lot of mileage there and the #3 rod tends to go first as they have one side unsupported by a main bearing for a distance, the crank warps slightly due to the primary chain load and why that rod tends to go bad. A real problem on the bigger 1100 with much more power, they have #3 issues if hotrodded at all. Why people racing them use 900 cranks instead of 1100, the 1100 uses a bigger chain which increases the likelihood of it happening. I tend to dog my 750s and have seen bad bearing come out of that location too, they show copper much faster there than any other location.
 
I appreciate your detailed answer and start to see things more clearly.. unfortunately. If I would trust my guts, I will also say it comes from #3

English isn't my mother tongue, therefore the maybe unclear description. As you guessed it: They noise is regularly cyclic, but not in a 4/4 beat, rather a 2/4, what threw me off the idea of a blown bearing.

When I come home in an hour I will set the piston a bit over TDC and try to push them with a piece of wood through the sparkplughole, to see if I can detect some wabble there already. When I had the head of the first time, I actually tried to rock the rods and couldn't feel any freeplay..
I will also face my fears in the weekend and drain the oil to see what expects me there ^^

I guess I learned (again), to not do things halfway. Should have split that case in the first place, to take a look at the bearings.

I will keep you updated.

PS: Quick question, since I don't have my workshop manual at hand: Could I change rod/crank bearings without removing the head? Or do I have to take the timing chain off the cam first?
 
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Little update: I made these tests.

1. Test: Piston a bit after TDC, then with a wooden handle, through the sparkplughole and trying to push the piston a bit down --> No wabble on either of the pistons.

2. Test: While the engine is warm and running, I unplugged one cylinder at a time. --> No change of the noise at any of the cylinders.

Maybe this could help with the diagnosis.
The oil pan review will follow on the weekend ;-) Thanks for the help so far!
 
The rod bearings can be changed but if spun the crank may well be junk. Engine has to be out of course, head can be left on for rods only. Crank coming out means cam chain has to be loosed, head comes off. To remove the lower case half the alternator rotor MUST come off, it traps the lower case on. You can get the case loose but not remove it completely, rotor blocks it.
 
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