Topic: noise in engine sohc cb750k8

[Quail "Owner of the comfortable k8"]

I just got this bike with 24k miles.  it has a chain type rattle in the engine. I adjusted the camchain and some of it went away, I adjusted the clutch and some more noise went away ( Iknew it was not the clutch because that noise stoped when I applied it).  I hear it when riding it at all speeds (and at idle) but maybe not as much when I accel.  I am a mech of 35 years, it is not piston slap,rod knock or anything like that.   My guess is the internal chain, is this common, how do I check it short of a tear down.  can it be adjusted?  Need some of your wisdom in my direction.

[Harry]

Unbalanced Carbs will cause your clutch to rattle - typical SOHC4. Check the clutch FAQ.

[Quail "Owner of the comfortable k8"]

Wouldn't the noise stop when you pull in the clutch if it was that?  Pulling in the clutch make some noise go away but not the clattering type rattle noise that concerns me.  you maybe right.

[Harry]

No, the rattling wont go away completely by pulling in the clutch. If you give it a bit of gas, though, it dissapears, right? This is one of THE classic SOHC4 questions. Trust me, they all do it if the carbs are a bit out of balance...

[Ibsen]

My 650 is like that at idle speed when the engine is started cold. But when it get's warm it is completely quiet.

[Quail "Owner of the comfortable k8"]

Lets just say yes it does go away and it is the carbs out of balance, what causes the sound.  I will not be able to start it and warm it up untill later on.

[Ibsen]

I have always believed it is the primary chain.

[Quail "Owner of the comfortable k8"]

Is it worth the time, $, and effort to check the chain. Can it be tightened or do you always replace it.  If you replace it does the noise go away. Does the chasin ever brake when crusing, (I am not a racer).  I am saying this knowing I must need to adjust the carbs, I just don't want a chain to snap because it is worn out and I ignored the sound that hen the carbs were out of sync.  Please tell me if I am fretting a no problem.

[bransonmowoodman]

Cam chain tensioner sometime sticks, clean it or replace it and see what happens.

[Harry]

Lets just say yes it does go away and it is the carbs out of balance, what causes the sound.  I will not be able to start it and warm it up untill later on.

In an unbalanced engine, the pistons "work against each other" and cause the clutch to rattel. I thought I had a terminal bearing problem on my 500 - dissapeared after I had adjusted the carbs.

[cben750f0]

its the primary chain for the crank t the transmission, and no it isnt worth messing with, the SOHC's have bad tensioners on theprimary chain and when your carbs are out of balance the motor tends to 'bounce ' the chains as opposed to just 'pulling' them along.... i replaced the primary chain on my bike, and tensioner... and a week later it was doing the same thing.... when the carbs are out i have had ppl ask when it was gonna blow up.... 20 min in the shed with the synchro gauges... sweet running CB.... just balance the old dear, and ride in the comfort of a bullet proof motor...peace

[Quail "Owner of the comfortable k8"]

cben750f1

Thank you.  I wanted to hear that.

Terry

[CHUNG]

Sorry if I repeated a reply,
The Primary Drive chain has a spring loaded tensioner. if the motor is in a poor state of tune, it will yank the chain back and forth. slapedy whack.

[CB750F2]

Sorry for the late reply. It just so happened that I had removed my clutch out of my F2 and was waiting for a splined washer. When I read your post I refitted the outer cover and started the motor minus the clutch centre hub and clutch plates but with the basket still in place. The motor was cold so it ran a little rough. The clatter noise is still there but reduced. This supports the claims that most of the clatter is coming from the primary chain. I was pleased to hear that someone had replaced the primary chain and the clatter was still there because I did not replace mine because it was nowhere near the stretch limit. Pat form Nth Qld

[carsholduptraffic]

the dulcet tones of a 750/4 at idle.....
i doubt the clutch is the cause, carby balance and primary chain noise are partners on a sohc4!
a brand new honda primary chain with a brand new honda tensioner is quiet for about 5000km. then it is back to the more usual chain slapping 'let me out' sound. i got comments of 'thats the quietest honda4 i've heard from any bloke that knew anything about them for those 5000km's!!
then i thought it was about to die for another 5000k's, and then i just rode it. went a couple of years before i got a carb gauge and balanced the carbys. oh the joy. it went from dirty idle (it has a big cam mate) to purr (highly tuned beast). the primary chain sounds settled down a bit, but still were there.
the design of the tensioner on the primary chain is ordinary, but it's the reason why gear drive is the go these days. chains lash about, esp with the distance the primary chain has to travel, and its only a roller chain anyway. it still works, and i don't see any posts about anyone snapping primary chains, so a bit of noise is normal.
 if you ride without a helmet on, it sounds like it's about to destruct, but it's just lots of little pieces working really fast to make you go forward.

[Jonesy]

Someone back in the greenspun board gave an in-depth description of what's happening. (Here's the nickel version) When the clutch is engaged, all the clutch plates are rattling back and forth in the clutch basket as a single unit, making for more noise. When the clutch is disengaged, each individual plate can move on its own, making for less noise. My 750 is nice and quiet until you let out the clutch in neutral- then it turns into a rattle trap!

If it still keeps you up at night, you can drop the oil pan and check the resting position of the tensioner with a caliper. If it's out of spec, it's time to replace the primary chain. The whole procedure is outlined in the Honda shop manual.