Topic: CB750 lower end "ting" or rattle. seems to be the primary chain

[bmikkalson]

Hi, workin on a 78 CB750,  the bike has a lower end "ting" or rattle..  It sounded like it was comin from the the primary chain..  I pulled the motor out to inspect the primary chain tensioner,, the tensioner is good. But one of the two primary chains is much loser than the other......  So my question is, what would make one chain strech??     Or is this a common problem? 

[Hondawggie]

Hi, workin on a 78 CB750,  the bike has a lower end "ting" or rattle..  It sounded like it was comin from the the primary chain..  I pulled the motor out to inspect the primary chain tensioner,, the tensioner is good. But one of the two primary chains is much loser than the other......  So my question is, what would make one chain strech??     Or is this a common problem? 

I had a primary chain rattling something fierce on a kz550 early this year.  It was a real mechanical sounding rattle that was loud when the bike idled but harder to hear when the throttle was open -- this was on a kz550 '81 with a very quiet stock exhaust.

Of course this is a different bike.  Maybe the same cause though.   It came down to my carbs being way out of sync.  The way the folks explained it was -- at idle:

- if one carb is open more then that cylinder will rise in revs and the primary chain tightens
- then the revs drop when the cylinder with the less-open carb fires and the primary chain goes slack
- then the more-open carb fires again and the primary chain goes taut
- then it goes slack
- so with out-of-sync carbs the primary chain is going taut-slack-taut-slack and forever.  The transitioning of the primary chain going taut-slack-taut-slack is what makes the chain so noisy.  When the carbs are in sync the primary chain is not speeding up/slowing down/speeding up, it runs at a more constant tension so it doesn't rattle.

I did not 100% believe them but sure enough when I pulled my carbs they were way out of sync.  I synced them up and the primary chain noise disappeared and the bike sounded great, no more mechanical noise.

Don't know if that's what part of your problem is but carb sync apparently has an impact on the primary chain noise.

[MCRider]

Hi, workin on a 78 CB750,  the bike has a lower end "ting" or rattle..  It sounded like it was comin from the the primary chain..  I pulled the motor out to inspect the primary chain tensioner,, the tensioner is good. But one of the two primary chains is much loser than the other......  So my question is, what would make one chain strech??     Or is this a common problem?

How do you know the tensioner is "good". The wheel may look OK, but what about the spring? Hard to know without comparing it to a new one.

As mentioned, running in a poor state of tune will make the Pri chains louder. And I think will promote ones wearing faster than the other.

Finally, did you jump the gun on your analysis? These bikes have a loud clutch rattle that can be misdiagnosed as Pri chains. If the bike has less than 40k on it pri chains should be fine. Did you test the slack of the chains per manual, indicating wear life remaining?

[bmikkalson]

The rattle was like a marrble rolling around inside,  it would come and go not constant...    There are two chains that ride right next to eachother,, and one side is much loser than the other.



This whole carb out of sync is complete BLACK VOODOO to me...  It sounds plasible but the noise was not constant,,,  I found some metal chunks in the bottom of the oil pan,,,,

[MCRider]

"I found some metal chunks in the bottom of the oil pan,,,, "

Ooops.   

[bjatwood]

That aint a good sign...metal chunks in da Pan!

[bmikkalson]

Ya, they are more like "shavings" I will try and get some pics..

[Hondawggie]

The rattle was like a marrble rolling around inside,  it would come and go not constant...    There are two chains that ride right next to eachother,, and one side is much loser than the other.

This whole carb out of sync is complete BLACK VOODOO to me...  It sounds plasible but the noise was not constant,,,  I found some metal chunks in the bottom of the oil pan,,,,

Yep I argued on the Kaw board 'sorry I don't believe having carb slides out of sync will cause the primary chain to rattle' then they kinda lit into me, called me a dooshy newb, they called me a Hambone-Sammy they called me this and they called me that. 

But I did what they said, I pulled the bank of 4 carbs. I set the slides all the way down as deep as they'd go then I adjusted the idle screw to lift each slide a bit then measure the gap between the the bottom of the slide and throat of the carb (measured the gap from the rear of the carb slide where the cutout is, not the front).  I made sure all 4 slides were open the same amount when at rest (this is on a Kaw kz550 but the carbs are similar).

Then I took 4 lengths of raw spaghetti and from the rear of each carb I slide a dry raw spaghetti spear into the carb throat and under the slide, under the cut out.  The spaghetti spears were long, they stuck out well past the back of the carb throats.  NOTE carbs must be dry inside no carb cleaner or gas else the spaghetti is affected.  I wanted to watch the spaghetti move when I opened the throttle and I wanted to see all 4 spaghetti spears start moving at the same time.

Then I opened the throttle with the butterfly linkage and made sure all 4 pieces of spaghetti started moving at the exact same time.  As I opened the throttle the 4 spaghetti spears moved downward at the same time.

If one carb is out of sync, ie. if one carb is open more than the others -- at idle/low revs, when that cylinder fires the engine will rev UP just a bit for that one carb and snap the primary chain tight.   Then the other carbs are not open as much so the engine revs drop a bit and the primary chain goes a loose a bit.   Then it snaps tight again when the out-of-sync, more-open carb/cylinder fire.   Tight/loose/tight/loose/tight/loose/tight/loose.

It sounded like 'CLANG-CLANG-CLANG-CLANG'.

After I synchronized all 4 crabs, no more tight/loose/tight/loose, clang-clang-clang of the primary chain.  I was sold.

I'm not sitting here saying for 100% sure that's the cause-- BUT CHECK THE THROTTLE SYNC.  It is a LOT easier than pulling/opening the motor.

[MCRider]

It comes under the heading of "power pulses". You have like 4 little engines tied together with the crankshaft. If one engine pulses a little harder than the others, for that moment its going to hit the crankshaft a little harder. Its fraction of a second responses, but its very real.

in a bigger sense, power pulses came top be known in a broad sense in flat track racing. It was easy to make a flat tracker with more HP than a Harley. Yet the Harley was so competitive, virtually unbeatable. it was finally discovered that the staging of the power pulses, 1-2-pause, 1-2-pause, was just right to give the bike a push, then a fraction of a second to regain traction, then power, then traction. The bikes with real smooth power delivery would just be in a constant tirespin.

Not an analogy to the pri chain problem, but the point being what's happening at levels that we can't grasp sometimes, is still relevant.

[bmikkalson]

Im gonna split the cases, cause im a gluten for punishment and hope I find something

[Hondawggie]

Im gonna split the cases, cause im a gluten for punishment and hope I find something

Go for it dude, you can't believe the pride and sense of accomplishment when she's back together running good.  You learn so much when you 'achieve closure' on a engine problem -- you start with a stubborn problem and if you tear into it and solve the problem, especially pulling motor/splitting cases, you are much better prepared to diagnose a problem later.  When you're out there buying your next project you know dang well if a seller's talking out of both sides of his mouth and no one can trick you.

It's a fun deal have at it man.

[fang]

It is good to remember that ClassicCycleCity (LINK) is the ONLY person on earth who is selling Upgraded/ Heavy Duty primary chains.  A few other folks are re-marketing their chains.


The heavy duty chain is of course the one on top.  (Thank you mick7504 for the pic.)
Ricky_Racer here on the forum is their USA sales rep.  I have bought from them and it was a good experience -- plus the chains are off-the-chart awesome!  There is a discussion about primary chains in the Performance Forum.

Regarding the carbs being out of tune making bottom end noise, that really can be a problem.  If the carbs are out of sync, the cylinders will be 'fighting against each other.'  When the carbs are synchronized the goal is for all four cylinders to pull together uniformly.  When they are out of sync, the individual cylinders pull differently.  Even if the cylinders (carbs) are slightly out  of sync it can cause back lash -- this means that the crankshaft does not rotate at a smooth speed, it instead (on a micro level) is being pushed forward and backwards in pulses.  That just one reason why out of sync carbs make you loose power -- your engine literally is fighting against itself.
This creates noise in your primary chains because of the how it creates slack and then re-tightens the chains, but extremely quickly... In general, primaries make nose and experience the most wear on deceleration.  Out of sync carbs can create the effect of 'micro deceleration' all the time, creating not only significant power loss, but also excessive wear and tear.  This sometimes can be heard in a fatigued bottom end.


Good luck!
-Fang