Topic: CB750 Chain Slack?

[Jim Harris]

I'm getting a bad rubbing sound when I first take off on my CB750. It goes away once I get up to speed. rt sounds like the chain is either too loose and surging or my rear sprocket gear is bend causing the chain to be pushed off axis. Before I take off the rear wheel, I thought I would check to see if the chain was too loose. How much vertical slack should I expect to find in the the chain at mid-span when the weight of the bike is on both wheels (not on the stand)?
Thanks!

[KRONUS0100]

ideally 3/4 to 1 inch slack in middle of chain with bike resting on wheels

[Jim Harris]

OK, thanks!
That must be the problem, currently I have almost 2" of vertical slack. 

[lucky]

You need to be sitting on the bike when you check chain slack.

[Jim Harris]

OK, so does everyone agree that 3/4" to 1" vertical slack at mid-point in the chain with the rider on the bike is the target?
Thanks!

[KRONUS0100]

yep

[lucky]

The reason to be sitting on the bike is that the rider weight positions the swing arm to be midpoint and that is when the chain is fully extended.
As the swing arm moves up or down in the arc the chain slack increases.

[Duke McDukiedook]

Yeppers. 

[Jim Harris]

No luck!
I've tried adjusting the chain to that tension and it still rubs on the rear sprocket when that tight. I can get the wheel to roll freely when up on it's stand, but the chain has to be looser. I remove the rear wheel and sprocket this afternoon thinking that the rear sprocket may be bent and it's as flat as a board. The wheel bearings look OK too. Now I'm thinking that the drive chain is twisted or damaged in some way and when that section gets to the rear sprocket it rubs. I'm ready to try replacing the chain to see if that solves the problem, but when I look on line to order a new one, the parts supply places want to know how many links are in my current chain. I'm suspicious that the current chain is too large, so I don't want to just order the same thing again, I want the right sized OEM chain. How many links would have been on the original chain? 

[Xnavylfr]

Are you sure your wheel is installed straight?? Put the bike on the center stand and take a look down the length of your chain from the back, There shouldn't be any curves or bends in the chain from rear sprocket to the front. I have seen sprocket bolts rub on the bottom of the shock.   To check alignment of the wheels in relation to each other , you'll need a little help from a friend.    Stand the bike up straight and take a string along one side of the front tire up about 4" from the ground, hold the string so you can have enough length to reach the back tire and lay the string along the same area of the back tire, if the tires are in line , the string should touch the sides of both tires with the string STRAIGHT!!

Xnavylfr(CHUCK)

[acollin]

Can someone please explain the procedure they use for making a drive chain adjustment on a cb 750 by oneself .

How do you put weight on that rear wheel while making the adjustment?

My process has been an inch of chain slack with bike on the center stand. I understand the importance of this adjustment and I want to do it right.

Thanks-- I guess I'm just more thick headed than most-- should this be a new thread?

[acollin]

Also-- have you checked to see if your chain guard is bent? And contacting the chain.

I would think you would be ale to tell where things are rubbing by simply rolling the bike around in neutral and listening for the problem.

[flybox1]

If you are not the original owner, check to be sure the chain is aligned with the front and rear sprockets, and nothing is offset.  The rear wheel might have been swapped at some point.

Then...set the chain slack with the rear axle adjusters.

[Jim Harris]

Thanks guys!
In response to Acollin, I was thinking it was rubbing on the chain guard at first also, so I removed the guard, but that wasn't the problem. It's definitely coming from when a specific section of the chain engages with the rear sprocket. I'm going to go ahead and order new front and rear sprockets and a matching chain set. I'll let you know if that doesn't  do the trick.
Thanks again!

[acollin]

I did think of another place to inspect before you spend any money. The surfaces between the rear sprocket and the wheel. Are the surfaces clean-- with good contact between sprocket and wheel?

When the bike is up to speed that rubbing sound is in competition with all kinds of sounds. I'm not surprised it goes away or is lessened. It's possible that it is still there and you just can't hear it.

Does the rubbing occur when the bike is on the center stand and you are turning the rear wheel by hand?

[acollin]

Are you certain it is the chain that is rubbing?

A loose or broken spoke could cause the wheel to be out of round and be responsible for irregular rotating contact of one part against another.

I hope you don't mind me enjoying your struggle , but I love these puzzles when the occur on my own bikes.

[bollingball]

Can someone please explain the procedure they use for making a drive chain adjustment on a cb 750 by oneself .

How do you put weight on that rear wheel while making the adjustment?

My process has been an inch of chain slack with bike on the center stand. I understand the importance of this adjustment and I want to do it right.

Thanks-- I guess I'm just more thick headed than most-- should this be a new thread?

It does not have to be weight. I use a ratchet strap under the swing arm as far back as I can then over the top of the frame under the seat then crank it enough to get the swing arm straight out. This can be done on the center stand. Or bags of sand,cement.gravel. Or take the shocks off and jack the bike up till straight out but that is getting a little over board. Hey find a fat girl and tell her you are doing a photo shot for a bike mag.
Ken