Mystery Clunk! (Is there a freight train in my engine?)

[Eli Z]

I have no idea what this sound is.  It sounds like a chain or a wheel trying to catch.  It clunks like a slow train on metal track.  It happens after I've been riding a while, then slow down and accelerate - happens when I accelerate.  Stops after a little while but starts again at any acceleration from a slow speed.

Usually it first happens after getting off the highway on the way to work.  I stop at a red light.  Turns green, go, and it seems like the chain is trying to come off or the transmission won't catch or something.  Doesn't seem to affect performance much or at all... but it's a really frickin' creepy sound.

The chain HAS come off in the past.  Could it be the heat expanding the chain or something when I'm riding fast...?

1993 CB750.

[Bumblebee]

The drive chain came off?   That won't happen with a properly adjusted and properly maintained drive hardware.

Have you adjusted the chain? Is it too loose? Has it been too tight?

What does the driven sprocket, rear sprocket and chain look like?

Where precisely is the sound coming from?

[Munkey]

The drive chain came off?  That won't happen with a properly adjusted and properly maintained drive hardware.

Have you adjusted the chain? Is it too loose? Has it been too tight?

What does the driven sprocket, rear sprocket and chain look like?

Where precisely is the sound coming from?

+1 on all of what he said.

A chain coming off at speed can cause some serious damage, not to mention a crash.

Did the sound start immediately after the chain coming off? If you don't have a center stand, you need to get the rear of the bike off the ground (while stationary) so you can rotate the rear tire and locate the source of this sound. Could be something simple, could be something major.

[Eli Z]

The sound is coming from the left side of the engine.  I had a diagram of the engine and it looked like it was in the transmission or cam chain riser.

Yes, the chain was too loose and it came off.  At low speed.  No idea about the sprockets or whatnot.  I am no mechanic and my mechanic can't figure it out (or maybe reproduce it).  It only happens after riding then slowing and accelerating.  so lifting it and rotating the wheel won't do it.

[cryber27]

I had the almost identical sound happen to me today! But on my 450, I can easily diagnose the issue is the chain and the sprocket not coming together properly. Sprocket is showing wear in a place there was no wear before I tightened the chain...then loosened it because I thought it was too tight...STUPID me. Anywho...Putting it on the center stand confirms I need new sprockets and a new chain...

[coffee_brake]

My '92 made this sound 1) when I had not centered the axle properly and the chain wasn't running perfectly straight and 2) again when the chain was worn out and needed replacing.
Get the sprockets too, you're shooting yourself in the foot if you just buy a chain and re-use the old sprockets.