Engine Knock

[RodneyCB750K1979]

I have an engine knock that is more noticeable at idle and less above 2,000 RPM.  Sound comes from the lower case area around the starter motor on top down to underneath the the gearbox area. I'm running Rotella 15W-40 oil. See video on YouTube.  

YouTube video of my engine knock sound


http://www.youtube.com/watch?v=aKnoEC2xa5Y&feature=youtu.be

[TOOLS1]

Are your carbs synced?
TOOLS

[750K8]

I don't have anything to offer, though I hope it's a simple issue.

It has absolutely nothing to do with Shell Rotella Diesel Oil. That is the best bang for the buck oil to use for a motorcycle.

Tools, help enlighten us less mechanically inclined....but what does syncing carbs have to do with a knocking noise?

[750K8]

I bet TOOLS is right.

Rodney, watch this video and take a look at what he did to fix it...

http://www.youtube.com/watch?v=YVPpScwzbfM

Same exact noise you have.

"Just tuned the carbs, adjusted the cam chain tensioner and tightened the three bolts on the starter gear freewheel, and 90 % of noise disappeared. A tiny clutch rattle at idle and some faint cam chain/valve rattle is all thats left!
Easy enough to pop the left cover (timing cover) with the 8 mm bolts and bam there it was. easily tightened the 12 mm bolts attaching the freewheel to the starter gear sprocket."

[TOOLS1]

Carbs out of sync can make an engine sound like it is going to fly apart at lower RPMs. Then as you give it throttle it will quiet down.
TOOLS

[RodneyCB750K1979]

I watched the Youtube video of the other 750 making noise like mine.  It said to tighten the three flywheel bolts.  I removed my cover and voila!  The bolts were loose and backed out!  I tightened them and 75% of the noise went away.  I still have some noise from the clutch and it goes away completely when I depress the clutch handle.

[Johnnylevi]

did you put some red thread locker on those bolts? If not they'll come back out soon

[RodneyCB750K1979]

No I didn't.  I thought about doing it but decided to wait and see if they loosen again.  I theorize they loosened from crank shock due to either carb sync issues or excessive use of the starter during the engine commissioning work after a rebuild.

[Black 750K8]

As far as carbs out of sync think about it like this. You have four engines with there own carb but they share the same crankshaft  if they don't all work together then the crank starts trying to go back and forth instead of one smooth rotation and it makes thing rattle the timing chain goes slack then tight sort of like timing but the cause is  fuel instead of spark. I'm sure that is clear as mud I have a hard time putting things into words sorry.

[RodneyCB750K1979]

That makes sense to me.  As the crank rotates, it should be receiving equal pulses of torque from each cylinder as it fires. If the carbs are not synced properly, the pulses will be uneven, thus causing the flywheel and cam chain to accelerate and decelerate.  This causes chain slap and possibly loosening of rotating fasteners.  

[750K8]

Thanks for the explainaton. Makes sense to me.

Seems like less of a problem with twins for obvious reasons.