Topic: 1974 CB750 – rattly sound in engine while running

[jembers]

Just a few days ago I noticed my 1974 CB750 making an abnormal rattle sound. The best way to describe the sound of it is kind of like a ball being bounced around the inside of an engine or even kind of like the sound of a typewriter. It's not crazy loud but noticeable enough to me that I know it doesn't sound how the bike normally sounds.

The bike runs fine mechanically and the only issue I have had with it is trying to figure out some carbon fouling of plugs. I loosened up the lock nut and screw for the cam chain tension and tightened them both back up thinking that would solve it but it hasn't.

Anyone have an idea of what it could be? Video is below and you'll most likely hear the sound better through headphones rather than computer speaker.

Thanks in advance.

https://streamable.com/6hs05u

[Pielz]

Have you removed your cam chain tensioner mechanism and checked that it functions properly? They have a propensity to become seized. I know mine was

[ekpent]

and did you have the cylinder number 1 tappets loose and the proper spot lined up on the points plate when you loosened the nut and bolt to set the cam chain ? A tapping sound could also be a valve adjusting nut and screw hitting the inspection cap which is not a rare problem and results in the nut falling off inside the top end. Might want to check those in your search for a cure.