Just picked up this 81 cb750c. Hearing a thudding knocking noise anybody lnow what it could be?

[Ghostxsn1per]

 

[Lagrango]

I had a knock like that once. My cam chain tensioner guide had broke. This is most likely not your issue but mine sounded similar.

What is the exact year and model of your bike? Is it SOHC or DOHC?

 

[BAHNST]

A 81 will be a Dohc

 

[Ghostxsn1per]

I had a knock like that once. My cam chain tensioner guide had broke. This is most likely not your issue but mine sounded similar.

What is the exact year and model of your bike? Is it SOHC or DOHC?

It's an 81 cb750c. Cylinder 3 isn't getting hot and spark plugs 3 is carbon black while 4 is black and oily looking. (1st pic is 3...)

 

[BAHNST]

FWIW. cam chain tensioners are a bit of a weak point in these bikes.
(Lagrango might be correct....)
There also seems to be a "ticking" there suggesting a loose cam chain.

The wet plugs might be a separate issue.....
Have you done a compression test/leakdown test?

 

[Ghostxsn1per]

FWIW. cam chain tensioners are a bit of a weak point in these bikes.
(Lagrango might be correct....)
There also seems to be a "ticking" there suggesting a loose cam chain.

The wet plugs might be a separate issue.....
Have you done a compression test/leakdown test?

No not yet ordering the tools this week. Got a link for a well priced compression tool?

 

[BAHNST]

Ive always been told a compression tester with the check valve near the end that screws in the spark plug hole will give you the most accurate reading in small engines.

With the check valve at the gauge's end (away from the engine at the other end of the hose) this will add the hose volume to the reading-so the absolute psi reading will be affected more so in smaller cc cylinders...
BUT these type will give you useful data to compare between cylinders.
Mine is a old Ryco of this type (made in OZ)...used for this comparison between cylinders.

 

[Davidson750]

Sounds like cam chain, and by the look of those plugs it hasn't been adjusted in a while or could be stretched, put new plugs in it to verify it's not an ignition problem or a bad plug, any compression tester will work to give you a ballpark compression reading