Unknown engine knocking/beats

Alright, every since I got this bike a year ago, I've had this odd puesdo-knocking noise that sounds like it is coming from the lower part of the engine, maybe transmission. The best way I can describe the sound is helicopter beats (i.e. the sound of repeated high/low intensity beats about 5 to 10 times a second). I did the screwdriver to ear thing, and as best I can tell it is coming from the rear, right side of the back of the engine, just begin the clutch plates.

I was hoping the sound would go away as I continued to tune the system. At first it seemed like it might be imbalance due to not all cylinders firing (i.e. the 1-4 pair stronger than just 2 with three not firing). Now, I'm fairly certain I have all cylinders firing, correct fuel levels, a fixed and strong electrical system, sync carbs (all synced at 9 inHg (23 mmHg) according to my NOSHOK guages). I was really hoping a good sync job would rid the sound, but to no avail.

Sometimes it is barely noticable, sometime it sounds exactly like a helicopter about to take off. It is always worse when warm.

And ideas on how to dianose? is this even a problem, really? what can I do?

If it does it in neutral sitting still I
would guess that it is camchain related.
I wound start with checking your adjuster
for wear and plunger travel.It could also
be something in the starter mechanism.When
starting do you get any unusual noises?

For starter noises, I don't think i hear anything unusual, but as this is my first bike and it was always done this, who knows. It seems smooth and without issue.

I have adjust the cam chain tension at least three times i the last few months to no avail. When I put the screwdriver up near the head, I don't hear the noise, so I don't think it is the cam chain.

However, on a different site I saw a comment about slapping in the clutch basket. Could this be it, how could I tell?

ive wondering if that bike has a primary chain like a 650?

Everything I've read seems to suggest that it is a worn primary chain. Apparently I can test by take the clutch cover off and using the screwdriver to listen to the 2" cover to see if it is loud right there.

Question though - how difficult is replacing the primary chain? Can it be done from underneath?

pull motor,split cases,pull crank..........:blink:

I have never worked on the 650/750 motor.
Does anyone know if the primary chain can
be checked by dropping the bottom motor cover?
Are there any adjustments? If not, it kinda
reminds me of an old Maico my buddy had.

How many miles?