Max, you speak of old gas, and you say the noise is on every other rotation. That is a classic symptom of stale gas an varnish on an intake valve. I would recommend you squirt some Marvel Mystery oil in each cylinder and let it set a week. Then try turning it over with no spark plugs and see if you still have the noise. If not I would put gas in a small container and suspend it over the carburetor to bypass the fuel tank, lines and pump. I have an aluminum go kart tank that works good for this. If you suspend it about 18" over the carb, you get about the the correct fuel pressure. I would sure recommend this before you start taking things apart that you may never get back together again.
