New situation.
Carb tech cleaned my carbs and tried to synchronize them. 1,2 and 4th carb were in 230-240 mm Hg range but 3rd was around 150 mm Hg. As he started to rise the value on that carb engine started to knock. Deep thumping sound was comming from the engine. He said that problem could be in brass ring in piston rod, piston or a piston pin that connects piston and piston rod. But most likely in that ring because it is soft metal.
I checked in parts list and repair manual and did not find any brass rings in piston rod.
Does anyone have some engine rebuilding experience or idea what to do?