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Not familiar with the 1200 but generally speaking if the throttle plates WERE returning to idle position no way the engine could get enough air AND fuel to support a 4k RPM. Your throttle plates would not be affected by junk in the fuel line or tank. The junk in the fuel theory would have to effect all 4 carbs at the same time, if one carb were plugging something it wouldn't pull the other 3 up with it IMO.
The air AND fuel required for a 4k RPM must be coming from a different carb circuit. If you have enrichment rods for a choke system I would be looking there.
As a test with engine warm crank up the choke and see if you can simulate the fast RPM. There could be other systems in the engine which will increase RPM if an overheat is detected which I would not know about and might include a change in Ign timing also.
The air AND fuel required for a 4k RPM must be coming from a different carb circuit. If you have enrichment rods for a choke system I would be looking there.
As a test with engine warm crank up the choke and see if you can simulate the fast RPM. There could be other systems in the engine which will increase RPM if an overheat is detected which I would not know about and might include a change in Ign timing also.
