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Hello. My name is Tan.
We need to start with a spark and compression test. The spark must jump a 7/16" gap on a spark gap tester. The compression must 90+ psi on all cylinders with no more than 10% difference between cylinders.
The engine in neutral will rev all day long up to wide open throttle with no load. It could be running on 2 cylinders and would turn up to wide open throttle. So in neutral is not a test and this is a good way to blow your motor up. Would you get in your car and with in neutral push the gas pedal to the floor?
The chances are you have an engine run issue not a control issue. To be sure the control isn't the issue put the throttle to wide open engine off. Now look down the throats of the carburetors is the throttle plate full open? If yes then control is fine.