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Hello. My name is Tan.
With no load on the engine it will turn up rpms so that isn't an accurate test of how the engine is running. The engine can be running on 3 cylinders and will turn up over 5000 rpms with no load.
We need to start with a spark and compression test. The compression must be 90+ psi with no more than 10 psi difference between cylinders. Then we need to check spark on all cylinders with a spark gap tester. The spark must jump a 7/16" gap on the tester.
You can purchase a spark gap tester and rent a compression gauge at your local auto parts store.
Post back with findings and the engine serial number please.