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Hello. My name is Tan.
The engine will rev with no load even if it is running on one cylinder but will not with a load. We need to do a spark and compression test. The compression must be 90+ psi with no more than 15% difference between cylinders. The spark must jump a 7/16" gap on a spark gap tester. Now since the engine runs you can check for spark by removing the spark plug wire from each cylinder.You should hear the engine rpms change and hear the spark jump or crack when you remove the spark plug wire from the spark plug. The compression has to be tested with a compression gauge.
Can you do these checks and post back with findings?