cub cadet 1650 r p m's

My 1650 cub's throttle cable came loose at the clamp on the motor. It calls for 3600 rpm's at full throttle. I can only get around 1500 to 1800 using a tach that raps around the spark plug wire. We tried the tach on two other engines and they measure 3600.Why can I only get 1500 without blowing up the motor while trying to get more. It runs ok setting it by ear. I want a reason that Im only showing half rpm's. Thanks.

cub cadet 1650 r p m's