1800 is not enough altitude to cause problems. 5,000, maybe! Once the RPM gets above 1,000 or so, none of the adjustments will have any effect on the mixture- - - -it's all internal after that. Do you have the correct carb for the engine? At least two thirds of "carburetor" problems are actually ignition or mechanical problems. There is a "transisiton" system as the engine makes the jump from idle to cruise conditions, usually a couple of metered holes drilled into the airflow from the idle jet, directly above the idle mixture adjusting screw. These are precision-drilled holes, and they're not adjustable. That's one reason (of many) that just any old carburetor won't bolt on and work well. The calibration is for a specific engine setup and sometimes it changes for a different transmission or rear end ratio, etc.
Adjust the mixture and idle speed for the smoothest idle. The rest of the performance is up to the internal calibration of the carb, and the mechanical condition of the engine.
Jerry
