You mentioned you have a new crab, Is your carb a stock Bing carb?
Does it have a choke stem that you push down? If it does, if the choke stem is pushed down, it should come up when you twist the throttle.
Try adjusting the throttle cable length where then cable enters the elbow on top of the carb. screw it out from the pipe a bit until the engine runs at a higher RPM.
Also screw the idle screw on the side of the carb in or out to fine tune adjust the idle.
Lube the throttle slide under the throttle grip on the right handle bar. The throttle should snap back to the idle position if it is working properly.
Check you clutch adustment to make sure it is not slipping.
Check your tranny fluid. 1 speed 4.5 ounces ATF type F.
, 2 speed 9.5 ounces ATF type F.
DON'T overfill if it is a 2 speed or it will bog down under a load.