Sorry I am not familiar with your vehicle, but I can see two different scenarios:

1/ Does the vehicle try to take off by itself? If so, sounds like your idle speed is too high so your clutch stays engaged. There's a idle adj screw on the engine that limits the closing of the carb. Unscrew it a 1/4 to 1/2 a turn and try again. Don't confuse it with either of the mixture screws.

or on a completely different take:

2/ It also may be that your brakes need adjsutment which should be done through the brake linkage. In this case, your idle may be dropping because your idle stop screw is set for too low an idle and you need to turn it in a bit.

It's difficult to say without seeing it, but from your description, it sounds like #1 to me.

If you have an instruction manual, look up idle adj and brake adjustment. It might help clear things up.