you should have an electric choke on your carb, i'd start there and make sure the choke isn't sticking, causing a high idle. It happened to me once on my old ski nautique, it can be adjusted with a flat blade screw driver.
also check to make sure all of the throttle linkage, throttle plates/shaft and the cable isnt sticky and moves free through out the entire operating range. It may me time for a carb rebuilt also. Definately sounds carb related to me.

what rpm does it idle at when it idles normally?