I was out on the lake this morning about to take off and when I stepped on the gas nothing happened. All my engine did was rev up like I was in neutral but I was in gear. I turned off my engine and then trimmed it up to see if something was wrong with my prop but it looked just fine. Then I trimmed it back down and cranked it again and put it in gear and the same thing happened. I can go about 5 mph at like 1500 rpm but once I past 1500 rpm it jumps to 4000 rpm really fast. My outboard is a 1994 Mariner Magnum III 150. Any thoughts?

Thanks