I have a 1988 mercury 25 hp 2 stroke that is stalling at idle rpms. When I first start the motor it will idle fine. When I throttle it up, it runs fine at high rpms but when I try to go back into idle it starts to bog down and makes a "knocking" noise. I can shut it down and let it sit for a while and come back and the motor will crank up and idle fine until the throttle is increase and the problem returns. Any help would be great. Thanks.