Bought a boat last fall with a 1980 3 cyl. 70hp Evinrude motor, 13.25x17 prop. The boat is a 14 foot fiberglass tri hull. Here's the problem. After the boat has been idling for a while, (like waiting for a skier to get ready), when you hit the throttle the engine just chuggs and dies. It does work pretty good if not towing a skier, but does also do it sometimes. Carbs have been cleaned, timing adjusted, and new plugs put in. Was wondering if this is a common problem, and how do we fix it. And could we have the wrong size prop on this boat. Can't test that as we do not have a tach on the boat either. Please help!