I have a '61 Evinrude Fastwin 18hp that 2 summers ago started and ran perfectly until I tried to go beyond trolling speed. That's when it starts to bang and clunk like its jumping out of gear. Afriend took apart the lower unit---we replaced some worn gears, replaced the gasket, and got the unit in ship-shape. Still clunked! We then replaced a burnt out coil, thinking we had it solved. Still clunked! My friend thinks it must be in the upper unit somewhere and something he didn't have time for. Basically, I can't afford a new motor. What could it be and is it worth takng in to a shop? Please help. Thanks