I have a 1974 mercruiser 302 which ran fine earlier this year.July 3 took it out and heard a noise from engine box (possibly backfire),motor never skipped a beat.next day took it out and couldn't get the boat on plane (no power) throddle all the way out.All plugs were carbon fouled.Mechanic has worked on the boat all summer.He has done,comp test,leak down test,which he says I have a tired motor.he has installed new plugs,wires,rotor,dist cap,just went thru the carb today per my request.timing is correct, firing order is correct,points set to spec.Everything seems fine but the plugs keep fouling up, wet carbon fouled and motor continues to run on 5 or 6 or 7 cyl.little hesitation on throttle. pull plugs out clean them up reinstall,run motor sounds good for 5 sec then back to running on 6 or 7 cyl.There is smoke coming from the exhaust (blowby) and gas or oil slick on the water after running. I will be pulling boat out for the summer soon.Any ideas would be great.mechanic says he will pull valve covers off next if I want him to.talking about cyl walls, valves, rods, blah blah.Idk. Thanks.