69 650 "Dropping 1 or 2 cylinders under load"

I've been trying to get this motor to run well for a couple years. Took it to a small motors mechanic who is more familiar with 25 hp and under, but he said he'd give it a try. Everything checked out fine on muffs, so he came with me in the water to try to "fine tune". With just me in boat, it gets up to full plane in about 20 secs. (14' glass 15" pitch prop) With 3 in the boat, would not get past about half rpm (no tach). Tweeking the mixture and even quick start in one carb at a time had little or no effect. Carbs were rebuilt last year. He is pretty sure it is ignition and it is dropping a cylinder or 2 under load. I changed plugs to little or no effect. My wife says dump it and get a different boat, but I'm not quite ready to give up yet. Any ideas where to go next? I don't want to throw more money at it unless there is going to be a reward. Is it worth it?