Ok so I have a 17ft 1973 Delta Trihull with a Merc 140 I/o. I bought this boat last summer early and had it out 6 or seven times and had a very hard time getting her to plane out. I have tried plenty of stuff but my symptons seem wierd. I am able plane out but only if its me and one other person. I dont have soggy foam or floors as i replaced them all last year. I have a hydrofoil. I replaced the plugs with correct type and gap. new coil. prop is like new with perfect bushing, had it repitched from 17 to 15. I have good compression. I did realize when winterizing that there was no thermostat, which I am now replacing. The outdrive oil that I drained after the season seemed fairly clean not chips or silver color. The boat requires high octane and leaded additive, which I use. Boat idles great no prob. I can get 4300 RPM at WOT but just wont plane with any load. I checked the WOT RPM specs and they say 4200-4600 RPMs. I was about to try and diagnose maybe a fuel problem or point issue, but this should not be a real issue if im getting the rpms. I'm scrathing my head here and dont want to spend another summer putting around at 3 mph on the lake. Also i did notice that the choak body was a little loose when I was winterizing, which I think was causing maybe a slight vacume leak but arent I geting enough rpms to plane out.