I had the same problem on my boat it has a 175 opti and it wouldn't get on plane without shifting weight to the front. the first time it did it was the power packs. I knew it was electrical then cause I blasted off fine and when I shut it down, fished a bit, and then tried to blast off again it wouldn't go but I never could get it on plane then. And it could also be an exhaust issue. My boat on one occasion would get up to about 2500 rpm and stay there until you shifted weight and it would run out fine. I took it to the shop and inside the motor under the power head something came loose and when you would go to blast off the plate that came loose would shift back and partially cover the exhaust port. I know its crazy the dealership said it as a common issue in evinrude motors and this was the second merc 175 he had seen do it. Lucky me huh... But when trimmed the motor up and down you could actually hear the broken part fall back and forth...