Re: 1976 mercury 500 50hp thunderbolt classic boggs out
Sounds to me like carb issues, probably a piece of dirt in the high speed jet. Even if it's just one carb, the other cylinders would be trying to drag dead cylinders. If the motor can't get the fuel that it needs at high RPM, it will just bog. It sounds like your low speed jet will provide enough fuel to sustain a lower RPM, but you can't get flow thru the bigger jet to maintain a higher fuel demand.
It's all about fuel + air, as soon as you run out of one or the other, you get no more power. You're not going to run out of air, that's the carb butterfly opening, so you must be running out of fuel. I would start by pulling the carbs and giving them a good cleaning, or just rebuild them while they're off, kits are cheap. Your fuel pump could be the problem too, just rebuild them too while they're off. Remember, a carb kit does not include all of the parts that you need for the fuel pumps, they're two different kits and you will have parts left over.