1986 Mercury dying at full throttle? 50hp

[OldMerc19]

I just bought a 1986 2 stroke long shaft outboard 50 hp and have been working on it for a while now. It has 4 spark plugs and 2 carbs. I have put a new starter motor on it, new spark plugs and replaced one carb. The motor doesn't run well and it sounds off. It starts right up but it seems like its only running at half power. It wont even put my 15 foot trihull (1966 sportscraft) on plane and when i push the throttle all the way down it dies. I have put in a new fuel line and ran a can of seafoam through it. When I put a pair of muffs on and run it out of the water, the engine throttles up and it sounds much better. I can push it to full throttle and it doesn't die.

I have tinkered with the jet screws on the carbs and checked to see if theres any air blowing out of the carbs which there isn't. Water comes out good from the cooling line and the lower unit has new oil. The solenoid is also new. This problem only seems to happen when i put the boat into the lake. It sounds much better when the prop doesn't have any resistance. All four of the plugs have good spark.

Background: This is my first boat that I got for a quarantine project. I repainted the whole boat top and bottom and put new fiberglass in the bilge and put a new floor. Im trying to get this motor working before I start college but I dont know what else to try.​

 

[SteveVT]

Are you firing on all cylinders? Pull the plug wire off of one cylinder at a time while running (using an insulated puller) and listen for engine slow down for each. If any pull doesn't effect the RPM, that cylinder isn't firing. That's where I'd start.

Running a can of Seafoam through a gas tank is a good way to cause all kinds of debris to clog the carbs from the tank and lines. Learned that the hard way. BIG difference between putting some in a running engine's carb throat, and putting it in a fuel tank to run through the whole system. In the first case you are attempting to put a powerful solvent in the cylinders only. In the second, you are putting that powerful solvent through the whole fuel system. If it acts as it should, it'll loosen up every bit of junk in the fuel system, and help deteriorate older fuel lines.

Seafoam won't clean chunks of stuff it loosens in the fuel system out of a carburetor. You have to take the carbs off, physically clean them again, renew all lines and filters, replace plugs, and get the tank cleaned, otherwise expect chronic fuel problems, unless everything was clean and in great shape to begin with. In which case Seafoam wasn't needed.

 

[racerone]

Maybe the exhaust passage ways are plugged.----More trouble shooting needs to be done.----All motors run and " bark and roar " on a trailer and that means nothing.