50 hsp johnson outboard loses power when speed is up.

I just purchased a pontoon with a 1992 50hsp Johnson engine. First time out last week and it ran ok on low speed but after speed was accelerated it died down to very slow on its own. It has new spark plugs and that is all I had done. Model is J50TLEN. A friend took it out to see if he could figure it out and said when it slowed down he took off the engine cover to look at engine and decided to return to the dock and the engine ran perfectly for a long time at full speed and never slowed down. Is there something in the cover that would make the engine slow down?? Please help! Liz

Perhaps a sparkplug wire is shorting to cover clampng mechanism.-----Oil injection in use , or running with 50:1 premix ??

We will check out the spark plug wire and not sure what oil the marina put in the engine. We will have the oil drained and put in new. Is it possible that the fuel filter needs replacing or it could need a new power pack? Strange how it ran well when the cover to the engine was off.

Thank you for the suggestion. We will do the test tomorrow when we go to the boat. Hopefully that will take care of it. Liz

Thank you for letting me know this information. I will get it checked out tomorrow. Everyone is baffeled about how it runs with the cover off the engine. Liz

Is it possible to just add a vent hole to the motor cover to allow more fresh air when the cover is on? Same problem with my 1975 40EL75C, runs great without cover, loses power with it on...
If so, would I make a hole right in front of the carburetor?