Problem with my 20hp mercury. Late 70's

I got ahold of a late 70's mercury 20 hp from a friend that was having issues with it. It was running good for him, and at some point during service, things went wrong. He took it to a local shop for service and his lower seal was bad. He had the whole lower unit replaced with a new prop. Bunch of other stuff done (which ill list below the problem) here is what the motor is doing....

Takes a little work to get it started. Once started, seems to idle and revv in netrual easy. When put into gear and applied throttle, it doesn't want to build RPM's. Boat goes 5mph max. Here is the list of what was done from the shop...

Water pump impeller
Inlet filter screen
Spark plugs
Good used lower unit with new GL5
Cleaned fuel pump
Cleaned carb
Fuel pump missing inlet screen (replaced)

Any ideas of why its doing this? I thought he said he had the carb worked to accept ethanol. Something with the jets? Not sure.... Anyways.... Any idea what's going on?

Thanks.

Check for spark on both cylinders. I bet one of them is dead. Either the stator is going bad or the switchbox or coil is dead on one cylinder. Heres a link to a service manual. https://boatinfo.no/lib/library.html

Steve

I'll have to do that 1st and see what happens. It has the thunderbolt ignition, and everything looks pretty simple to work on.

Buddy, the guys at this site eat and breath Mercury. They are a very knowledgeable group in regards to Mercury motor.

https://johnsoldmercurysite.com/phpBB3/

good luck