About 4 yrs old 90? hp 4 stroke outboard runs

Yours sounds like a fuel problem... however did you try disconnecting the shifter lines and operating it manually. I'm actually running down a short in my remote shifter box that appeared to be a fuel issue but is isolated to the shifter box as I disconnect the cables and was able to make the boat go to full throttle but as soon as I use the remote it dies.

sounds pretty normal. If it's loaded down, displacement hull physics come into play.

idle too low

water in the engine crank case oil --major seal or corrosion problem in the motor
water in the leg gear box --
pump seal on the drive shaft

check when its on full throttle that the carb butterfly is at 90deg and not over as this will reduce throttle but max advance on ignition !

remove carb and clean jets etc !

I have a problem with my 2 stroke 20hp mercury 1989,when I run 10-90% throttle it's works fine but when in 100% its only runs in 90% and doesn't sound normal like it's not burning right.

My outboard motor is old and used.When I went to run it for the first time everything worked except when I went to put it in forward gear the boat went slowly but when I gave it more gas the engine just reved up but the boat wasn't going any faster.The motor is 40hp gale model.I think because it is bought used the prop is not the right one and it seems like it's slipping as I go faster.
I would appreciate any feedback on this problem.

Thax, Don

Marine electronics should never be powered through an ignition switch. A sudden drop in voltage will cause it to shut off. If your boat is powered by a four stroke outboard it probably is not generating enough current to meet the demands of the engine and the electronics. Four stroke outboards generally have to run at high rpms to generate enough current to charge the battery. Old or poorly maintained batteries could also contribute to your problem.