1980 60 ae outboard

[garethmciver]

hi all
can anybody tell me what else to check?
i have a approx 1980 60 ae outboard which i got 2nd hand.
on land(i.e. not under load), with ears on, the motor will rev until i think i may damage the motor. however when i put the boat on the water, the motor wont gat over 2000 rev's.
i have cleaned the carbs,changed the fuel filter, cleaned the sparks checked the air intake and am running carb cleaner in the fuel.
any ideas what to check next?
ps. i am not that techinical yet, so please keep things pretty simple.
thanks

[boats.net]

Do a compression test to see if low on compression, check to see if engine is running too hot and activating the overheat warning and limiting rpm's to 2000 rpm's.