mercury 4hp 2 stroke runs and ides but wont rev up at all

[boss hoss]

Hi looking for some help please

I have a mercury 4hp 2 stroke outboard that starts easy and idles but wont rev up at all i have checked/ done the following

1 changed the fuel
2 changed the fuel filter
3 replaced the diaphrams in the fuel pump
4 removed the carb and both jets and cleaned in a small ultrasonic cleaner this has sorted others before
5 checked throttle response on carb linkage aswell as cable through tiller

it only revved up when i turned the fuel tap off and ran it until fuel ran out would this point to something Heeeeeeeeeelp.

 

[racerone]

Check float and float valve.----Serial # is ?

 

[boss hoss]

hi thanks for replying, float and valve looked ok but ill have another look . ser no is 09930283 its 1997