carbs acting up???

[snowstud98@hotmail.com]

75 merc 500 2 cycle, The engine runs fine and has good compression but when i put it in the water it only goes about 10mph, It sounds like it isn't getting alot of gas,however I took the carbs apart again and the directions say to adjust the main jet screw 3/4 of a turn however the shop manual says you can't adjust these,what gives???? what can i do??