70hp merc will not rev

[Justjoe]

I have a 1989 70hp merc that has been setting for many years. I got the motor running without any problems. It is very sluggish when I give it gas in neutral. Today I put the boat in the water to see what it would do. At idle and very low speed it's not bad but when I give it gas it bogs down if I hold the choke in it will take off like a raped rat for a second then die down and after several seconds take off again so on and so forth. I tryed pumping the ball on the fuel line while it's running but could not seem to get it hard but seemed to improve the rpms and bursts of speed while holding choke at full throttle. When the motor takes off its running very smooth. The motor is also very smooth at idle so I don't think it's ignition. I'm thinking fuel delivery. When the motor is off the ball pumps up nice and hard. Like I said this motor sat for many years. Where do I start any advice would be great.
I thank you in advance

[boats.net]

You should remove and thoroughly clean/repair carburetors. Be sure to properly link, sync and adjust when reinstalling.

[jlbates@nctv.com]

i had the same thing with a 6hp 2000 merc, tryed everything rebuild carb nothing worked. replaced carb and everything was great again work and runs good. it had a very small hair line crack in it that i had to use a mag lins to fine. thot this mite help.