Weird issue truck accelerates faster at half throttle??

[yellow400ex05]

well it'll only kickdown at 45mph or less. and it wont do it everytime. it seems to be okay in 1st and 2nd gear but when it hits 3rd it'll bog way down, and when i let up from WOT it'll make a hissing or sucking sound and start to accelerate faster, then if i lay back into it sometimes it'll keep accelerating better, almost like my secondaries aren't opening. plus i cant figure out why its running so damn rich. you can see it idling black smoke. i love this truck but it runs like crap! everybody i talk to wants to set it up differently.
when i put it into 2nd and floor it it'll pull fairly decent, but still when i let back on the throttle it starts to pull harder. this truck is supposed to run like hell broke loose, but instead it runs like garbage.