Carb disassembled, cleaned, still runs poor please help!

[Black70c10]

Okay guys, I'm beyond perplexed, any insight is appreciated. So out of no where one day, my truck had difficulty idling, used to idle fine around 500-600 rpm. Then out of the blue, it would not hold idle until around 1000 rpm... It would jump around and hesitate on low end light acceleration..driving it was fine but as soon as I coasted to a stop or let off the gas it would almost die and I'd lose power breaks.

So while I had the truck in the shop getting some body work done, I asked the mechanic to take a look at the engine issues figuring it was a vacuum leak or something (I couldn't find it though). He ends up saying he couldn't find the leak and on the way home I have the hardest time keeping it running.

So, I spent the day yesterday taking the carb apart and cleaning it.. What made me nervous is the fact that overall it was really clean- clean gaskets, accelerator pump looked good, all jets were clear... So I throw it back on the truck and sure enough, same story.. It will idle at 500-600 now but wants to die as i let off the gas and has a nice hesitation off the line!

The other weird thing is that the idle mixture screw has little to no effect on the rpm until you turn it one or two whole revolutions- this is odd, normally quarter to half turns have big effects...

Please help!