strange engine issue from 900-1400 rpm

[Bocephus69]

i recovered my C10 from a slow death in a tree-row at my in-laws farm. it was sitting for 10-15 years. it has a 350 engine that was donated from a 76 caprice. it has all new ignition parts and i rebuilt the quadrajet carb with a simple kit from napa and the help of google and youtube .


the engine now runs great except while in the approx rpm range of 900-1400 rpm. at idle, it runs good, at highway speed it runs good, but low rpm it surges. i dont know how to explain it other than it is "the opposite of cutting out." if i am driving 25 mph in town with my foot barely on the go pedal, it surges. it jerks your body a little bit and you can hear the exhaust get louder. my wife jokingly says "well, the old gal just wants to GO!"


even when it is in park it will do it (right when you start it and the choke is on.) it is idling higher, around 1000rpm, and you can hear the engine "burst" for a second, approx once every 5-10 second. it isnt a backfire, it is just a simple burst (almost as if you would lightly burp the throttle).


all i have tinkered with are the 2 A/F mix screws on the front side of the carb. can anyone provide me with some insight as to what could be going on here?