Holley 1920 problem

That lever is part of the "economizer". Similar to a "power valve" in 4 bbl carbs, it opens ball ports to richen the mixture at high manifold pressure (high throttle). My 69 slant idled like that for decades, thru many 1920's. I finally got a good 1920 and it ran unbelievably smooth. The metering block tends to clog internally. One guy here knows how to blow air pressure thru to unclog them.

You can get rebuilt 1920's on ebay. I found an almost new one in the trunk of an old M-B at an LA junkyard. Someone must have stashed it, assuming no M-B diesel owner would have an old Mopar. Sold it to slant guy here who lives nearby. His car was idling the same. The swapped carb helped but still shook a bit, which might have been ignition issues too. Before you get too deep, think long-term. Many here changed to a 2 bbl Super Six setup or a 4 bbl intake w/ 390 cfm carb.