idle?

EGR stuck open?

Put a vacuum gauge on the dipstick tube , flip the PCV out of the grommet and seal both valve covers. If the engine doesn't make pressure almost instantly at 900 rpm, something is wrong. If it pulls a vacuum, the intake gaskets are blown out into the valley, and you should be seeing oil in the tailpipe.

If you have a brake booster, clamp the line and note any rpm change.

Put a timing lite on it and note any timing change going into gear. At 900 rpm the mechanical advance may already have started. Make sure the Vcan is on the spark-port and is not commanding advance at 900 rpm

If nothing turns up, pull the carb off and look for a crack from atmosphere to either port thru the carb platform. I guess check the base gasket is correct for the application, and correctly installed.

If you have a lean burn, make sure the plumbing to it is 100%. If the ECU senses low vacuum it might think the car is working and pull timing. The Vcan should not receive vacuum until the thermo-sensors warm up and relay it. The sensors must be sitting in liquid to properly trigger. On early systems ( I don't know the years), the Ecu was tied to the transmission and no advance was allowed until the trans shifted into hi.
Once all the hoops are jumped thru and the vacuum is "normal", then the Vcan works "normally".
I don't know all the ins and outs of the lean-burn system, as I only ever worked on a few of them. Up here we didn't get LB until the '80s; but this I know; if you pinch the line to the Can, you will not get any advance,lol.