check all vacuum hoses behind your AFM easy way to do this is get some starting fluid and a fire extinguisher just in case and spray all major vacuum lines behind the AFM including the rubber boot directly behind it. If your rpms go up briefly then you found your leak you could also try adjusting your TPS and your AFM. The AFM in my 77' was bad so i bought a new one and it came out of the box calibrated to lean.