Tighten idle speed screw a bit, the one that should touch the bellcrank to throttle cable. If not, clean carb, readjust idle, new plug. Adjust valves, if any. Might possibly be loose rubber boot to carb, check it and clamp. Spray WD 40 to see if any microscopic holes--the idle speed will change. Also look for loose, broke vacc lines.
The screws, take one to the hardware store, probably a 6 x 1.0 metric, use a stainless screw. Cost a few cents more but never seize, corrode, look good against chrome.