B&M Converter is bad

Make sure the engine is FULLY warm / up to temp when you adjust idle

You MAY have a vacuum leak causing a lean condition. I agree--- sounds way lean.

Do NOT retard timing to "fix" chugging--the opposite. Set timing, adjust idle screws best you can for highest vacuum/ best idle, then diddle idle speed and adjust timing for now I'd LEAVE it at the 13/ 38 unless the engine seems to like more at idle. More idle speed/ more vacuum is an indicator. Keep backing the idle speed down as you improve idle.

Try pulling the idle screws clear out when running and stick your finger over the hole. This tends to pull out any dirt. If no effect, get some aerosol carb cleaner with a snorkel and blow some cleaner through the idle screw holes. Watch your eyes, it should come up out of the air bleeds int the throats.

The question is "have you improved it?"