Back in the days when the tp engines were more popular---------It sometimes took a small hole in the throttle plate butterfly to allow enough air at idle-We generally used one of those step drills and started with a smallest hole------then we could easily step it up size little bit at a time----- you can do this on the engine if you are care ful-put paper towem under open butterfly------drill hole , remove paper towel -chips-

If you do eventually think you have gone too big-you can use a rivet to close hole and start over-----

Another way is to take a saw sharpening file( 3 sided) and file a notch in the edge of the butterfly going larger until it runs good enough-------
Make sure engine is up to temp---------