The difference may be that it was run on a bench and your running it in a cowl. A friend of mine had a similar problem and he found that turbulent air was interfering with the diaphragm on the carb. He attached a small piece of plexiglass over the diaphragm hole. Not covering it completely but just a small gap between the hole and the plexiglass. problem solved. I have heard of guys soldering a brass tube over the hole alos and that seems to work also.