My M715 has a AMC 258 in it, and I can not get it to run right. It runs fine if it is wide open. but it won't idle very well and mid throttle it kind of chugs then tries to die. If I keep bumping the throttle it will stay running. I have checked for vaccum leaks and have turned all the screws(probably made it worse). Any tips? The carb is a Carter if I remember right(everbody said it was not he most desirable carb).