If it has a dyno jet kit in it, the numbers wont match. This would come into play if you are not the original owner and dont know its history. Second a 142 main jet has absolutely nothing to do with idle and comes into play at half throttle and up. 3rd a 142 isnt that much larger than the 138. A starter jet and a slow jet can be put in the wrong spots, actually flip flopped as they are the same threads. On the exterior of the carb bowl is the pilot jet adjustment screw. If its more than 2.5 turns out, it may idle rough and rich. A dirty needle and seat may cause a rough idle. Carb machines are dinosaurs as everything is EFI now. Nothing wrong with carb machines though. Hows the air filter? if its falling apart, it will cause havoc to a carb. I would look at the electric choke first and see if its fully seated while its running, if not a rich condition would exist.