Lets start with something simple instead of all this guessing. Now, you said the boy run it quite awhile with the choke half out, so let pull the plugs and see if there all sooty. If so the engine will run ruff. The only way to clean the plugs is have them sand blasted with fine bead so you can blow them out good, course bead will stick up inside the plug. While you've got the plugs out it's a good time to check the compression. It's very simple and you'll never know if you have a bad valve until you take a comp test. If the comp is all good your stepping in tall cotton. So, do those simple little test first. So ain't that simple. Walt
