I installed this engine on my trike and is starts just fine but it won't rev up at all, and I'm baffled? First thing I did was go to set the mixture and there seems to be nowhere to set it. I see pics online of these carbs with a mixture screw at the top, but mine has a welch plug where it should be. Not saying I'm a pro but I've worked on these most of my life, even took an unnecessary course in small engines, so If I can get some ideas maybe I can fix it. I should add I did try the choke in every position as I tried to rev it since sometimes that will act as a temporary fuel/air/ adjustment for trouble shooting and it made little to no difference so, can't necessarily say it's a mixture issue. As I now understand it Harbor Freight won't allow shipping of anything that had a combustible fuel in it, so can't return it that way and this being my only transportation, I can't really go the 80 or so mile round trip to their store and back, so not sure what they will do about it. I did email them. I'd be grateful for any ideas on what could cause this.