Lean stumble for sure. Pulled the plugs after 500 miles and they are just as clean as when I put them in. I will read the plugs again after a week or so and see if they get any color to them. It runs great on the throttle once you get past the secondarys opening...and it will idle clean for hours on end. I've got it pretty close, I think with a little more fiddling it will be good to go.