I never had a stumble when secondaries opened only at tip in off idle. I have an 800 thunder and was going to play with that adjustment. Haven't really looked at it to see how it adjusts. Maybe get a little more power. Bought the carb new but it came with performer series owners manuel?????? That's not adjustable on a performer carb. I'll be looking into that tomorrow and see if i can figure it out. Yes i have a thunder not mistaken performer.