What brand of pipes did you install? Most of them only require changing the low speed jet setting (idle mixture). Also, how far do you have the throttle open when you notice the stumble. That has alot to do with which throttle "circuit" is not performing correctly. If it on the first quarter, it is the idle mixture. 1/4 to 1/2 is the needle jet. After that it could be your main jet is too rich or too lean. Check your spark plugs too for either a lean or rich condition. White plugs are too lean, black plugs are too rich. Tan to brown color is perfect. Dave