[Mr.Fixit]

The stumble could very well be a mismatched powervalve which isn't opening at the right time. You don't need to buy the rear fuel metering block kit, you can just drill and tap the metering plate to accept jets and fill the holes that it used to use with loctite quick set epoxy. MSD boxes do "go out" rather frequently, that's why they sell a harness that allows you to run two MSD boxes parallel to one another and just flip a switch to go from box A to box B during a race. Mounting it upside down is "bad" lots of heat is "bad" vibrations are "bad" it's six-of-one , half-dozen of another. I like to put the box inside the pass compartment as I fell that heat is worse than the possibility of condensation build up. (personally, I run a Jacob's box, not MSD)