Your deal, pump gas at your altitude, needs to be tuned on all the circuits properly to avoid the spark plug fouling your having Work on one circuit at a time, I would start on the idle circuit first, does your current carb. have four corner idle? If not try opening the rear throttle blades more to help lean out the front, close the fronts up to set the idle RPM I would probally restrict the idle fuel feed and the transition fuel feed holes into the main body to be about half the diameter the holes are now Carbs can be a pain in the butt But thier all tuneable You can fix it Do it one step at a time