It's not just the octane- fuel purchased from a pump in the colder months in northern climates has better cold start characteristics than summer blend fuel. It's more volatile, even for the same octane rating.

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

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

I was out of town all weekend. I have had the slots at all reasonable positions. It is a two corner carb. I think my throttle shaft is so sloppy that the exposure of the slots changes every time I blip the throttle. It runs great off idle and no dead spots, so it is just the idle circuit that is a problem.