Help me get my slant running right

I still can't figure this Fn thing out. Here is some more info:

I checked the vacume advance function with a pump and it worked fine.

The timing mark is not bouncing around a lot, though it is nowhere near the right position so it can't be correct.

The idle mixture screw seems to do nothing. If you turn it all the way in it stays running and turn it out and it doesn't seem to change anything.

It starts easy and idles fine and can turn the idle down very low.

It sounds real smooth at high RPM under no load.

Under load it backfires on acceleration, frequently and sometimes out the carb when you let off the gas after trying to run it.

If you push it past it's low RPM problems and open it up it stops backfiring and you can open it up and it sounds OK.

The fuel filter on the carb is not plugged.

It is not getting hot even after sitting running.

I think I have ruled the valve adjustment out.

The compression is good so I am doubting valve issues.

Lastly, it is pissing me OFF!

Conclusions:

Possibly carb or dist?

The dist. was remanufactured so I don't think it would be that. I need to check the bushing again. I am thinking carb at this point.

Any thoughts?