I have an old Craftsman 6HP vertical shaft chipper/shredder that is giving me
trouble. The engine idles great, but at any higher speed, runs really rich,
making black smoke, and even spitting gas out the carb if I take the air cleaner
off. It does not rev properly in the process, except when it first starts or as
it runs out of gas.

The diaphram in the carb looks excellent. It looks new. I might have replaced it
a couple years ago when I acquired this unit used. The gas valve it operates
seems to operate also. If I blow into the fuel inlet, no air passes until I push
up on the needle that opens the fuel flow. I even tried installing a layer of
plastic from a bag "outside" of the diaphram (between it and the cover) to
eliminate the possibility of any leaks through the diaphram, with no effect.

Would a bad diaphram allow the idle to work great, while higher speeds are all
super rich?

The idle needle adjustment works properly. The high speed needle adjustment
seems to have no effect, other than if I remove it completely, at which point
the engine will quickly speed up like it should work for a few seconds before it
then dies.

Any ideas? Is there something I haven't thought of that could be repaired on
this carb/engine?