Bone:

Some of your questions. Yes, the 72 will sit in the place of the 80, but the mounting screw holess will not match.

Not the air bleed carb.

Another possibility is a massive intake leak. Get the engine running as slowly as you can, then use a spray can of WD-40 or anything similar, spray around the outside of the carb, and where the intake goes into the cylinder. There will be a reaction if you hit a leak point.

And yet another possibility. Take the carb off and look into the bore from the engine side, holding the throttle full open. You should see a slit in the spray bar, the slit should be centered. If it isn't centered the engine will go lean at part throttle, and you will not be able to adjust for it.

Let us know what you find.

Bill.