Taking up where MasterSmasher left off:

Turn the neele inwards to make the engine leaner (closing the valve, reduces fuel flow, which makes a leaner run).

Norvell BigMigs are known for the annoying muffler mounting arrangement. Jurry rig as necessary to get it to stay in place. If I recall, they have a gasket that should be in there to take up some of the space. A lot of guys go at it with high-temp RTV (from your local autoparts store) to make a gasket and seal it up. I've seen guys make muffler mounting straps from nose clamps for the Norvels as well. If you have a field that allows you to run with out the muffler, then, yes, you can do it. It's loud though.

Plugging up the tank isn't really a great idea. You will cause a lean run, as the fuel is pulled from the tank something has to get in there to fill the vacume. So the engine will find itself pulling harder and harder to draw fuel as the fuel level goes down. However, I have seen the Norvels run better when really small fuel tubing was used for the muffler vent line, apparently sometimes, they don't like the fuel to be too pressurized from the tank. Try leaving the vent line open but not attached to the muffler.

I've also not run this particular engine, but I've seen lots of BigMigs with the above issues.