this is just a couple shots in the dark.. but a lot of grandes i have seen (well, 2) and heard about tend to get a huge build up up of oil and whatnot on the cylinder head around the decompression assembly. mine was running a little weird so i took the cylinder head off and soaked in engine de-greaser and then rinsed it off and used a soft detailing brush to clean all the crap off.
also, is your spark strong? what color is it? if its yellow/orange, thats no good... weak spark. you should have a big blue spark...
also, when you cleaned your carb did you use a fine piece of wire to make sure the jet is completely clear? as well as the float needle (rocket ship liiking thing under the float..) that should be completely free of varnish and everything. im sure you saw zippys photo essay on cleaning a vespa carb, right?
how about your airbox? did you clean the filter element (little screen thing) with brake cleaner or something? and make sure there is no debris in the airbox itself..
i know other people have already mentioned a lot of this.. but it pains me to know someone has a grande that isnt running.. hopefully someone else will mention something else to try also...
-casey