I reread all of the post here and found something that did not make sense to me...
It is not too easy to change jets in and jets are impossible to find for it. I can use jets from stock 1200 carbs in the high range but the mid-range is the driving section and the jets are impossible to find for it. I think I will have to solder up the jets and redrill them to smaller venturi.
I'm going to assume the 'high range' jets are the ones in the float bowl and the 'mid-range' jets are the ones on each side. As I understand it the jets in the float bowl are the main jets and the ones on the side are the Idle jets. The Idle jet on the primary feeds the Idle Mixture Screw and ,as the throttle begins to open, feeds the Progression holes. "these progression holes smooth the transition between closed and open throttle." thats right out of the Weber Carb book by HPBooks.
Anyways, I would change the main jets to something bigger ,even if the engine runs real fat, just to see what happens. Before you throw away that new carb.
Good Luck
angelo

Mario sorry if i missed someone else telling you this and it probably isn't it but have you checked your hoses. Somtimes old ones can suck flat at high revs (don,t know why )and restrict the flow.
Hope this helps

i didnt know so many bloody things could cause overheating, my attitude is if it overheats for seamingly no reason add more fans or bigger radiator :), if that dont work buy another car :)