G2 makes throttle tubes for specific models. Should fit if it was made specifically for the MV....they didn't make many, and when I last contacted them for one to fit my SPR they were out of stock. Low demand.
You probably already tried the obvious but is it in the correct orientation? ie are you putting it in upside down? You should loosen the cables up so there is as much free as possible and then tighten them after you get everything back together on the handle bar.
Never put one on an MV but I just did one on the supermoto. Didnt fit until I loosened the cables...a lot.