I had a M900 in this week (1993, carby) with a coupe of issues, one of which was not so nice WOT performance. I went through the basics before getting too into it: belt tension, drained the float bowls (LH with lots of water), set the balance and idle mixture (screws were 2, now 4 1/2 turns out) and it seemed quite happy. Back on the road it ran quite well, but still not so flash at WOT.
I did notice the airbox lid had the snorkels removed, and knowing that this puts an SS on the edge mixture wise and Monsters tend to need more fuel comparative to an SS I figured this might be contributing. So I grabbed a lid with snorkels and headed for the dyno. I figured I'd road test it on the way down with snorkels, and it certainly felt better. A much more linear power delivery, and pulling better to the top end. The dyno runs with snorkels looked good and much as expected. The runs without snorkels not so, and somewhat cringe inducing. We did two, the second to confirm the first, which it did.
As below. It didn't feel that bad on the road, so maybe the dyno exaggerated the difference somewhat. The carbs had std needles, so I would assume it was running all std jetting. Looks like it's a bit rich on the needles and lean on the mains. Both the SS and M run the same 140 mains, I think I have some runs somewhere showing a std jetting 900SS being rather on the rich side:
I found with my M750 that there was very little difference (that I could notice anyway) replacing the open lid with a lid without snorkels. I don't recall why I did it now, but it confirmed to me my thoughts that as a halfway step on a carby bike the no snorkels thing doesn't help anything much at all. More so on a Monster than an SS as it turns out.