You could do that Alex, maybe a small pinch clamp on the solution hose would be the cheapest route to take. Or maybe even a smaller fan jet.
But let's look at your perceived problem from another direction. The water delivery on a small spotting machine is quite low. If being used as intended, you will be flushing out mucho contaminants from a carpet. If the carpet is too wet, is it because you have over wet it, or under dried it? There are many of us who would argue the latter.
Safe and happy cleaning:)
Ken