Low speed screws....
On the bottom of your carbs you'll find the low speed idle screws, open them on both carbs 1/4 turn.
You will have to push the throttle all the way open to see them (I can feel them with my eyes shut) with a mirror, looking from the bottom, backside of the air box cover.
If your un-familar with these, they will have a slot in them for a screwdriver to fit and also, a "t" to use like a thumb screw. If you adjust one, you have to adjust the other.
Know their position before you make the adjustment. That way, if there is no difference, you can put them back in their location. But from what you are describing, this is probably your cause. As long as the ski runs good in all other areas....if not, may be obstructions in your carb(s) and need to be cleaned. Make the adjustment first and take out for testing. You can't check the throttle performance on land with no shaft torque.
Just re-read your post. I haven't looked in the manual (got to go pick up the kids from camp, no time at the moment) but if you have the carb system with the accelerator pump, you may have blockage in one of those lines. The pump will be on your PTO carb and has small, 1/4" tubing, that is attached to each carb on the forward side.
Good luck...........:cheers: