MANY years ago after purchasing my 1st Invader in 1982, I battled with the sled being very unresponsive from a standing start.
I tore into the carbs several times. I had the center to center distance set at precisely 12"
I was subscribed to 2 of the snowmobile magazines from back in the day. A new issue had an article on clutch tuning. One thing it discussed was belt deflection. At the time, something totally new to me.
Over the years, I have found that several of the manufacturers use the belt deflection spec rather than the center to center spec.
I probably had a fairly new Dayco 1078 on the sled back then, but when I pressed down at the midway point of the belt, it was so soft I could push it down and make contact with the bottom of the belt.
"As a general rule" it seems that pressing down at the midway point between the clutches with about 20 lbs. of force, you should see about 1 1/4" deflection from a straight edge spanning the clutches.
I know once I moved the jackshaft back to get the deflection near that measurement, the Invader was transformed.....I had it set so loose before that the belt was probably engaging way too high on the drive clutch, rather than at the inner circumference of the clutch.
Once the drive belt has worn some, it is no longer 1 1/4" wide and it tends to sit down in the secondary clutch.....which makes it behave like a belt that is too long, and the belt deflection becomes more than 1 1/4".
Here is a belt that I believe is an absolute bargain:
https://www.mfgsupply.com/hp3012-w2.html I tried one of these belts on my "new" trailer queen Invader that I got last year. We have received just enough snow around here a couple times that I just barely tried it out, but it looks to be equal in quality to the Dayco 1078's that I have used in the past.
And, adjusting the jackshaft to get 1 1/4" belt deflection with that belt left a lot of room to move the jackshaft forward or backward more, if needed.