I too purchased a new Morini S6S 9.4HP motor, and full throttle would barely move the bike. But adjustments are easy, and in less than an hour, you can get your machine to behave as it should.
Steps as follows
1. Drain the oil (small phillips screw at the base of the cover) and remove the side cover off with 5mm allen wrench -leave the kickstart lever on, do not remove it.
2. Remove the clutch nut with 19 mm wrench, Note: it has reverse threads. Either use a piston stop to hold the clutch, or slip a small phillips screw drive thru the hub (it has small holes at edge of the drum -peek between the shoes and you'll see 'em) and into the large drive gear. Once you have it braced, remove the 19 mm nut.
3. After the nut is removed, you should be able to wiggle the clutch off the end of the shaft -you don't need a puller.
4. Wipe off the clutch real well, and remove each of the three clutch shoes using 4 mm allen, be careful to make sure the washers stay on the bolts (makes it easier) and make sure you get all the flat washers out of the hole too.
5. Adjust stack height. I used {}{}{}{}{}/ (using thinest flat washer) for a stack height of 0.455 inches -yours maybe different, but this should get you close. I measured each stack to ensure they were the same. A dial caliper works great for this. If you don't have one, buy one, its an important item in any tool box.
6. Reinstall the three shoes -be sure to use lock-tite! Tighten the bolt till it bottoms. Don't be surprised if the shoes will not clamp firmly to the hub, mine had about 0.040 inches of play. Fear not.
7. Reinstall the clutch nut -again use lock-tite (remember, reverse threads).
8. Reinstall the cover -just re-use the gasket.
9. Fill with MTL 75 wt transmission lube (says 'wet clutch approved') available at your favorite motorcycle shop.
10. Take your bike for a ride that will amaze.
If it still slips too much, take it down another 0.005. If it hooks up too much, add 0.005. You'll find you can dial in very well and if its not exactly right, don't settle. Make another adjustment.
I had the cover off probably 6 times to settle into my setting -mostly because I made many small adjustments, each time slowly reducing the stack height until I had it just right. The bike was absolutely transformed and 0.455 was the best for me. I took another 0.010 out of it today, and I went just a bit too far, the bike rides great, but it hooks up a little too early so it's harder to start as the clutch grabs just off idle, so tomorrow, I will add the 0.010 back in and be done with it.
Once adjusted, the Morini is as tame as the little China motors, don't let anyone tell you that these motors can not be made to behave, I can ride my bike as slow and easy I like.
The first time might take you an hour, but after a couple of times, you can get it to about a half hour.
Happy riding!
Rich