Mixture screw doesn't relate in any way to the main Jet in how you are talking. If you turn the mix screw in all the way and engine doesn't stall then your idle jet is possibly too rich.
I used the following steps posted earlier in this thread. Are they not referring to the 7mm mixture adjustment on the back of the carb?
"To set the mixture screw correctly, here's what you need to do :
Start the scooter and go for a 1-2km run to warm the engine up.
1. With the engine running on the stand, take the engine side panel off. Turn the idle speed right up, the long screw with the flathead screwdriver fitting that pokes out of the carb box top, turn clockwise in.
The engine will be racing now!
2. Immediately turn the mixture screw on the back of the carb all the way in, the engine will get choppy and the idle will drop. On PX200 models it will require an 7mm spanner, there's not much room in there.
3. Immediately then turn out the mixture screw from closed in 1/4 turn increments, the idle will increase and the engine will start to smooth out. Take a few seconds wait between each 1/4 turn out. Count the number of turns as you open the fuel screw out.
4. You'll get to a point where the the engine will have smoothed out and the idle stops increasing when you turn out the mixture screw. This is close to where it should be set.
5. Adjust the idle speed back down to an acceptable running level. Then listen to your engine when you blip the throttle.
6. If the engine 'bogs' and feels flat when you blip the throttle it is probably set too rich. If the engine 'hunts' and takes more than 2 seconds to come back to a steady idle after blipping the throttle, it is probably too lean. A lean idle that 'hunts' the revs will make a 'pip, pip, pip sound.'
You should be able to blip the throttle, the engine should rev. clean and it shouldn't either bog, or 'hunt out the setting'. It should rev. and return to a good idle within 1-2 seconds.
7. Make a small adjustment here if necessary. Then adjust the idle speed slightly.
8. On tuned Vespas, if it takes more than 4 complete turns, then pop in a richer idle jet, and repeat. This is the smaller jet on the left. The PX200 runs a standard 160/55 idle jet. The T5 runs a richer 100/50. The richness of the idle jet is the ratio of the two numbers, the lower the number, the richer the idle jet. The 100/50 is 2.0, the 160/55 is 2.9. The 100/50 is therefore richer.
If it takes less than 2 turns on a tuned Vespa, consider popping in a leaner idle jet and repeat."