The notch is a front-end problem of MP3s. It feels like there is a detent in the steering which makes it difficult to turn slightly from straight ahead. It is most annoying at low speed and when "filtering" through traffic, where you have limited freedom to wobble.
The notch is caused by the rollers of the lower steering head bearings creating dents (notches) in the bearing races. It occurs primarily because the original bearing materials are too soft. It is aggravated by improper adjustment and poor initial lubrication. Unlike normal roller bearings which are rotating most of the time, these bearings spend most of their time in the one position (straight ahead), and that's where the damage is done.
The complete fix for the notch is a replacement of the steering-head bearings. If you have the problem and the bike is under warranty, get your dealer to to it. Unfortunately, it is a big job for the average rider. This article describes a simpler method of fixing the problem. It enables you to put off the day you have to bite the bullet and replace the bearings.
1. Lubricate the bearings.
If your notch is small, this may be all you need to do. In any case, you need to do this first. Get or make a lube bolt for the bearings. Piaggio now have a spare part for this. Essentially it is an M10 bolt with a hole down the axis and a grease nipple on the end. It enables you to pump grease into the steering head. The grease eventually finds its way out via the bearings. Here are some articles:
A modification to increment the live of the bearings sistem
Piaggio release steering 3 Wheeler grease bolt tool...
Easy to Grease Those Bearings without a Special Bolt
2. Loosen the steering head nuts.
Put the bike on the stand. Release the tilt lock. Jack up the front so the front wheels are off the ground.
Remove the front plastic side panels.
Start with one side, say the right side, and loosen the upper lock nut on the top of the steering. Back it off a turn or two, then undo the lower lock nut the same amount. Repeat this a few times. You will find that you can't simply take off the top nut, then do the lower one because the hydraulic fitting bracket on top restricts your movement.
3. Expose the bearing rollers.
Continue loosening the nuts and you will notice the bearing rollers coming into view at the bottom of the steering head tube.
From this point on do not turn the steering until the nuts are locked up again.
When you can clearly see the rollers, use a white marker or correcting fluid and mark the position of the roller cage on the tube (see pic above). Keep undoing the nuts until the roller and cage are free to turn relative to the bearing races.
4. Adjust the position of the rollers
Now the rollers are free to move, while keeping the steering from moving, use a punch and gently tap the rollers in their cage to one side or other until a roller lines up with the white mark you made earlier. What this does is aligns the indentations in the races with the GAPS between the rollers, rather than with the rollers themselves.
5. Lock it up
Now tighten the nuts at the top in stages. First the bottom one a couple of turns then the top one. The rollers will disappear back up inside the tube.
There is now a special tool to tension the locknuts. I don't have one, so I do this: When the lower nut starts to get tight to turn by hand I use a punch to tap it gently until it stops turning. Mark this point. Back off the nut 110 degrees. That's almost one third of a turn. Hold the nut there, and tighten the top locknut against it.
6. Repeat steps 2-5 for the other side.
7. Reassemble the bike and test ride.