1) Open the seat, remove the tub, and center the handlebar.
Seat up, easy access.
2) Position a jack beneath the engine-pivot, being careful to balance it properly. BE VERY CAREFUL or you could hurt yourself or your scoot! I used a wide-base jack with a piece of 2x4 across the pivot, and jiggered it until I found good balance. A safer way to do this would be with a proper motorcycle jack, but I had no problem.
3) Remove the lower bolt of the rear suspension unit. The locknut is 17mm and the bolt is 14mm.
That single bolt at the end of the shock is the only thing holding your engine and rear wheel in place. A leeeetle scary to consider.
4) Remove the spark-plug wire or else it'll tear while you jack the scoot. I suspect the people who last serviced my machine neglected to do this (more on that in a second...). I found that it was easier to remove the coil section than to try to pull the rubber boot - no space! See what I mean?
Just barely visible at the upper-right of the engine stuff is a black nub: That's the spark plug wire. Good luck accessing that with the scoot flat on the ground.
4a) To remove the coil unit, first pull the engine-head service cover, a little panel that sits between your heels as you're riding. It's a single screw. Now you'll see the single bolt that hold the coil in place; remove that, then pull the two color-coded wires. See this pic for details:
Here's where you'll find what appears to be the coil - where the spark plug wire leads. Big silver finned thing to the right is the engine head.
5) Jack the scoot slowly while holding the body or else it could tip. It'll remain pretty stable if you have the jack centered, because the rear engine unit and wheel remain on the ground. Also keep an eye on the wires and other hoses, checking with each jacking step to ensure nothing is binding or pulling. I found I didn't need to move anything, but things could get tight depending on how your Vespa was manufactured. Here's what it looks like all jacked up:
Jacked up and ready to service.
6) Now you can easily access the spark plug, the air filter, and many bits and pieces that are filthy but you couldn't clean due to their being hidden by bodywork.
Now you can access the plug! And here it is, yoinked from the head. Notice how much more room is available with the scoot jacked up.
7) While working, don't forget that the Vespa is balanced on a jack! Hold the body if you're torquing on something.
8) Clean the air filter with soap and water, dry, then re-oil with air-cleaner oil.
9) Replace the spark plug: The S150 uses an NGK CR7EB or Autolite 4303. The Autolite is more available at an auto-parts store, but you'll need to keep the wide tip from the NGK and screw it onto the top of the Autolite before installing it. Use a little anti-seize lubricant on the middle of the spark-plug threads to prevent it from rusting in place and to ease installation and later replacement.
10) SLOWLY lower the scoot, watching to make sure nothing gets pinched while keeping it balanced and centered. It'll wobble as its center of gravity shifts: Don't panic! Watch the bottom of the shock unit, as that can get caught in the inner fender. I balanced the scoot by holding the shock-unit's spring and guiding it into its mount on the engine unit.
11) When the holes line up, slip the bolt back in place and re-attach the locknut.
Ready to rock!
Viola, you've just serviced your modern Vespa!
This is my primary transportation, so when something goes wrong, I fix it right away. Last night, it stopped running in the middle of a busy intersection and I had to push it home about a mile. Turns out that the culprit was a broken spark-plug wire, which I stripped, soldered, then wrapped and re-installed. I suspect that the last folks who serviced it (suring the 1000-mile service last fall) neglected to pull the wire before jacking the scoot, thus tearing the wire and leading to the frequent poor running that I've had to deal with since. Well, it's fixed now.
Now let's see if I have the energy to work on the Newport....