Modern Vespa

Hey all, I recently scored a nice 2009 LX150 from a pal who's moving. I've only had a new (2016) bike up to now so haven't had to do much, ok ANY, of my own wrenching! So now is my time to learn. Anyway, about the bike:

Runs great, starts immediately, good accel, good top speed. Does not backfire when cold, but after warm up it will backfire pretty much every time I decelerate. I can minimize it by babying the decel, but still annoying. Combined with really poor gas mileage, I am guessing it's running way too rich. Although could be wrong there -- tell me! Bike has a couple of mods: gravity feed fuel pump, performance exhaust. That's about all I know.

Ideas? Diagnosis flow chart for me to work? Thanks!

Edit: the exhaust is a SITO PLUS.

It sounds like you are going to chasing down an air leak at the intake and exhaust. The idle circuit sounds like it needs work as well. Start by checking the number of turns out on the idle mixture screw. Check the intake rubber manifold for cracks - it's probably due for replacement. Go over the header pipe and muffler and see if there is a leak starting at the connection to the head and work towards the tailpipe. Many aftermarket exhausts will back fire as there is not enough back pressure built in to the pipe for the sake of the sound or it was modified and the baffles were removed. And I don't know why it would have a fuel pump on a late LX model carbed scoot when the ones I know off were just gravity fed from the petcock.

Get Rid of the EVAP system, block off the SAS, replace fuel and vacuum lines and report back.

Thanks. Let me clarify, for my ignorance: 1. Assume this means turn it till it stops and count? Which direction?
2. Hmm, is this just a visual check, i.e. cracks/gaps, or something more "scientific" for air leaks?
3. So you mean gravity feed was stock on these? My mistake, I assumed a pump.

1) you may not be able to do this as it may have the original "D" shaped head on the mixture screw, unless someone replaced it or cut a groove in it so a small screwdriver can be used. Anyhow, you would count the turns in, that is, clockwise and make a note. Stock is about 2 1/4 to 2 1/2 turns out from bottom. Turning it in will lean it at at idle and slightly above.
2) visual and you should be able see and feel it when the engine running.
3) they were gravity feed until fuel injected came in around 2010 - 2011
Abner also had some great suggestions and you should probably do those first.

Another possibility is the condition of the rubber intake hose between the carb and the cylinder head. They are known to perish.

There is a nice video in the Wiki section that explains the process to take out the evap system. Mitch's video. Also, he has a video that goes through the restoration of an older LX which I found invaluable. I have an LX150 too.

Good choice of scoot, by the way. We all love photos...

Many of the last few carbed LX/S 150s have factory fuel pumps (including mine, an 09 S model).

I had heard that but never saw one in person. And I can't figure out why Piaggio would do that except to make it more complicated. Honestly, the gravity feed worked just fine.

I think that '07-'08 was gravity feed, '09-'10 was carb'ed with the pump and the '11-on models were fuel injected.

Can confirm that each variant has its own wiring harness.

Yup, we sure do. Here's the blue LX in front, the black Prima in back.
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