Modern Vespa

Long time lurker, new member here!

Took the LX150 out for a ride last Sunday. Everything seemed normal. Started right up, idled on the stand as normal and off I went...

After about 10 miles of zipping along as normal, I lost all usable power on throttle. The scooter basically slowed to a stop. Didn't stall, just bogged down/fuel starved on any throttle...crept along at less than walking speed. At a standstill, the bike idled as normal. Letting off the throttle returned the scooter to a normal sitting idle. Turn off the scooter, then she re-started back up easily as normal and idled fine at a stand still. But applying any throttle bogged/starved the engine and...no go.

Initially, I thought maybe some gunk found its way into the main carb jet (though I doubted the main jet was clogged while the smaller idle jet was obviously clear). Anyway, scooter wasn't rideable in this condition so I had to trailer her back home.

This week I let the scooter sit on the battery tender until I could tear into her today (Sunday - 1 week later). So, she started right up, idled as normal on the stand, gave her some throttle and...off she went! I zipped around the neighborhood a few times just like before the strange incident of last Sunday. I did not have the time, weather (raining) or daylight to ride her further to see whether the loss of power under throttle would return. But, I think this rules out a clogged carb jet.

Anyone experienced this with your carbed LX? Any shared experience or recommendations on where I should look to solve the problem will be GREATLY appreciated! Thanks!

That sounds like the immobilizer. On the carbureted scoots, it holds the revs to about 2K RPM.

Maybe the battery was low enough that the immobilizer reset and failed to reread your key?

You might have your battery load tested and connect a digital voltage gauge to ensure that the scoot is properly charging the battery.

Thanks for your reply!

Just to confirm I understand it, are you suggesting the battery may have discharged while I was riding the scooter and wasn't being sufficiently recharged by the stator (or couldn't hold the charge) so the immobilizer lost connection with my key? When I put the bike on the battery tender, that topped up the battery to eliminate the problem (at least temporarily).

So I should check the stator to ensuring it's charging and check the battery to ensure it's holding the charge. Do I have that right?

Yes on both counts.

Hoping it's an easy issue to run down.

1) Sounds like the coil is dying, to test this, run it like you did and when it goes kaput, let it sit for about a half hour and try again.
2) Or choke is staying on,
3) fuel starvation
4) or what the previous poster said.

Don't think there is anything else in the fault diagnosis flow chart for this symptom.
If something pops into my head I'll post it. Good luck, and remember to test every hypothesis and make sure before moving to the next, so not to chase a monkey by the tail.

Thanks to you both for your helpful (and prompt!) replies. I was also suspecting a weak/failing coil (my father used to say most "carburetor" problems are usually electrical problems...) The fact the scooter had no power, but wasn't stalling and easily returned to idle confounded me. I'll start with the battery/stator. If that doesn't resolve the issue, I'll check the coil. Thanks, again!

I had a similar problem on my 2009 LX150. It was caused by tears in the carburetor rubber diaphragm. It's easy to check without removing the carburetor. Simply remove the 4 cover screws and remove the cover and spring. Then carefully pull out the diaphragm/ slide assembly. Attempt to Look through the diaphragm at a strong light to check for tears or rips. On reassembly I use a small amount of silicone dielectric grease to stick / seal the diaphragm back securely to the carb body. Don't use regular grease.

Thanks for the reply. I'll check that out if the other electrical causes don't do the trick!

Following up on my my original post. The battery checks out (holds a charge and passed the load test). Charging system appears to be functioning properly (connected a digital voltmeter to my charger harness and watched it charge as normal when running). I've now ridden the Vespa twice without the "no power under throttle" condition repeating...though I haven't yet ventured away from "push back home" distance. I'm running out of riding weather here in NE Ohio but still hope to arrange a day when the GF can follow in a recovery vehicle. i'd like to resolve this now and not wait until spring.

I think I've ruled out carb issues, as well as the battery and charging system. Thinking about the coil, I assume failing could be sporadic initially, maybe even weather dependent (hot days vs cold days)? Also, it possible for a failing coil to allow the scoot to start and idle fine, but then not allow it to travel under load? I would think a bad coil wouldn't allow the scoot to start or idle?

Any other ideas?

Thanks for the reply!

What did you do to rule out a carb problem? From where I'm sitting it sounds like a problem with the diaphragm.

The last two runs the scooter started, idled and ran as new. Also, when I was experiencing the no power under throttle, the scooter never stalled. After sitting, the scooter ran as new. I figured all that means the carb is functioning as intended. I suppose it could be an intermittent diaphragm sticking issue, but I'd think the scooter would stall if starved for fuel at higher throttle opening? Perhaps not.