Modern Vespa

I searched for an answer to this problem, but didn't/couldn't find an answer. My 2008 Vespa will only start with the throttle. It will keep running with full throttle, but then stalls when you let off of it. I've had this scoot for 3 years now, and have had this problem every year. The past 2 years, we'd finally get it started and idling and after riding it for about 1/2 hour, it would shut off and then start fine.
I tried adjusting the idle screw, but to be honest, I have no idea if I even had the right screw! I'm trying to save some $$ before I trailer it back to the dealer - AGAIN!

What do you mean "I've had this problem every year."?

Has it ever run normally? Does this only happen when you take it out of winter storage, or something similar?

Have you solved the problem the previous years?

Bad gas, clogged carb....?

It could be electrical, but I'll go with fuel feed. IF the GT sits for a long time between rides, the fuel may be deteriorating and creating sludge in the carburetor float bowl. Those particles will get sucked up into the jets and passages. In your case it appears the pilot/primary jet [low speed and idle] is being blocked.

Try the cheap and easy fix first and add some carburetor/injector cleaner to the gas. Sea Foam is good. If the blockage is only sludge, the stuff will dissolve it. If no joy, have the carburetor removed and cleaned in an ultrasonic.

If you've tweaked the idle screw and/or the air screw, they will need to be reset after the carburetor is clean.

Use StaBil in the gas during storage.

Every year, when I take it out of storage. I use stabil every winter like the dealership told me, and it's in a heated garage all winter, The past 2 years we would finally be able to keep it running by me actually riding without stopping for about 20-30 minutes. (not too hard to do on the back roads around here). I even took it to the dealer last year, told them the problem, and they didn't know what was wrong. They said it ran fine for them.
This year though, I can't even start it.

Could it be the battery?

While it is in storage is the battery on a trickle charger? Maybe the battery is to weak to idle and only has enough juice when revved at full throttle.

Do you charge your battery before starting it?
Depending on the battery type it will lose 1/3 of it's charge or more on a four month lay-up.

After adding StaBil, did you ride the bike for ten miles to get the StaBil down into the carburetor? If not, only the tank content was protected and the gas in the float bowl deteriorated.

Is the air filter clean ? or maybe over-oiled ?

Thanks all for the suggestions, they're very much appreciated. I'm going to get some carb cleaner today, and see if maybe that's the problem. I have a feeling maybe there's a piece of crud in there. I'll let you know how it goes!

Berryman B-12 chemtool fuel treatment available at Walmart and most auto parts stores its much cheaper than Seafoam and I'm told is the same thing. I always treat my fuel in one way or another since we have 4 scoots sometimes one or two sit and I dont want to pay to get the carb cleaned.

If you use STABIL use marine grade (its blue) the regular stuff isnt as good.

I had stalling sputtering issues with my GT but after using additives for a while they went away.

Check the vacuum hose that operates the fuel tap. I've had them develop cracks over time.

+1

I've had this exact problem with my LX150 several times when its sat for more than a couple weeks (which isn't often, but I digress). It's a clogged idle jet. The fact that it will run with throttle, but die on idle gives it away. Typically, what also happens is that I end up flodding the carb while trying (stupidly) to get it restarted by holding the throttle open.

I don't know how the GT's car differs from the LX's, but all you should need to do is remove the idle jet, blow it out with some compressed air, and re-install. No need to adjust any mixture settings.

Best of luck to you.

I bought some Seafoam, and I don't know what they put in that stuff, but God Bless 'Em! The scoot is back up and running, I did have to tweak the idle screw a bit, but it seems to be running fine. I'm going to take it out again tomorrow, just to keep everything clear.
Again, thanks for all the advice, so much better than the dealer!

SeaFoam to the rescue again!

Seafoam 2 oz every month and 3-4 oz's for winter storage