Modern Vespa

Howdy folks, I'm Charlie. I work as a mechanic at a motorcycle repair shop up in the frozen wasteland also known as Minnesota, and I have a 2006 piaggio fly 150 on my lift table that's absolutely driving me up the wall.

I've been through the carburetor numerous times, it's the same basic model keihin carb that the kymco mopeds we used to sell as a dealer run; so I'm no stranger to the process of cleaning them. The vacuum diaphragm is intact and soft, slide needle isn't overtly worn, spring isn't bound under the cap, fuel flow from the vacuum petcock is adequate and I'm not getting any bleeding fuel down the vacuum line leading to the intake, and I have also bypassed all the emissions equipment with either unhooking lines or capping them. This is a four stroke two valve engine, but the goofy little thing will (regardless of idle screw setting) either barely idle at about 500 rpm than suddenly swing way out to nearly 3000rpm and vibrate across the floor like an unbalanced washing machine, than slowly sink back to 500 rpm or less and continue this rpm swing till it stalls. Hard starting as well, I cannot find any vacuum leaks at the intake manifold, the rubber boot itself isn't dry rotted and I have replaced the intake manifold to cylinder head gasket. I've basically exhausted everything in my arsenal of ideas for this and I'm getting quite mad that I can rebuild about any Harley Davidson short of the brand new Milwaukee eights from end to end but I can't make a moped run correctly. I've called numerous piaggio/Vespa dealers ranging from Manitoba Canada down to Florida and anywhere else between that'd answer their phones and trying all the tricks they've advised and I can get no changes. I have had the carburetor in a sonic cleaner, and also in a solvent cleaner for numerous hours and sprayed through all passages with carb cleaner and compressed air, but still no change. What am I missing here???

Have you checked the choke system by removing the bystarter, running 12 volts to it and checking the rod for changes? It should lenghten by a small amount (don't know offhand) when working properly to cut off the enrichment circuit once warmed up. That's all I got.

I'm in the process of checking that now, thanks! Although I did try it with it unplugged and the problem persisted, I assume that choke off would be when it's fully extended? I would've thought that if it was stuck " on" choke it'd have a constant rpm instead of the wild fluctuating. Maybe I'm overthinking it and missing something simple.

It does go up with 12v power applied. Takes about 3-4 minutes so it is functional

Hi Charlie, I feel your pain. I've been in a similar place 3 times. Each time the solution was somewhere else. #1. Honda Metro that would idle and die upon throttle. Went through the fuel system 3 times. Ended up being a bad pickup coil ignitor off the stator. #2. Honda Ruckus would idle, but would die upon throttle. I cleaned the carb twice. Plug would be black as soot after a couple of attempts. Ended up being previous owner/mechanic drillied out the main jet in an attempt for more speed. My untrained eye did not pick up on this. Took another set of eyes to figure it out. #3. Genuine Buddy 125, running great. Out of the blue started to die when it would drop to idle. Replace fuel and vacuum lines including fuel filter and petcock, went through the carb 3 times, check valve gap. Ended up being a bad spark plug cap. At this point, you might look elsewhere than the carburetor. Can you buy a cheap ebay carb to throw on, and see what happens? Good luck.