Modern Vespa

Hi,
I did a cursory search on my topic, but I'm struggling to find the right words.

2009 S50, has been sitting untouched for 7 years. A week ago I flushed fuel, new battery and sprayed the hell out of the carb.

Problem: Vespa starts ok. Heater/choke starts it up and it seems to keep going once it calms down. Turning the throttle gives a nice bump in acceleration. It lurches forwards, but then it hits 5mph and bogs down and just feels/sounds starved. Can't get going above a crawl/walking pace.

Specifically I...
1. Replaced fuel and vacuum lines. Sucking on the vacuum and the fuel flows.
2. Sprayed through main and idle jets. Ran a length of copper wire through them.
3. Checked the diaphragm seat/seal and condition.
4. Soaked everything in carb cleaner
5. Done all of the above twice.

I have not:
1. Removed the spark plug (can't access the thing in its tight corner)
2. Cleaned the air filter. Blocking the air intake makes the RPMs climb. Seems ok
3. Adjusted mixture - don't have the tool
4. Replaced any jets/gaskets
5. Checked for a mouse-house in the exhaust (I read about that somewhere). But there is exhaust felt.

Things that I wonder about.
1. The sensor hose that runs to the ECU? It is in pretty bad condition/bad seal. I have read about others removing/blocking this hose. I have left it poorly attached.
2. The needle-slide seems to have a slow response on the way down/closed. When I blow on the vent (? oval-segment above throttle intake) the slide moves up, and then takes a second to slide down. Maybe that's fine? I don't know.
3. I read on another post that the variator (sp?) was a likely problem? I'm nervous to open the case before ruling out anything that might be obvious/easy.

Thanks in advance for any help.

I would have replaced the fuel valve while you was replacing the hoses. you are probably looking into a new carb. The spark plug is hard to get to but it will come out .check the vacuum line and fitting on the intake

Thanks Harry,

Fitting on the intake? I tried the "spray carb cleaner on the intake while running" technique. I didn't notice a change in RPM.

I just bought a stubby spark plug socket that I hope will make it around the bend to get at it. I'll try pulling it and see if it needs a clean/replacement.

Could the fuel valve still be an issue? Like I said, it seems to respond correctly to a suck on the vacuum line.

There should be a brass fitting on the intake where the vacuum line goes to from the gas valve .I have had to replace a new valve because of a vacuum leak.Use a good quality valve.( Scooterwest).I still think it's a carb problem as mine was doing the same.I ordered a new carb from Scooterwest and problem solved.Mine sat for 6 years and was loaded with old mice nest in the frame and at the trans cover.They also chewed some of the wire harness.

Thanks again Harry,

I guess I am loathe to buy a $300 carb when I can't find the issue. I'm getting a new plug in, but still haven't solved how to get the plug out. I think I need a 13mm spark socket, which I can't find online? I assume that buying the original vespa tool kit will solve that.

I don't suppose anyone can tell me what these wires (image) are for and if this is their correct state?

I believe one is for the cold climate switch that is located under the choke heater ( one wire) if it is tape it you do not need it .

Try using some seafoam in the fuel

Hey Graham! Did you ever find a solution to this problem? Did you end up replacing the carb? I'm dealing with a similar issue with a 2013 s50, changed the fuel line, cleaned the carb and its jets, replaced float gasket, replaced spark plug and replaced the air filter and still have occasional bogging down. Would like to hear how you solved this...

I have not solved it.

Last weekend I finally fixed one of my long standing issues, which was not having a carb mixture adjust wrench. I found a solution was to hacksaw a slot into the screw and then a simple flat head works!!!

But, this has not fixed my issue. She starts, she idles, she revs up fine to 10-12mph, then tops out. Still very frustrating.

I have not replaced carb.

I read this morning that the variable speed transmission could need a rebuild? I dunno.

I would be too. New OEM Vespa carb for that scooter is under $200. .

https://scooterpartsco.com/s50-air-and-fuel-c-3637_3656_4025/carburetor-for-50cc-vespa-piaggio-aprilia-4-valve-p-4791.html

With my issue, I ended up biting the bullet and bought a new carburetor and intake manifold, which didn't change the situation much. After doing everything I possibly could to fix the problem, I gave up and decided to take it to the local Vespa mechanic. As I was "riding" it there (going about 10mph with it constantly cutting out the gas and me having to pull over to avoid being hit from behind), I randomly noticed that the issues also seemed to be triggered whenever I'd go over any sort of moderately bumpy terrain... turns out it was a bad ignition switch. Long story short, I dropped $300+ into the scooter only for it to be a $15 part that needed replacing lol . On a positive note, I pretty much rebuilt the engine and it runs great now lol

Yeah yeah yeah. I appreciate the better estimate. I'm still nervous of keepbouncing's experience, even at $200. I wish I could do some proof positive investigation, rather than just throwing money until something sticks. My time isn't free either, but I enjoy spending time more than spending money.

Thanks for updating your experience. In my case, I recall that this issue always happened after riding in the rain, rather than bumps. So I wonder if that is related? But, it hasn't been rained on in years at this point

Closing out my own topic with an answer. This was the issue. I'm kicking myself because I kind of always knew it was.

The hose was cracked with a bad seal. Taping over the port on the carb fixed the problem initially. Replacing the hose with 1/4" ID hose also seems to be working just fine.