Modern Vespa

LX 50 4T 2V model.

Starts up easily. Goes to idle without problem. Can drive it for about 10 minutes, then whenever goes to idle, it will stall. Every stop sign/light, it stalls.

I removed the air filter, doesn't look dirty.
I removed the carb, cleaned the idle jet, the main jet, and the bowl. Noticed that the accelerator pump moves up and down, but doesn't really squirt fluid. Also noticed that when the carb is disconnected from fuel line, fuel flows out of the fuel line without stopping. Also noticed that when engine is running, if manually depress accelerator pump lever, engine bogs.

I have ordered new idle and main jets. I have ordered new air filter. I have ordered new petcock(vacuum model).

Where is the fuel filter? Is there one? I have an inline fuel filter and hose i can install.

Does anyone have any ideas other than what I have come up with?

You might as well replace the fuel and vacuum lines while you are there. Adjust the valve clearance if is time, check the spark plug and cap. If you are able, get the carb in a sonic cleaner.

Be sure your spark plug cap is making solid contact with the plug. I had this problem on a Buddy 125. I went through the fuel system twice with no success. Plug cap was worn. At idle the scooter jumped enough to cause intermittent loss of contact. At higher rpms, it was smooth enough to make solid contact. A new NGK cap, and all was fine.

Thank you both for your suggestions. I will do as you say.

One thing I noticed which is strange relates to the petcock and the vacuum line. I previously had the fuel line disconnected from the carb, and fuel was flowing non-stop out the line - in otherwords the vacuum valve on the petcock was in the full open position, despite the engine not running. I thought this was a petcock problem, so i replaced the petcock. When I did the petcock replacement, I removed the fuel line first expecting fuel to flow as it had before, but no - the petcock now seems to be operational! So in order to drain the fuel from the tank (necessary for petcock change), I had to suck on the vacuum line until all the fuel had drained out the fuel line from the tank. While doing this, i noticed that there was fluid inside the vacuum line. Strange. In any case, I got all the fuel out of the tank, changed the petcock, and refueled. Unfortunately, the stalling problem was not rectified.

However, in my vacuum line I noticed that it is not just a hose, but that there is an inline filter on the line. Perhaps that is causing some problem. So now i have changed out the vacuum line and filter for a new line (no filter).

I got looking at the parts schematic for the LX50, and noticed that there is a secondary airbox on the engine, and that airbox has a diaphragm in it. Does anyone know what the secondary airbox is for, and does the diaphragm fail?

The secondary airbox and filter is for the SAS system which injects air into the catalytic converter to make it heat up faster to burn any unburned gas in the combustion process. You can remove the line going to it and seal up the T connector on the vacuum line that the SAS line comes off of, but I don't think that will fix your idle problem, but it's worth a shot. The petcock needed changing out, so it's good that you took care of that. Check the cap as suggested and if it still stalling, I'm guessing the idle circuit is still clogged up.

Thanks for the explanation about the SAS. As you say, it likely has no relationship to my stalling problems.

I would to like to clarify the exact particularly of the stalling since it's so strange. The engine only stalls when the motor is hot AND the motor is brought from a high rev down to idle. So, what happens when you slow down to stop at a stop sign - then it stalls. I can then easily re-start the engine and it will not stall. Until, i repeat the process of high revs down to an idle. The engine bogs at this and then stalls.

I am starting to think the symptoms indicate something with a sticky needle in the carb or the mixture screw or perhaps the CDI.

Check the plug cap, they are a wear item.

My sparkplug cap is very tight, could hardly remove it.



I have been told that the carb is not transitioning properly into the idle cycle. Perhaps I need to clean it better, or adjust it better,.......or buy a new carb.

What I find so strange is that there is no idling problem until at least 10 minutes of engine running. And then when the engine is warm, if I let off the throttle, the engine will reduce RPMs until idle and then stall. It really bogs. I have found that if i keep the throttle open a bit, i can prevent the stall.

This may be because it is running off the enrichment circuit while cold. Once the engine is warmed up, that is when the idle circuit comes into play. Honestly, the idle jet is so tiny that it is easy to clog up and clean. I usually have to run a stand of copper wire thru the opening to make sure it isn't blocked. And then check by looking thru it with a strong light from the other side. If you take the carb off again try to get it sonic cleaned. It works better than spraying carb cleaner everywhere. Any local motorcycle shop would have one, especially the ones that cater to dirt bikes. They're always cleaning the carbs on a regular basis.

A 19mm open-ended wrench (spanner) works to prise it off. The fact it's tight is good. A loose (or easy to remove) spark plug cap should be replaced.

Follow up: The stalling problem has been resolved. I replaced the CDI unit with a new model from ScooterpartsCo (actually an unrestricted model). There is no stalling at all now. There is no bogging at all. It runs like a new machine. Top speed has also increased.

If you have stalling problems with a LX 50 or 150, read through the posts about how the stalling happens only when the engine is warm. If that's your case too, then you need a new CDI. Also, do not buy a CDI from China - I did and it was defective. In the end I bought a CDI from ScooterpartsCo, and it worked perfectly.

There is a troubleshooting guide on this site https://scooterpartsco.com/lx-50-electrical-c-3637_3647_3891/cdi-coil-for-vespa-lx-50-piaggio-fly-50-2v-unrestricted-p-1526.html

Thank you for reporting back.