Modern Vespa

Hi guys, weird issue with my scooter that showed up: It stalls when coming back to idle, but only sometimes.

It does WOT, full acceleration, max speed (90kmh+) just fine, no issues, but as soon as I come to a stop, it sometimes stalls. I can hear the rollers rattling around and then it either stalls or idles perfectly.
It also becomes hard to start sometimes, but it eventually does and idles.

If it does idle, it will never stall. It's really whenever I give it gas and it comes back down.

I've checked:

The idle jet - super clean, no issues whatsoever.
Fuel flow - comes out with just a little tap of the kickstarter.
Valves - all within spec, unless they got out of spec in just a few hundred kms.

I've tried spraying some wd40 around the front of the inlet, didn't notice any rpms change and it was covered in the spray.

Could the rollers have something to do with it? I admit I did reuse the ones that were in there. But I don't see how this would happen even on the stand.
Maybe a bad auto enricher?

This happens to me when the fuel gets a little stale. Is your fuel truly fresh? Fuel companies change fuel formulas in the US in winter and summer, and this can affect carbs a little bit. Is it possible that you're running it now on a slightly different formulation than the one the carb was tuned for?

Clean and/or replace the air filter.

Then, if you're truly satisfied that the carb is clean (thinking this when it isn't really true is a very common problem), consider increasing the idle speed just a little bit.

I'm in Europe and the fuel is 100% fresh. I'm using 95 which is what it was rated for. Ironically, though, I started noticing these issues after refilling from another gas station. Vent tube is unclogged, I checked.

The idle jet was clean. I couldn't see where the hole went though, so maybe it isn't clean where the idle fuel passes. No idea how to clean inside those passages, it's a pretty old carb

I'll clean the air filter and check, thank you. If that doesn't work, I'll try messing with the idle speed. It's already somewhat high, which is what makes this all the weirder.

If you have any other ideas, I'd love to try them out.

Cleaning the air filter did nothing, same goes for the idle screw. I checked for leaks again at the intake, nothing.

Anyone have any ideas?

Is your spark plug relatively new? My Stihl chainsaws did the same thing (stopped running sometimes after coming to an idle). Fresh spark plugs solved the issue.

I wouldn't mess with the idle speed. I've had this same problem twice on my Buddy 125. Went through the carb twice, ended up being a bad spark plug cap. At cruising speed, the engine was smooth, and contact wasn't a problem. At idle it would bounce around enough to lose contact. 2nd time, the stator was going bad. Both times, looked like a carb problem, both times not.

It's a few months old, spark plug color seemed perfect (black around, brownish middle). Thinking about ordering a new one though, just to see. Thank you. Thank you, I just ordered a new coil and a cap. I was also suspicious about it. I wiggled it around a bunch, but it never seemed to misfire. Still, like you said the vibrations from the engine itself might break the contact loose.

"Maybe a bad auto enricher?" Same symptoms and checks on ET4 150. I went through all the same steps as you. It was the auto choke (enricher) sticking intermittently.

I had a similar situation with my GTS, turned out to be the battery. Replacing that and all returned to normal.

Nice, we have the same engine.
I'm going to order a new one in a bit, already have a new cap and coil coming. I've noticed it acting weird around cold starts (only revved up intermittently, meaning it only works sometimes). Thought it was a quirk.

I pulled it out and it seemed extended. Can you confirm whether it's able to be pushed back in when cold? What did you do to test it? Thank you

Thought about that but I put in a new battery around two months ago. Volts on it are at 13v. Don't think I have any bad cells. Thank you.

"I pulled it out and it seemed extended. Can you confirm whether it's able to be pushed back in when cold? What did you do to test it? Thank you"

I popped it in the freezer for 15 minutes and it should have retracted somewhere around 8 mm., if I remember correctly. Did this three times and let it warm back up to room temp. Out of the three times, I got three different measurements. All falling short of the of the required specification.

This is an old scooter?
Have you checked the intake manifold for cracks?

I just performed this test and I think you were right. From room temp to 15 mins in the freezer it only dropped 1-2 mm. The changes are so small but I think I got a few different measurements too. Do you know where I can see the specifications? Thank you

Yes I sprayed WD-40 around the intake. No rpm changes

Well, after some more tests and checking out the workshop manual for the et4 (also has the walbro carb), I think it's safe to say it's done for. Resistance should be 30-40 ohms at room temp, read 24 at first, then it suddenly shorted and stayed that way.

What's more, once extremely heated, the max the piston gets up to is 16-17mm, rather than the specified 19mm. Cold it read like 12mm which is close to the 13mm. Read it from the rubber to the top of the brass only and not including the tip.

If anyone else has anything for me to try out, let me know, just waiting for the parts to arrive and will edit the post if it's fixed (or not).

After an infernal lottery game, opening and closing the carb for a thousand times to the point it's cleaner than when it came new, I seem to have fixed it. It no longer stalls no matter how slightly I touch the throttle.

I installed the new enricher and increased the idle some more for the wheel to be spinning slowly. I now have a higher idle on cold starts no matter what.
Still no air leaks, my scooter is basically a mirror at this point with all the wd40 I threw around the carb/intake. Valves are within spec, I checked again.

My float was set slightly incorrectly, though I dunno how much that even contributed to the issue. I set it according to the workshop manual, parallel to the carb upside down.

I do hear it cutting off before recovering when switching from the main jet to idle jet, but from what I can tell, that's normal. Only happens once the enricher is off/wax is heated and only sometimes. If anyone can tell me this is not normal behavior, please do while the thing is dismantled, the Liberty is a pain to fix. Still waiting for my spark plug cap and coil, will also swap out the plug as its been through enough tests.

I'm pretty sure I might have to mess with the mixture screw if it happens again, which I do not want to do. If the enricher turns off and the scooter struggles when switching to the idle jet, it's most likely running a bit lean if I'm thinking correctly.

All I know is this old carb is a major issue. This has been a total pain in the ass, especially since there are no rebuild kits. It's basically a praying and guessing game hoping the old parts still function and seal. For anyone else with a Walbro, just save yourself a headache and get a KEIHIN CVK 2600A. Sooner or later everything on it will fail.