OP
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ET4
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I have 2001 Vespa ET4 (150cc).

I have a question concerning AF (A/F Air Fuel) mixture screw.

As I turn this clockwise and screw it more tightly into the carburetor body, does this make the scooter run more rich or more lean?


Please let me know from your concrete experiences instead of random guesses. Some motorcycle AF screw controls fuel flow and some controls air flow.


Thanks a lot,
@wheelman-111 avatar
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2010 S50 Modified "Punkin"; 2003 ET4 Malossi 187 "ISO"
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Greetings:

Walbro WVF carb, right? OUT is Rich. I have that exact carb from my 2003, swapped for Keihin CVK that was introduced in 2006. Both work fine, but improved considerably with deletion of U.S. Canister evap recapture system.
@jixaw avatar
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2003 Vespa ET2
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this tip has always helped me:

if its on the airbox side of the carb, its for air...out is more air

if its on the intake side (out) its for fuel...out is more fuel
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Weird 80's Vespas & Cool Vintage Lambrettas
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here's a quickie: run the screw out until it is just flush with the carb body orifice where it threads in.

do this while the motor is warm, and double check the function of your choke.

but that seems to be the sweet spot.

you may need to bump the idle speed down a hair, but other than that you should be rock and roll.

-g
⬆️    About 3 years elapsed    ⬇️
@technomama avatar
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Vespa et4 leadier 125, Vespa Lx 50 2t
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I've got the same carb, Walbro WVF, and it works perfectly but isn't the mixture screw discussed here is for idle? I play with it and it affects idle - where is the l mixture screw then? Most likely it doesn't exist even in the hidden form. Many thanks if you come back to this old post, no problem if not.
OP
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I got this image from another posting on Modern Vespa. This is a pict of a carb for a different model Vespa. But certain things are still the same. The Air Fuel mixture screw is on the left side of the carb.

But you shouldn't mess with it, because it takes a lot of work to set it just right. Also there is a small washer that you can lose if this A/F screw is totally unscrewed out.

One should read up on the role of Idle screw vs A/F mixture screw. Also A/F mixture screw to adjust fuel vs air.

If you messed up A/F screw adjustment, then first screw it in all the way. And then unscrew it by around 3 complete turn. This will be a good starting point. After that, the optimal will be somewhere within +/- (180 degree, 1/2 turn).

Good luck. If your scooter was working fine for years but feel the need to adjust A/F screw (make richer), then I would say first search for any vacuum leak first. The rubber intake manifold is known to go bad (cracks & leak). And that large thick vacuum hose, you should feel by hand all sides from beginning to end. My large hose, there was a hole from years of rubbing against side of engine.
Forum member supplied image with no explanatory text
Forum member supplied image with no explanatory text
@technomama avatar
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Vespa et4 leadier 125, Vespa Lx 50 2t
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Vespa et4 leadier 125, Vespa Lx 50 2t
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Thank you a lot for your advice. I found a crack in the intake manifold and ordered a new both - manifold and air filter hose. The engine works fine on any screw setting (also, I have disassembled this carb for inspection reasons) but I thought, if I could lean the mixture reasonably, I could add some distance and/or mileage. My longest trip so far was 200 km with 50 cc 2T engine and now with the new 125 4T I look into some greater distances. Going back to the screw - most likely it is the one screw for idle and mixture rich/lean adjustment.
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If lean condition because of a vacuum leak, you might
experience following symptoms.

After starting the engine, while auto choke is still
partially ON (extra fuel hence rich), your engine will stay ON.
But about 2-3 minutes later, auto choke ends, and
Your engine will want to die. You will find yourself
Restarting your engine several times over and over.

This whole symptom gets worse after a night of rain,
moisture in air…. , not optimal engine running condition.
But once you get your engine going,
regardless of rain, your engine will work fine.
This is because "hot" engine will run even with lean (vacuum
leak) condition.
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@greasy125 avatar
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Weird 80's Vespas & Cool Vintage Lambrettas
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@greasy125 avatar
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adjusting your A/F mix isn't going to alter your mileage. I mean, maybe if it was way out of adjustment it could have an effect running around town.

it's more for ride-ability, anything past 1/4 throttle it doesn't have much bearing on the operation.
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Vespa et4 leadier 125, Vespa Lx 50 2t
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A/F mixing - I used tampering with that while riding 2T 50 cc a couple of years ago. I've been making it as rich as possible because I was afraid to overheat the engine running WOT for hours. I noticed another thing - nowhere in the pictures you can find people using air duct hose that runs from under the seat - to the air box. The newer models most likely don't even have that, although LX body surprisingly still has fixing places inside the tip of the seat. Adding a photo from the internet to be clear. It most likely was designed for the first series of "modern vespa" to make sure air turbulence doesn't affect air intake. And with time they found alternative airbox designs containing fewer parts. My airbox is from ET 4, not even sure Leader or pre-Leader.
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If you turn the screw in all the way, the engine should die. If it doesn't, the idle is up too high.
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