Vespa ET4, AF (A/F Air Fuel) mixture screw
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Joined: 10 Mar 2007
Posts: 15
Location: MA
Thu Oct 15, 2020 4:41 am quote
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,
2010 S50 Modified "Punkin"; 2003 ET4 150 "ISO"
Joined: 18 Feb 2017
Posts: 542
Location: Deepinnaharta, Texas
Thu Oct 15, 2020 12:56 pm quote

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.
2003 Vespa ET2
Joined: 05 Apr 2019
Posts: 357
Location: Cambridge, MA
Fri Oct 16, 2020 10:22 am quote
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
Sergeant at Arms
Weird 80's Vespas & Cool Vintage Lambrettas
Joined: 21 Oct 2005
Posts: 8047
Location: The state of insanity, SoCal
Sat Oct 17, 2020 1:03 am quote
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.

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