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Happy New Year All! My 05 PX is running a bit rich and I am going to tackle adjusting the fuel/Air mixture on the carb. Just wanted to know which way to adjust the mixture screw (tighten or loosen) to lean out, allowing more air to the carb. Thanks in advance for your help. Safe Riding
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bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
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Tightening the screw reduces airflow to the carb, making the mixture "rich" in fuel. Loosening the screw allows more air in, making the mixture "lean" in fuel.

So: In = richer, out = leaner.

Oh and don't turn wildly. Turn in tiny increments and give the engine a few seconds after each turn to process the change.

Enjoy!
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Thanks! That helps a lot. I just realized that the mixture screw is inaccessible. The electric starter is in the way..Will I need to pull the carb out to perform the adjustment. I was hoping to adjust while the engine is idling. Easily performed on my dirt bike but this has got me scratching.
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SoCalGuy wrote:
Tightening the screw reduces airflow to the carb, making the mixture "rich" in fuel. Loosening the screw allows more air in, making the mixture "lean" in fuel.

So: In = richer, out = leaner.

Oh and don't turn wildly. Turn in tiny increments and give the engine a few seconds after each turn to process the change.

Enjoy!
the exact opposite: Mixture screw is after throttle: this is a fuel screw... tightening the screw makes idle mxture leaner, loosening it makes mixture richer....
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Molto Verboso
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Re: Vespa PX 150 Carburetor Tuning
edoinfl wrote:
Happy New Year All! My 05 PX is running a bit rich and I am going to tackle adjusting the fuel/Air mixture on the carb. Just wanted to know which way to adjust the mixture screw (tighten or loosen) to lean out, allowing more air to the carb. Thanks in advance for your help. Safe Riding
unless you are having problems with your idle i would not touch that screw. it only affects below about 1/8th throttle and is there to fine-tune the mixture to eliminate hesitation off of idle.

adjusting that screw if you think your bike is 'running a bit rich' yet has no problems idling or accelerating from idle will only make your bike run worse and introduce more problems for you to chase.
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bodgemaster
63 GL, 76 Super (x2), 74 Primavera (x2), 79 P200, 06 Fly 150
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UTC quote
fastfrog wrote:
SoCalGuy wrote:
Tightening the screw reduces airflow to the carb, making the mixture "rich" in fuel. Loosening the screw allows more air in, making the mixture "lean" in fuel.

So: In = richer, out = leaner.

Oh and don't turn wildly. Turn in tiny increments and give the engine a few seconds after each turn to process the change.

Enjoy!
the exact opposite: Mixture screw is after throttle: this is a fuel screw... tightening the screw makes idle mxture leaner, loosening it makes mixture richer....
Yup, I stand corrected. Started celebrating New Year's a wee bit early... hiccup ...

Cheers!
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Ya I accept defeat. I cant possibly see how to get a screwdriver in such a tight position to adjust anyway. The scoot does idle well> Its just a little hard to start after I have been riding a little. I do shut off the fuel valve whenever the engine is turned off. Whenever I turn the fuel valve back on it takes me about 10 kicks to start her up. I pulled out the plug and noticed that the plug is wet.
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I used a small 6mm ignition box end wrench to move this mixture screw. Bending it near the box end would make it really easy to use. I did notice that the center part of the mixture screw with a very shallow line on it does not seem to move as the outer 6mm hex part is turned? Seems odd. Does anyone know if this mixture screw a two piece part? I did get crisp off idle throttle response. Had a cool slightly faster Stella before but now I really like my classy 2005 Vespa PX150 even more !
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edoinfl wrote:
Ya I accept defeat. I cant possibly see how to get a screwdriver in such a tight position to adjust anyway.
A couple thoughts at the bottom of the page...

https://modernvespa.com/forum/post1548957#1548957


THANKS G!!! vvvv It worked!
⚠️ Last edited by Vader19 on UTC; edited 2 times
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Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and '66 Blue Badge Smallframe
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Vader19 wrote:
edoinfl wrote:
Ya I accept defeat. I cant possibly see how to get a screwdriver in such a tight position to adjust anyway.
A couple thoughts at the bottom of the page...

Random tricks, share your best little tips & shortcuts
Just a little fyi Vader... if you copy the link from the little box at the left of the day & date of the post (look directly above the writing) you can link directly to the post.

https://modernvespa.com/forum/post1548957#1548957
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Useful input, Thanks! One thing that has led me to a lot of confusion is regarding the screw that adjusts the fuel and air mixture. Many previous posts indicate that by turning the screw clockwise will increase air, hence leaning the mixture. Yet many other posts indicate the opposite, hence introducing more fuel and richening carb. I seem to agree with the latter since I noticed this screw referred to as a an "air mixture" screw. Can anyone please confirm.
One more thing....The mixture screw has a rubber boot which is sleeved into the grommet on the case. Do I simply pull on the rubber boot to remove. Looks like I will need a needle nose plier to access. Any thoughts?
Thanks, Ed
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edoinfl wrote:
Useful input, Thanks! One thing that has led me to a lot of confusion is regarding the screw that adjusts the fuel and air mixture. Many previous posts indicate that by turning the screw clockwise will increase air, hence leaning the mixture. Yet many other posts indicate the opposite, hence introducing more fuel and richening carb. I seem to agree with the latter since I noticed this screw referred to as a an "air mixture" screw. Can anyone please confirm.
One more thing....The mixture screw has a rubber boot which is sleeved into the grommet on the case. Do I simply pull on the rubber boot to remove. Looks like I will need a needle nose plier to access. Any thoughts?
Thanks, Ed
on later electric start motors, you do not remove the grommet; you use a wrench to turn the screw on the back of the carb that sticks out thru the grommet.

regarding rich./lean, it honestly doesn't matter. you're overthinking it.

the procedure on adjusting that screw is to go 1/4 turn one way. wait a few seconds and blip the throttle. does it respond better or worse? if better, repeat. if worse, undo and go the other way. repeat.
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this is not an "air mixture" screw but a "fuel mixture" screw. The idle system starts around the idle jet which mixes air and fuel, then walks along the carb till its 2 outting locations in the venturi:
-The first is the one controls by the mixture screw just under slide(first 1/8 of opening)
- The second (the main one) in the slide groove.
The only way to modify air/ fuel balance is to change your jet: bigger or smaller air hole correction at top of it... Mixture screw only deals with a pre determinate gaz mix. That's why when you screw in you get a leaner mix at idling and vice versa...
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Thanks guys for the clarification.
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Molto Verboso
'61 GS150Cushman '63 GS160, '74 Primavera, '77 Rally 200,'80 P200E '05 PX150,'13 946,'64 Vyatka VP150,'77 Vyatka-Elektron,'07 GTS250, '63Tula200M
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Mix screw access MAGIC TRICK :)
Many, including myself, scratched their heads trying to find an easy way to adjust mixture screw on PX with elestart. We all saw bent wrenches, wrench heads welded to screwdrivers, etc.
I am sure I am not the only one who found another way ( however I haven't seen any reports by others . )
Everything genius is SIMPLE: remove the right cowl, of course, take the appropriate size wrench in your left hand, slide you hand under the fender , between the wheel and the metal body; the wrench will naturally land on the mix screw. I am right handed but I had to problems holding the wrench in my left hand.
⚠️ Last edited by vyatka on UTC; edited 2 times
⬆️    About 1 year elapsed    ⬇️
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air mixing screw mod
I like the idea of working your hand & wrench through from the left hand side- I'll need to try that one- I thought about that, but figured you'd need to remove the tire.

Anyway- what a friend / tech does is he takes the screw out, puts it in a vice, and cuts a slot in the head for a screw driver! After reinstallation, he then uses a long narrow screw driver to reach down and make the adjustments.
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Molto Verboso
'61 GS150Cushman '63 GS160, '74 Primavera, '77 Rally 200,'80 P200E '05 PX150,'13 946,'64 Vyatka VP150,'77 Vyatka-Elektron,'07 GTS250, '63Tula200M
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UTC quote
Re: air mixing screw mod
stgeorge wrote:
I like the idea of working your hand & wrench through from the left hand side- I'll need to try that one- I thought about that, but figured you'd need to remove the tire.

Anyway- what a friend / tech does is he takes the screw out, puts it in a vice, and cuts a slot in the head for a screw driver! After reinstallation, he then uses a long narrow screw driver to reach down and make the adjustments.
I just corrected my mistake: remove the right cowl' of course. Removing the wheel would make the job easier but I found that it is possible to do the job without removing the wheel. My hands are not small, I did the job nevertheless .
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