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ok I get the 'jet high then come down' bit but how do you work out a starting point, particularly concerning the ac and the mixer, do you start with a 160 and b3 and see where your main jet is or start with a 120 and a b4?
Just trying to work out in my head a methodical way of working with the carb.
The above may seem a bit of a stupid question but the more I read about jetting the more confused I become.
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A neat document someone linked here was the SIP carburetor tuning document or something. It's one of the free pdfs at the bottom of the webpage when you look at carb stuff. They've got the jetting of common setup listed. I'll find something close to those and upjet another 10 points and start my testing with a CHT.

I don't know if SIP lists for peak power or a little conservative for engine life, but im definitely conservative on my jetting as I have long country highways and hills with hot weather. Others will jet for peak power and short city jaunts. There can be a big difference!

https://www.sip-scootershop.com/download/article/1/pdf/a9f5954f-6fff-43fe-9682-38199a5444f4/Tuning+summary+largeframe.pdf?contentType=application-pdf
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MJRally wrote:
A neat document someone linked here was the SIP carburetor tuning document or something. It's one of the free pdfs at the bottom of the webpage when you look at carb stuff. They've got the jetting of common setup listed. I'll find something close to those and upjet another 10 points and start my testing with a CHT.

I don't know if SIP lists for peak power or a little conservative for engine life, but im definitely conservative on my jetting as I have long country highways and hills with hot weather. Others will jet for peak power and short city jaunts. There can be a big difference!
Agreed, as I'm hauling a sidecar, conservative jetting is the way to go. Ill have a look at the sip page, thanks for thr heads up.
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Andy - would be helpful if you remind us what if any modifications to the motor have been made.
Stock carb?
Cylinder?
Exhaust?

I noticed you are using a large main jet.
I'm not sure though, if you have drilled the float bowl hole.
If not - the float bowl hole will be restrictive rather than the main jet.
In other words - you can put a 150 main in - but it won't really be any different than a 125 - because the carbs hole is the restricting element.

One way to help circumvent that issue - is to use a smaller AC.
Main jet is always relative to AC.
I like to use the 120 AC - to allow smaller main jets and avoid carb restrictions controlling flow.

That said - if there is an air leak - like the fly side seal - then jetting is not going to fix.
Need the motor air tight first.
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charlieman22 wrote:
Andy - would be helpful if you remind us what if any modifications to the motor have been made.
Stock carb?
Cylinder?
Exhaust?

I noticed you are using a large main jet.
I'm not sure though, if you have drilled the float bowl hole.
If not - the float bowl hole will be restrictive rather than the main jet.
In other words - you can put a 150 main in - but it won't really be any different than a 125 - because the carbs hole is the restricting element.

One way to help circumvent that issue - is to use a smaller AC.
Main jet is always relative to AC.
I like to use the 120 AC - to allow smaller main jets and avoid carb restrictions controlling flow.

That said - if there is an air leak - like the fly side seal - then jetting is not going to fix.
Need the motor air tight first.
Hiya, not after specifics(I think my jetting was pretty close at ac160, be3, mj130, 55/160 idle in a bgm fastflow carb with fast flow tap on a vmc super g just bolted on, mated to a polini box), more a formula , if you like , so anyone could go 'this is where i start'.
For instance I'm using a 160 ac (Guy who supplied the kit helped me jet with vids and sound clips I sent him) but most on here use a 140 or even a 120 and I'd like to try and understand why.
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I was trying to explain exactly that.

In my experience - the carb is itself becomes the restricting force somewhere between 120 and 125MJ size when using a 160 AC
So that 130 you are using - is probably doing little to nothing different than a 125MJ.

The benefit of the 120AC or the 140AC - means you a smaller MJ is required.
Example:
A 130MJ might be matched to a 160AC, (IF the carb is drilled).
That same carb, with a 120AC would likely be matched to a 120MJ.

Not sure your carb size - but sounds like it's not drilled.
That means you are likely not getting what you think you are out of that 130MJ - because the carb is the limiting factor, not the MJ.

To specifically answer your question:
Quote:
most on here use a 140 or even a 120 and I'd like to try and understand why
1. Because it helps ensure that the jet stack is controlling the flow - rather than one of the carb restrictions - which there are a few of.
2. Puts you in a smaller MJ range - rather than trying to jet with 140+ MJ to compensate for the 160AC.
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Real long story short, @ 160 AC sometimes you can't address a specific part of the rev range with atomizers/jets on the market. So you can mess with the air corrector to richen/lean that issue but then that throws other things out of balance which you need to tinker with. Throw in lots of new performance cylinders, exhausts, 3 speed vs 4 speed and it's not one size fits all anymore.

Is it something you must do? Nah. We've got some real particular folks here who want to get the most out of their machines. That's why there's 80 page threads of hair pulling back and forth jetting. I say keep it simple and reliable and go ride.

If you're super bored, swap to a 140 air corrector and see how your idle and revs have changed.
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andyx wrote:
ok I get the 'jet high then come down' bit but how do you work out a starting point, particularly concerning the ac and the mixer, do you start with a 160 and b3 and see where your main jet is or start with a 120 and a b4?
Just trying to work out in my head a methodical way of working with the carb.
The above may seem a bit of a stupid question but the more I read about jetting the more confused I become.
Forgot you have a sidecar.
No such thing as an SI related stupid question.

Rough "starting point" AC (it depends, depends, depends):
160 un-drilled Piaggio filter, 140 drilled, 120 bellows only as filter.
No venturi. No filter inside the cowl.

The atomizer at WOT/high rpm is cleared out, so at the splutter point it's more MJ & AC.

Btw, what issue do you have with your current jetting?
Do you have a CHT gauge?
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charlieman22 wrote:
I was trying to explain exactly that.

In my experience - the carb is itself becomes the restricting force somewhere between 120 and 125MJ size when using a 160 AC
So that 130 you are using - is probably doing little to nothing different than a 125MJ.

The benefit of the 120AC or the 140AC - means you a smaller MJ is required.
Example:
A 130MJ might be matched to a 160AC, (IF the carb is drilled).
That same carb, with a 120AC would likely be matched to a 120MJ.

Not sure your carb size - but sounds like it's not drilled.
That means you are likely not getting what you think you are out of that 130MJ - because the carb is the limiting factor, not the MJ.

To specifically answer your question:

1. Because it helps ensure that the jet stack is controlling the flow - rather than one of the carb restrictions - which there are a few of.
2. Puts you in a smaller MJ range - rather than trying to jet with 140+ MJ to compensate for the 160AC.
https://www.bgm-tuning.com/en/product/bgm8571-vergaser-bgm-pro-faster-flow-dellorto-spaco-si24-24e-vespa-px200-typ-ohne-getrenntschmierung/

If the carb fuel supply is not the restriction, why do we all not use a 160 ac?
because if ( and its a big if), as I understand it ,the 160 flows the most air compared to the 140 or 120 why would we choose a smaller ac.Just trying to understand the science
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Ray8 wrote:
Forgot you have a sidecar.
No such thing as an SI related stupid question.

Rough "starting point" AC (it depends, depends, depends):
160 un-drilled Piaggio filter, 140 drilled, 120 bellows only as filter.
No venturi. No filter inside the cowl.

The atomizer at WOT/high rpm is cleared out, so at the splutter point it's more MJ & AC.

Btw, what issue do you have with your current jetting?
Do you have a CHT gauge?
This is what I'm trying to understand, the rules if you like surrounding the choice of ac, atomiser and mj ( and idle ) as the starting point for your/my jetting journey.
I dont think I have an issue (apart from an air leak lol) I believe my jetting was pretty good before this air leak developed.
Yep I have a CHT one of the signals that all was not well with the air tightness of the engine.
The trouble with a sidecar tug is the engine is under strain most if not all the time so jetting is even more important/ critical( if thats possible) than a solo
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MJRally wrote:
Real long story short, @ 160 AC sometimes you can't address a specific part of the rev range with atomizers/jets on the market. So you can mess with the air corrector to richen/lean that issue but then that throws other things out of balance which you need to tinker with. Throw in lots of new performance cylinders, exhausts, 3 speed vs 4 speed and it's not one size fits all anymore.

Is it something you must do? Nah. We've got some real particular folks here who want to get the most out of their machines. That's why there's 80 page threads of hair pulling back and forth jetting. I say keep it simple and reliable and go ride.

If you're super bored, swap to a 140 air corrector and see how your idle and revs have changed.
This is what I'm trying to do, MJ, because on balance I prefer riding to getting dirty and reliability is important to me. and understanding the thought process that goes into jet selection again is important thats the reason for the original question. Maybe one day give Jack or charlieman22 a run for their money ( no chance ROFL emoticon )
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andyx wrote:
This is what I'm trying to do, MJ, because on balance I prefer riding to getting dirty and reliability is important to me. and understanding the thought process that goes into jet selection again is important thats the reason for the original question. Maybe one day give Jack or charlieman22 a run for their money ( no chance ROFL emoticon )
Hats off to the folks who dive deep into the science of jetting. It's definitely not simple. I tried but lost interest.
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andyx wrote:
https://www.bgm-tuning.com/en/product/bgm8571-vergaser-bgm-pro-faster-flow-dellorto-spaco-si24-24e-vespa-px200-typ-ohne-getrenntschmierung/

If the carb fuel supply is not the restriction, why do we all not use a 160 ac?
because if ( and its a big if), as I understand it ,the 160 flows the most air compared to the 140 or 120 why would we choose a smaller ac.Just trying to understand the science
Well - the good news is - both the main known restrictions are enlarged on your carb.

So you can certainly utilize a 160, up to a point…

That point being, available jet sizes in small increments over the 130+ range, and the potential of finding other restrictions as your main jet continues to get larger.

Why not put yourself in a bracket that has jets widely available in single increments, and avoid the risk of discovering a new restriction or fuel starvation points?

In your case - your motor should likely be tunable within the 160 range from a flow standpoint (guessing) - tho you may or may not readily find the perfect mj size to match.
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andyx wrote:
I dont think I have an issue (apart from an air leak lol)
Yep I have a CHT one of the signals that all was not well with the air tightness of the engine.
Sort that out or you'll be a cat chasing its tail.
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charlieman22 wrote:
Well - the good news is - both the main known restrictions are enlarged on your carb.

So you can certainly utilize a 160, up to a point…

That point being, available jet sizes in small increments over the 130+ range, and the potential of finding other restrictions as your main jet continues to get larger.

Why not put yourself in a bracket that has jets widely available in single increments, and avoid the risk of discovering a new restriction or fuel starvation points?
In your case - your motor should likely be tunable within the 160 range from a flow standpoint (guessing) - tho you may or may not readily find the perfect mj size to match.
So the point being If I can get it to run well with the 160 then thats the best? option, if not go down to the 140 to give me more options, and avoid possible restrictions in the 'supply line'
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andyx wrote:
So the point being If I can get it to run well with the 160 then thats the best? option, if not go down to the 140 to give me more options, and avoid possible restrictions in the 'supply line'
I think your looking for a recipe like a cake, 350° for 35 minutes, in a 13 x9 pan and your done. If you change anything from factory that recipe is no longer correct. Like adding a convection oven to the mix. When using convection you can shorten cook time, turn temperature down or do both.

Jetting is the same, you may find more than one recipe that works and gets the job done.

I like other like to use a smaller air corrector to keep jets from getting to large and running out of fine tuning adjustment. Theres also keeping the rest of the passages within design limit of carburetor that wasnt designed for that large of a main jet. Last time I jetted an SI carburetor, I ran out of larger main jets and was going to be bigger than a 130, I put in a smaller air corrector. Less air meant I could use a smaller main jet and continue with main jets I already had.
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Christopher_55934 wrote:
I think your looking for a recipe like a cake, 350° for 35 minutes, in a 13 x9 pan and your done. If you change anything from factory that recipe is no longer correct. Like adding a convection oven to the mix. When using convection you can shorten cook time, turn temperature down or do both.

Jetting is the same, you may find more than one recipe that works and gets the job done.

I like other like to use a smaller air corrector to keep jets from getting to large and running out of fine tuning adjustment. Theres also keeping the rest of the passages within design limit of carburetor that wasnt designed for that large of a main jet. Last time I jetted an SI carburetor, I ran out of larger main jets and was going to be bigger than a 130, I put in a smaller air corrector. Less air meant I could use a smaller main jet and continue with main jets I already had.
I like the cooking analogy, I think you're spot on and just like cooking if you change one of he components it affects other things, I was just looking for a definitive starting point, i,e should I start with a 120 a be4 and some main jet and work down or come at it from the other direction. When Jack and others give out advice( and jet their own motors) They must go, this is the engine thats been built,so this would be a good starting point. I want to know how they know that.
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andyx wrote:
I like the cooking analogy, I think you're spot on and just like cooking if you change one of he components it affects other things, I was just looking for a definitive starting point, i,e should I start with a 120 a be4 and some main jet and work down or come at it from the other direction. When Jack and others give out advice( and jet their own motors) They must go, this is the engine thats been built,so this would be a good starting point. I want to know how they know that.
Years of trial and error I would bet. I try to look on Google and for a close setup in a forum such as this one. Go a bit richer on all the jets and start dialing it in. Can be a very frustrating experience at times.
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Quote:
I was just looking for a definitive starting point, i,e should I start with a 120 a be4 and some main jet and work down or come at it from the other direction.
Couple things.

1. The "definitive" starting point isn't a 120/BE4/something-main jet. It's what the factory decided to install in the engine when it left Pondetera with the stock cylinder, stock piston, stock exhaust, stock carb, stock air filter, etc. for maximum reliability, minimum fuel consumption and optimum drivability - after thousands of hours of testing. Any time you change one of those components you have to rethink how it's going to affect the jetting.

2. Don't think there is any "other direction" for working out your jetting. Like Greasy says, if you don't start rich, you'll be poor real soon!
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andyx wrote:
I was just looking for a definitive starting point, i,e should I start with a 120 a be4 and some main jet and work down or come at it from the other direction. When Jack and others give out advice( and jet their own motors) They must go, this is the engine thats been built,so this would be a good starting point. I want to know how they know that.
Here's a list of setups. Unfortunately there is no specific section for the VMC.
If you go through the BGM & Polini sections you'll find that most ride with the 160/BE3 combination. Click 177cc's on the top left scroll down button. http://lfs.alexander-hepp.de/
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Ray8 wrote:
Here's a list of setups. Unfortunately there is no specific section for the VMC.
If you go through the BGM & Polini sections you'll find that most ride with the 160/BE3 combination. Click 177cc's on the top left scroll down button. http://lfs.alexander-hepp.de/
Cheers for this, I'll have a read, looking forward to furthering my education.
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Christopher_55934 wrote:
Years of trial and error I would bet. I try to look on Google and for a close setup in a forum such as this one. Go a bit richer on all the jets and start dialing it in. Can be a very frustrating experience at times.
I'm finding out that setting up a side car rig can be just as frustrating, never mind the jetting😂
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Starting to get a handle on this jetting lark, having a read on this forum, amongst others sime66, it seems that trying to set up an si becomes more difficult if you go over a 130 mj so lowering the ac say from 160 to 140 brings the mj down to a better range.
Thanks for all your input folks its appreciated.
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One additional data point that I ran across this weekend is that the advantage of the larger air corrector is you get better atomization of the fuel mix when it enters the carb, thus producing more efficient burning (and power).

This is a slight change to my own way of thinking, in that it would indicate you should aim for the biggest (leanest) AC you can get the rest of the stack tuned for.

Of course, this then runs into the challenge that the SI carb was never intended for the level of performance that a modern long stroke 177 cylinder produces (nor was the rotary intake). Hence the need to do things like drill out the float bowl passage and otherwise increase the potential throughput of the carb's fuel delivery system. Yet another bottleneck in the overall Vespa motor to take into account when designing a performance build around the limitations of the basic design.

Put another way: There are reasons that modern two-strokes don't use SI carbs or rotary intakes.
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chandlerman wrote:
One additional data point that I ran across this weekend is that the advantage of the larger air corrector is you get better atomization of the fuel mix when it enters the carb, thus producing more efficient burning (and power).

This is a slight change to my own way of thinking, in that it would indicate you should aim for the biggest (leanest) AC you can get the rest of the stack tuned for.

Of course, this then runs into the challenge that the SI carb was never intended for the level of performance that a modern long stroke 177 cylinder produces (nor was the rotary intake). Hence the need to do things like drill out the float bowl passage and otherwise increase the potential throughput of the carb's fuel delivery system. Yet another bottleneck in the overall Vespa motor to take into account when designing a performance build around the limitations of the basic design.

Put another way: There are reasons that modern two-strokes don't use SI carbs or rotary intakes.
Seems that 'best practice' would be pick the largest ac that gives you a mj of 130 or under?
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Definitely all about the main jet. Takes much practice to do quickly. And without doing any engine damage.
I like my o tune 200 because it's approx 20 bhp and still runs autolube.

If performance is required and is beyond autolube, then the SI carb should be replaced.
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Jack221 wrote:
Definitely all about the main jet. Takes much practice to do quickly. And without doing any engine damage.
I like my o tune 200 because it's approx 20 bhp and still runs autolube.

If performance is required and is beyond autolube, then the SI carb should be replaced.
Jack for us noobs out here what do you consider to be the easiest carb to jet on a small block?
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andyx wrote:
Jack for us noobs out here what do you consider to be the easiest carb to jet on a small block?
I'm no Jack, but I can still tell you it's going to be a 20/20.
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andyx wrote:
Jack for us noobs out here what do you consider to be the easiest carb to jet on a small block?
The BGM you have
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Hooked
px 125 disc
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Posts: 314
Location: Essex, GB
 
Hooked
px 125 disc
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UTC quote
Ray8 wrote:
The BGM you have
Thats good to hear Ray, normally I pick the worst ROFL emoticon
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Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
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Jet Eye Master
PX221 MHR, O tuned PX200, PX125 and some motorbikes
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Location: London UK
UTC quote
andyx wrote:
Jack for us noobs out here what do you consider to be the easiest carb to jet on a small block?
It's like learning Japanese. There's difficult and increasingly difficult, up to impossible.
SI carbs have a way of making it feel like it's all good, then first long wot they hole the piston.
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Jack221 wrote:
It's like learning Japanese. There's difficult and increasingly difficult, up to impossible.
SI carbs have a way of making it feel like it's all good, then first long wot they hole the piston.
ROFL emoticon ROFL emoticon ROFL emoticon
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Molto Verboso
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Molto Verboso
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chandlerman wrote:
One additional data point that I ran across this weekend is that the advantage of the larger air corrector is you get better atomization of the fuel mix when it enters the carb, thus producing more efficient burning (and power).

This is a slight change to my own way of thinking, in that it would indicate you should aim for the biggest (leanest) AC you can get the rest of the stack tuned for.
Larger AC (with a given atomizer) delays when the circuit initiates and leans the mix.

Foaming is the atomizer (with a given AC). A BE4 (with no holes above the fuel line) delays foaming and a BE2 (bunch of holes up there) accelerates it.

I've got 2 Lemarxon atomizers stuck in Koeln, DE
They're essentially BE2's with the lower holes (XX34/X234) blocked. The idea being the air flow at WOT from the lower holes (not designed for tuned engines) impedes fuel flow through the well.
Easy enough to do with epoxy, but honestly that guy deserves the $!
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Molto Verboso
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Molto Verboso
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UTC quote
Bits arrived.
I'll try 'em out Friday.
X234 atomizer and medium slide first.
X234 atomizer and medium slide first.
22mm drop-in sleeve
22mm drop-in sleeve
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Molto Verboso
Vespa
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Molto Verboso
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Vespa
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Trying to understand better with the X234 mixer does this allow for less air mix at the 1/2 to full open range in turn a richer mix in the higher RPM range?

Is the sleeve to reduce a standard 24 to 22?

Is the slide minus some of the normal cut aways making it a richer slide in lower range?

Correct carburetion any non- stock vespa is black magic.
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Molto Verboso
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Molto Verboso
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UTC quote
hibbert wrote:
Trying to understand better with the X234 mixer does this allow for less air mix at the 1/2 to full open range in turn a richer mix in the higher RPM range?

Is the sleeve to reduce a standard 24 to 22?

Is the slide minus some of the normal cut aways making it a richer slide in lower range?

Correct carburetion any non- stock vespa is black magic.
Better fuel flow, as air from the cleared lower holes at high rmp impede the flow of fuel through the well.

Yes, the accelerator sleeve is 22mm ID.

The slide minus that corner cutout delays initiation of the progression channel. Less overlap with the spray from the mixture screw at barely open throttle.

Installed 'em today. Need a few days to play with the other brass bits.
So far I can say these guys are just wow. More power at low rpm and smooth as F.
Cutout opens and closes earlier in throttle positions
Cutout opens and closes earlier in throttle positions
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Molto Verboso
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Molto Verboso
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Still much work to do, jetting wise
Went up a steep-ass climb in 3rd (1/4 throttle) where I used to have to ride slower and buzz up in 2nd Clap emoticon

There's a post of Andreas' (aka Lemarxon) on the German scooter forum, regarding his experience jetting a bike with a sidecar using his velocity sleeve.

https://www.germanscooterforum.de/topic/454558-lemarxon-spezialteile-f%C3%BCr-24er-si-wer-hat-welche-erfahrungen/page/2/

Posts there about other's experiences as well.
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Hooked
px 125 disc
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Posts: 314
Location: Essex, GB
 
Hooked
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Location: Essex, GB
UTC quote
Ray8 wrote:
Still much work to do, jetting wise
Went up a steep-ass climb in 3rd (1/4 throttle) where I used to have to ride slower and buzz up in 2nd Clap emoticon

There's a post of Andreas' (aka Lemarxon) on the German scooter forum, regarding his experience jetting a bike with a sidecar using his velocity sleeve.

https://www.germanscooterforum.de/topic/454558-lemarxon-spezialteile-f%C3%BCr-24er-si-wer-hat-welche-erfahrungen/page/2/

Posts there about other's experiences as well.
Thank you, any sidecar related info is always very welcome.Just read the article and its very interesting( once I got used to google's literal translation), but I had to laugh at the Line,
'At full load orgies on the test bench, the influence of the slider is NOT visible.' ROFL emoticon

Reducing the carb throat size to improve torque is something I did with my Classic series 2 landrover, there was/is an after market weber carb for the Landy that had a smaller venturi that noticeably improved low down reponse and improved fuel consumption at the expense of a few mph at the top end.
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