Lets talk atomizers
Post Reply    Forum -> Not-So-Modern 12Next
Author Message
Ossessionato
ET2, PX150
Joined: 15 Jun 2007
Posts: 3410
Location: Denver CO
Wed Jun 19, 2013 7:14 pm quote
Oh I've wrapped my head around the details of jets and what they do in the stack but I look at my B5E and have no clue what that means. Can some one toss out an explanation?
Ossessionato
1970 Vespa Rally 180
Joined: 04 Dec 2008
Posts: 4144
Location: Denton Tx.
Wed Jun 19, 2013 7:24 pm quote
Post Re: SI carb tuning info
swiped from Stella speed which was swiped from elsewhere:

"

Si 24 mm Carb. Summary
A carburettor is simply a device for mixing fuel and air and delivering it to the engine. The Si 24 mm Carb. is a very simple carb. however it does differ to other carbs used in other scooters and motorcycles.

There are two 'circuits'. The idle or slow speed circuit and the main jet circuit. Both these two circuits overlap more so than many other carbs, which usually have three circuits ; an idle circuit, a needle jet for mid range and the main jet circuit.

Both the idle jet and the main jet on the Si feature an upper section that lets air in at a fixed rate, a lower section that lets fuel in and both this fuel and air mix together in the central mixer/atomiser tube.

The idle jet is a fixed jet. It either has to be swopped for a richer or leaner idle jet OR the mixture screw on the back of the carb. can be adjusted to control the flow of the idle mixture into the venturi.

The main jet stack can be altered in three ways, seperate air correctors, mixer/atomiser tubes and main jets are available.

The idle circuit works from idle right through to full throttle but works mainly in the 0 to 1/2 throttle range, tapering off between 1/2 throttle to full throttle but never actually stopping delivery of mixture! It's influence on the mixture between half and full throttle is not that great however. When you come off the throttle at high speed/revs, the only fuel and oil your engine is then getting will be from the idle circuit.

The main jet circuit works in the 1/3 to full throttle range. As you can see, there is significant overlap of both circuits.

The air corrector on the main jet stack and the top opening in the idle jet determine the flow of air to be mixed.
The main jet and the bottom part of the idle jet determine the amount of fuel to be mixed with the incoming air.
The mixer tube/atomiser and the middle part of the idle jet determine the amount of mixed fuel and air to go into the venturi of the carb to be burned. In the case of the idle jet, the mixture screw determines the amount of mixture from the idle jet to go to the venturi.
There is significant overlap in where in the throttle position the two mixture sources operate.
The first section describes how to adjust the slow speed or idle circuit and is followed by adjustment of the main jet circuit.

Neither should be adjusted in isolation because of their overlap.

Tuning the standard Si 24 Carb. Setting the mixture screw and idle
Tools required : 7 mm small spanner, or the specially modified NH spanner (see below) or in the case of a Vortex, a flathead screwdriver.

It is too simplistic to say "Set the mixture screw on the back of the carb. to 1.5 turns from closed." In practice it depends on a whole host of factors which include the weather, state of tune etc as to what the settings will be.

The Si E 24mm carb is found on all Vespa P 200s, save for a few models in the USA where the 20mm was used for a short while. The Si G version is found on the T5. The Si G version has different jetting, a different slide profile (larger cut aways allow more air in at idle and hence it can be set for more fuel, for a 'crisper takeoff').

It has a shorter body height that allows a bigger air filter and hence a quicker throttle response! Newer versions are advertised as 26mm, they actually have a 25.5mm venturi. You'll need to match the lower carb. box that sits on the cases to take advantage of the larger venturi! A test a few years ago, Si G 25.5mm V's Si G 24mm in Scootering Magazine by Taffspeed, showed no performance enhancement on a T5. The article was biased in that:

1. The carb box lower wasn't opened out for the 25.5mm carb!

2. The jetting wasn't altered!

3. Had the 25.5mm carb. been compared to the Si E 24mm on the PX200 and not the Si G on the T5 then there would have been a difference I believe!


The idle speed screw is located to the right of the main jet stack in the above photo. You can't miss it, it's very tall and on a standard PX sticks through the top carb box and can be adjusted without taking the carb box top off!

The mixture screw on the back of the carb determines the amount of mixture from the idle jet to go to the venturi. The idle jet supplies fuel 100% of the time the engine is running, however it's contribution to the overall mixture between half and full throttle is small.




Many people have modified this mixture screw that is plainly too long to adjust easily, by cutting it in half and adding a flathead screwdriver grooved top. The Vortex Carb does just that on the left!

The other alternative is to modify the adjusting spanner!

Take a cheap, forged 7mm spanner, heat it up on either a gas stove flame or BBQ flame, holding it with a pair of pliers. When it gets hot, simply use another pair of pliers to bend it into shape, as shown below. Thanks to Nick for the NH Si Carb Adjusting Spanner!


To adjust the mixture screw hold the open end, and use the closed end coming from above, with the downwards section of the spanner pointing from above onto the fuel screw, to adjust. You will find you can adjust the fuel screw 1/4 of a turn at a time. It's much easier than a standard 7mm spanner!!!

The Vortex fuel screw is easy to adjust, it's shorter, has a flathead screwdriver thread and is easy to get to. Use a screwdriver rather than an 7 mm spanner. Also, make sure that you still use the rubber bush on the carb box where the fuel screw slots through the lower carb. box. This rubber bush is not designed to keep air or weather out, it's job is to stop the fuel screw vibrating loose! Make sure it's on!

To set the mixture screw correctly, here's what you need to do :

Start the scooter and go for a 1-2km run to warm the engine up.

1. With the engine running on the stand, take the engine side panel off. Turn the idle speed right up, the long screw with the flathead screwdriver fitting that pokes out of the carb box top, turn clockwise in.

The engine will be racing now!

2. Immediately turn the mixture screw on the back of the carb all the way in, the engine will get choppy and the idle will drop. On PX200 models it will require an 7mm spanner, there's not much room in there.

3. Immediately then turn out the mixture screw from closed in 1/4 turn increments, the idle will increase and the engine will start to smooth out. Take a few seconds wait between each 1/4 turn out. Count the number of turns as you open the fuel screw out.

4. You'll get to a point where the the engine will have smoothed out and the idle stops increasing when you turn out the mixture screw. This is close to where it should be set.

5. Adjust the idle speed back down to an acceptable running level. Then listen to your engine when you blip the throttle.

6. If the engine 'bogs' and feels flat when you blip the throttle it is probably set too rich. If the engine 'hunts' and takes more than 2 seconds to come back to a steady idle after blipping the throttle, it is probably too lean. A lean idle that 'hunts' the revs will make a 'pip, pip, pip sound.'

You should be able to blip the throttle, the engine should rev. clean and it shouldn't either bog, or 'hunt out the setting'. It should rev. and return to a good idle within 1-2 seconds.

7. Make a small adjustment here if neccessary. Then adjust the idle speed slightly.

8. On tuned Vespas, if it takes more than 4 complete turns, then pop in a richer idle jet, and repeat. This is the smaller jet on the left. The PX200 runs a standard 160/55 idle jet. The T5 runs a richer 100/50. The richness of the idle jet is the ratio of the two numbers, the lower the number, the richer the idle jet. The 100/50 is 2.0, the 160/55 is 2.9. The 100/50 is therefore richer.

If it takes less than 2 turns on a tuned Vespa, consider popping in a leaner idle jet and repeat.

On a standard PX200, with an Si E Carb. there should be no reason to change the idle jet of 160/55 unless you have done some tuning work. This could include fitting a genuine expansion chamber system where the 160/55 idle jet may be too lean. Concentrating solely on the main jet may not be good enough because of the huge overlap with the two circuits.

In colder climates you will find that the mixture screw needs to be set at more turns out, ie richer to cope with the dense cold and greater oxygenated air entering the carb. In warmer weather it will need to be set less rich as the air will be less oxygenated.

You may find that a standard PX200 will only need to be set at 1.25 turns from out in Queensland all year round but in a Tasmanian winter it may be at 1.5-1.75 turns from out. On tuned PX200s the settings will be very different and could probably be in the range of 2 to 3 turns from out, depending on the tune and conditions!

The Si 24mm carb. has two types of throttle slide. The Si E found on the PX200 has smaller cutouts compared to the slide on the Si G carb. found on the T5. The throttle slide cutouts allow air to pass through and mix with the fuel at idle. The T5 G carb. with it's larger cutaways allows more air in at idle compared to the Si E cutaway. This will also have some effect on takeoff as it will enable more air in and hence with the jetting, more fuel. This is the reason why the Si G carb has a richer idle jet than the Si E carb.

Setting the Mid Range and Main Jet
Firstly, as was stated earlier in the setting the fuel screw and idle section, there is a huge overlap between the components on the Si carb.


Main Jet Stack:

Air Corrector

This is the hole that a set amount of air comes in to mix with the fuel coming in. The larger the number, the more air is delivered. A 160 air corrector is therefore 'leaner' than a 140 because it has a larger air hole and lets more air in. Changing the air corrector will affect the mixture throughout the rev range from 1/3 throttle to full throttle!

Atomiser/Mixer Tube

This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. *** Therefore, the BE4 is 'leaner' than a BE3.***

Main Jet

This allows a set amount of fuel into the atomiser/mixer tube to exit the carb and out the spray bar into the venturi then into the case inlet. On the Si Carb. the fuel exit on the main jet is a metric size. Eg a 116 main jet has an exit hole of 1.16mm. The larger the number, the 'richer' the main jet. A main jet of 118 is therefore richer than a 116. The main jet stack has its effect from about 1/3 throttle to full open. This is a much wider range than most other carbs that have three jet circuits and not two, like the Si.

Summary

As you can see, there is a huge overlap between the two circuits that supply fuel/air to the venturi!!! Many people merely concentrate on the main jet when tuning but fail to look at the idle circuit, which overlaps. An example would be someone putting an expansion chamber on a PX200. They usually up the main jet by 2 or 3 points but fail to increase the richness of the idle circuit. Many engine seizures happen in the low to mid throttle range, I'll leave you to draw your own conclusions..........
Ossessionato
ET2, PX150
Joined: 15 Jun 2007
Posts: 3410
Location: Denver CO
Wed Jun 19, 2013 7:32 pm quote
Thumbs up and you're awesome.
Ossessionato
1970 Vespa Rally 180
Joined: 04 Dec 2008
Posts: 4144
Location: Denton Tx.
Wed Jun 19, 2013 7:44 pm quote
since I moved up to a 24 from the old 20, I have been doing a lot of research. glad to help.

I got this from Scootrally.com btw
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Thu Jun 20, 2013 12:25 am quote
spiderwebb wrote:
larger holes = leaner due to more air involved

http://www.scootercentral.net/forums/viewtopic.php?f=15&t=17999&p=182339&hilit=atomiser#p182339
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Thu Jun 20, 2013 12:28 am quote
This guy (David Vizard) is awesome. It's about Weber emulsion tubes, but it's the same thing that we call atomisers.

Banned
2:6
Joined: 11 Jan 2007
Posts: 7575
Location: San Francisco
Thu Jun 20, 2013 6:11 am quote
I really hate the SI carb! Has to be the most convoluted thing ever produced. wonder if adding a power jet or dial a jet would make it more tractable.
Lurker
Joined: 20 Jun 2013
Posts: 2

Thu Jun 20, 2013 11:52 am quote
spiderwebb wrote:
larger holes = leaner due to more air involved

The text at the top of the image is incorrect (common misunderstanding, actually) and Spiderwebb addresses this in what appears to be an edit of the post you linked to on SS
spiderwebb wrote:
Stop...
I was wrong...

this is correct about the holes in the atomizers in the stock si carbs...!!
larger holes = richer = more mixture
This is also backed up by the text quoted by jamesjohn
Quote:
Atomiser/Mixer Tube

This is where air from the air corrector and fuel from the main jet mix before going to the venturi. Moving from a BE3 to a BE4 mixer tube reduces the amount of mixture from the main jet stack to be introduced into the venturi to mix with straight air from the inlet. This reduction is caused by the BE4 simply having fewer holes for the mixture to pass through. So a smaller amount of mixture goes to the venturi where it is mixed with the normal amount of air coming through the carb throat, thus producing a leaner running condition. *** Therefore, the BE4 is 'leaner' than a BE3.***
To add, the placement of the holes and their size affects the mixture in different rpm ranges. The fuel in the main jet well rises and falls. It's at it's highest with the engine off and lowest when the engine has the highest fuel demand. At low fuel demand with the fuel level in the main jet well high, the lower holes do not add much, if any, air to the mix. At high fuel demand, the fuel level in the main jet well drops below the upper holes, taking them out of play.
Moderibbit
1980 P200E - "Old Rusty", 1976 ET3 Primavera
Joined: 12 Jun 2008
Posts: 8129
Location: Atlanta, GA
Thu Jun 20, 2013 1:32 pm quote
So damn complicated. So - some P200s came with BE4 and a smaller (112 if I recall) main jet in their 24/24E carb. This is in contrast to the much more common BE3 with a 116 combo. I had always assumed that the BE4 was richer to compensate for the leaner main in that weird combination. But both were leaner? Seems like a dangerous combo - better to run a 20/20 carb where the overall air intake is smaller I would think.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Thu Jun 20, 2013 7:30 pm quote
raysorenson wrote:
spiderwebb wrote:
larger holes = leaner due to more air involved

The text at the top of the image is incorrect (common misunderstanding, actually) and Spiderwebb addresses this in what appears to be an edit of the post you linked to on SS
spiderwebb wrote:
Stop...
I was wrong...

this is correct about the holes in the atomizers in the stock si carbs...!!
larger holes = richer = more mixture
Actually after that comment he realised that he was right in the first place. This is the whole point of Vizard's little lecture, where he says:
David Vizard wrote:
Where are there are no holes in the emulsion tube, it will cause richness. Where you want to lean it out, you drill holes.
It's worth watching the video as the guy really knows his stuff... back in the 80's his book "How to modify your Mini" was easily as popular as the Sticky DVDs are.

There's also this drawn by someone else to illustrate Vizard's concept.

Lurker
Joined: 20 Jun 2013
Posts: 2

Fri Jun 21, 2013 5:22 am quote
I stand corrected. I got to reading again last night and ran across that great pic you posted with the inverted jet stack. This is from Weber's site:
Quote:
For mixture enrichment at low rpm or during slight
accelerations (tubes without orifices at top)

For mixture weakening at low rpm or during slight
accelerations (tubes with orifices at top)

Tubes with many orifices for high rpm mixture
richness reductions when air bleed jet is larger
than 2.00 mm

When mixture enrichment for slight accelerations
is needed, the fuel reserve in emulsion well must
be increased: this is obtained by fitting a tube
having small outside diameter, orifices located
prevailingly in the lower portion and a larger size
air bleed jet to prevent excessive mixture richness
at high rpm
Lurker
Joined: 21 Jun 2013
Posts: 2

Sat Jun 22, 2013 8:49 am quote
You have no idea how wrong you are.... for a start an emulsion tube is full of petrol. A si mixer tube isnt.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Sat Jun 22, 2013 2:50 pm quote
nwolcuoy wrote:
You have no idea how wrong you are.... for a start an emulsion tube is full of petrol. A si mixer tube isnt.
Thanks for joining up to Modern Vespa.


The SI carb. In this image below (thanks pdxjim) you can see the atomiser (that's No.4, "Emulsionatore"). It's between the main jet supplying petrol, and the air corrector, supplying air. The mixer (atomiser or emulsion tube) mixes the air and petrol together.



The Weber emulsion tube.
Quote:
5) Emulsion tube
Its task is to emulsify the previously metered air issued from the bleed jet with the fuel coming from the main jet.
http://www.webercarburatori.com/?p=handbook&s=2
Quote:
Main jet, Emulsion Tube, Air correction jet Assembly.
This assembly screws into a fuel well having three delivery points.
1) Bottom - inlet hole through which the main jet draws fuel from the float chamber.
2) Top - Air inlet through which the air correction jet supplies air the emulsion tube.
3) Side - mixed or emulsified fuel and air outlet to the secondary or auxiliary venturi.
www.lotus-europa.com/newsletters/lotus_west/00FU038.PDF
Quote:
Emulsion tube, so what the hell does it do? The easy answer is that it mixes air with fuel as it enters the aux venturi and is sucked into the engine.
http://www.rx7club.com/1st-generation-specific-1979-1985-18/weber-emulsion-tubes-805950/
Quote:
The main jet is stuck into the bottom of the emulsion tube and sits in the fuel on a tapered seat. As the carburetor begins to work, the main jet meters the amount of fuel allowed to pass through it and up into the emulsion tube. Air enters the top of the emulsion tube through the air corrector which meters the amount of air to be mixed with the fuel coming in from the main jet at the bottom. The air and fuel are mixed, and this blows out of the emulsion tube through a series of holes along its length, and this aerated mixture is sucked up the emulsion tube "well" and into the delivery nozzle as the velocity (and suction) in the carburetor increases.
http://jiminglese.com/weber5_005.htm
Lurker
Joined: 21 Jun 2013
Posts: 2

Sat Jun 22, 2013 9:47 pm quote
Ok if your sure.

An emulsion tube is full of petrol, si mixer isnt.
That statement should be enough.

Heres another.
The atmosphere has no linnear effect on petrol in an emulsion tube, the si mixer exploits this effect to work.

But if your sure. I know they may look the same, but they behave differently.

Google abit more.

Note
Ive been very simple re atmospheric flow in relation to emulsion tube, there is influence at the venturi, within the tube its more turbulence than influence.

Fact
A be3 mixer is rich at higher rpm, lean at lower rpm
A be4 is lean at higher rpm, but rich at lower rpm.
All the mixer holes are exposed to the effects of the venturi all the time. There is no petrol inside the mixer tube.
Explain how this fact relates?
Lurker
Stella 2t
Joined: 13 May 2014
Posts: 2
Location: Athens, Ga.
Tue May 13, 2014 4:43 pm quote
I was wondering if with all the variables is it possible to have multiple setups to get proper jetting. Like using a smaller air corrector with a smaller main jet or a bigger air corrector with a larger main jet. If so what is the benefit of either setup?
Addicted
Joined: 01 Jan 2012
Posts: 629

Tue May 13, 2014 6:56 pm quote
thank the lord I am now only using modern carbs

those stock carbs have a learning curve as do the modern carbs.

I sure love my new carb...!! It seems more strait forward to me even though it has way more jetting overlaps compared to the old SI's or SH's.

33mm KEIHIN
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Tue May 13, 2014 7:03 pm quote
Are they any easier/harder to set up?
Mr. Clean
P,SUPER,V90, 50 Special
Joined: 27 Jul 2010
Posts: 10207
Location: This is't my locker!
Tue May 13, 2014 7:13 pm quote
spiderwebb wrote:
33mm KEIHIN
That's what I'm running in the small frame.. Large frames have 35 Air Strikers.. they're super easy to tune...

... in my O.
Lurker
Stella 2t
Joined: 13 May 2014
Posts: 2
Location: Athens, Ga.
Wed May 14, 2014 8:49 am quote
That's a good looking carb. I kind of enjoy all the difficulty of the SI carb, until it blows up on you. So I posted above about the possibility of multiple set ups. I'm trying to dial my wife's bike in. It is a Stella, stock bike except for the addition of a SIP road 2.0, TDR reeds and a 24/24 carb. Jetting I have in is a 50/160 for the slow idle, and a 160/BE3/122 main stack. I run it at WOT for a fair amount and it's still lean. However I do get four strocking in the lower revs witch means it's rich in that area I believe. SO I was wondering if it would be crazy to change the main stack to a 140 air corrector and take the main down as well or should I still be looking at the atomizer still? A BE1 or BE2?
Hooked
86' Vespa PX 200, 59' Lambretta LD 150
Joined: 23 Mar 2014
Posts: 108
Location: Brazil
Mon Mar 09, 2015 3:10 pm quote
Any news?
Addicted
Rally 200, SS 180, ET3
Joined: 21 Feb 2010
Posts: 683
Location: UK
Mon Oct 05, 2015 1:29 pm quote
And news just in....
This may end up long winded but hopefully you get the jist of what im attempting to understand and explain.



The rough sketch above is with the motor not running, using a BE2 mixer/mixer tube/atomizer/whatever you like to call it, as an example.

Fuel level is at the top of the lower mixer hole 'A', nearest the main jet.
At the same level is the top of the horizontal passageway that leads to the idle jet. As fuel is demanded, the level drops, which also uncovers hole 'A'

Holes 'A' and 'B', in the bottom half of the mixer, are common in all 5 x common mixers (omitting the BE6 for now).

From low to WOT rpm, the bottom half of the mixer, with the 2 x lower holes, as shown ^^^, are where the fuel and air are mixed together and 'atomized', partially at least.
This carefully ratio-ed mix will always be sucked into the venturi regardless of whether there are holes in the top half ('C' and 'D') or not.

Having more/larger holes in the top half will always add more air to the mix and will mainly affect the pick up/responsiveness at low to mid+ rpm. This is regulated by the air corrector and the size of these upper holes amongst other things.

Holes in the top half will also make WOT leaner.

No holes in the top (BE4 for eg) - less air - richer

So you can alter the crossover period responsiveness/take off from low to mid rpm by swapping just the mixer/atomizer. You can also swap the idle jet to achieve similar results at idle-low rpm. Slides can also alter this. Slides are lot more expensive and more of pain to swap when compared to jets and mixers. But each does its own job.
To make this low to mid+ rpm range leaner, you can use a mixer with more/larger holes in the top half.

Someone mentioned earlier that the mixer is not sitting in fuel. (well, the lower most holes do until fuel is demanded) but this is correct. So there can only be air there, yeh? So its going to be leaner.


So, consider the top 2 x holes 'C' and 'D', nearest the air corrector, at low revs, when the main stack starts to cut in.
The horizontal passageway into the venturi from the mainstack is right next to these upper holes.
It stands to reason that with more/larger holes at the top of the mixer, nearest the source of the air intake, the air corrector jet, then at low to mid+ rpm, the mix will have more air - leaner.

Omit the 2 x upper holes all together for a moment, eg, a BE4.
Fuel and air will still be 'atomized' by the 2 x lower holes of the mixer tube but this time with the BE4 there are no holes to add any extra air to the mix before it enters the venturi.


There are guidelines for the 'perfect' ratio of fuel and air for various rpm.
Simply put, the fuel:air weight ratio goes around 1:9 from idling, gradually up to around 1:15 for mid range/upto 3/4 throttle
Then to around 1:13 for WOT

I'm sure someone else can put that into much simplier terms than I can.

With exception to the BE6, the order of lean to rich mixers, in low to mid range and to a certain degree including WOT, is as follows;
BE2 leanest, then 3, 1, 5, BE4 richest.


I've made it easy for you all to edit all/any mistakes
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5331
Location: So Cal
Mon Oct 05, 2015 2:47 pm quote
Great explanation ... I think you're right on as far as how the different atomizers affect low- and mid-throttle.

How they affect WOT still isn't clear to me though.

According to this chart, a BE2 atomizer, which has two top holes and seemingly allows the most air in, is richer at WOT than the BE4, which has no top holes.

http://modernvespa.com/forum/post1830774#1830774
Mr. Clean
P,SUPER,V90, 50 Special
Joined: 27 Jul 2010
Posts: 10207
Location: This is't my locker!
Mon Oct 05, 2015 3:49 pm quote
I really don't even use them now, (not to say I won't again in the future) but what bugs me the most about these f'in atomizers is why they just couldn't go BE1, BE2, BE3, BE4, BE5 Leaner to Richer.. what's the sense in not just going in order? If there's some complicated answer Baaahhh.. they should just go up in number lean to rich.
Addicted
Rally 200, SS 180, ET3
Joined: 21 Feb 2010
Posts: 683
Location: UK
Mon Oct 05, 2015 3:59 pm quote
Hot air
Yeh i remember that diagram
Im not doubting it but so far i still fail to see how that its correct.

If for eg. a BE2 is rated at 7 (v rich @ WOT) because it has more/larger holes at the top
And a BE3 is rated at 6, whch has 1/2 the amount of upper holes of a BE2
Then how come the BE1, 5 and 4, which have progressively smaller holes, all have the same rating of only 4?
Yet the lower holes are all the same size?

Using the BE4 as an example
With more revs a greater vacuum is created, sucking even more fuel from the bottom of the main stack, through the main jet.
The only air that can mix with that fuel is coming via the AC and out through the bottom 2 holes of the atomizer.
Thats it, until it hits the venturi.
Thats as rich as you can mix isn't it?

Or is it??

With a BE2 for eg. the air being fed via the AC is also directly feeding the venturi with unmixed air

Damn, now i have more questions than answers....
Does this mean that there is less air going to the atomizer, therefore making the atomised mix richer?

How many times does the fuel and air get mixed? - Once, in the bottom half of the atomizer, thats for sure. But then what exactly?
Do the upper holes mix more air with the mix?
And then when it reaches the venturi it mixes with even more air?

Or is it that all this extra air (mostly coming outta my arse atm) but actually coming from the top holes, is effectively 'leaking' into the venturi, so giving less air to the mix in the bottom half, making that intial mix richer?!

Is that it?!


I'm still mulling it all over in my small noggin'.....
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5331
Location: So Cal
Mon Oct 05, 2015 4:35 pm quote
I hear ya. It's like the freaking DaVinci Code ... maybe the numbers make sense if you hold em up to a mirror or something.. who knows...

Just based on trial and error - and I've tried a BE1, 2, 3, 4 and 5 - I think atomizers have much less affect on WOT than on low- and mid-throttle. At least I couldn't tell a lot of difference between them at full throttle.

In low- and mid-throttle, your ranking of lean to rich is exactly the same as the chart. It's what I've noticed too, just trying the different sizes in the scoot and going for a test romp.
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1826

Mon Oct 05, 2015 6:28 pm quote
I remember reading somewhere, either in German or Italian, that the reason the mixers have a crazy numbering scheme is because they weren't designed all at once but instead as the need arose. For example, first there was #1, then another called #2, then another sort of in between 1 and 2 called #3, and on it went, or something along those lines.

What's confusing everybody is the fact that any given mixer is a specific combination of lean/rich effects at different throttle positions. I've seen charts that describe those effects, but I still have difficulty grasping it and I suspect it requires a lot of experience to get the hang of. If you prowl around air-cooled VW forums you'll find discussions of Weber carb mixer tubes making people crazy the same way.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Mon Oct 05, 2015 6:30 pm quote
Fantastic post Bholinath! Thanks so much for putting in the time to do it.

Notice I said post and not "posts" because it was so clear in my mind until your subsequent post!
I feel like you can probably ignore the bit about pulling more air than fuel and where it's actually mixed and concentrate on the effects.

I did wonder about the spacer you buy to create more volume in the float bowl. This would have the effect of lifting the 'normal' level of fuel in the tube and possibly throwing the 'calibration' out. Maybe.
Addicted
Rally 200, SS 180, ET3
Joined: 21 Feb 2010
Posts: 683
Location: UK
Tue Oct 06, 2015 12:32 am quote
Richie Rich
Yeh! I was quite pleased with the first post! Kinda made sense.
Thats what i should have stuck to!

Then i started thinking too much about how it could possibly be reversed..ie, more holes at top = richer.

The numbers dont mean anything to me except as a way of I.D.

I also find it very difficult to notice the subtle differences between the mixers at WOT

Low - mid range however is more noticable and can be 'tuned' via the mixer

I would never use those float bowl spacers due to, like you say, they alter the level of fuel and would no doubt alter the characteristics of the mixer, especially at low rpm take off, where i think it would be slightly richer, maybe be even hestitant? as the lower most hole would be covered for longer, not allowing the same amount of air into the mix.

I better leave it there before i talk myself into another confusing theory

If only i had the time, money and a science lab
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 7705
Location: Victoria, Australia
Tue Oct 06, 2015 1:58 am quote
Re: Richie Rich
bholinath wrote:
I would never use those float bowl spacers due to, like you say, they alter the level of fuel and would no doubt alter the characteristics of the mixer, especially at low rpm take off, where i think it would be slightly richer, maybe be even hestitant? as the lower most hole would be covered for longer, not allowing the same amount of air into the mix.
Hmmm. Might take mine out and see what difference it makes!
Molto Verboso
Joined: 15 Nov 2008
Posts: 1896
Location: Seattle
Tue Oct 06, 2015 8:12 am quote
Just wanted to throw this out there, as the atomizer is one of the most commonly misunderstood bits in the SI.

One significant function it serves is to move the transition point where the carb goes from the idle circut to the main circut. They are not strictly 'leaner' or 'richer', but more actg as a modifier that act on the other jets in the carb, similar to the taper of the needle in a more conventional carb.
Addicted
Rally 200, SS 180, ET3
Joined: 21 Feb 2010
Posts: 683
Location: UK
Tue Oct 06, 2015 10:58 am quote
Air brakes
Now its slowly coming back to me! Thanks rob

Its more about how the air, fed from the AC and into the tube and out of the holes, acts as a brake on the fuel supply from float bowl.

Generally, having more holes in the tube would limit more fuel from entering the venturi, hence making the end mix leaner.

Having holes in the top of the tube will bring on earlier 'braking' of the fuel supply.

Having less holes would not be so restrictive on the fuel flow from the float bowl and therefore will result in a richer final mix

Float height/fuel level and at what stage this level drops, which uncovers more holes, could have a big affect on take off/ the switch between the idle and main circuits.... Such as making the switch earlier or later.
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5331
Location: So Cal
Tue Oct 06, 2015 2:31 pm quote
The brake analogy is a good one... as the throttle opens and fuel starts rising up in the main well, air that's drawn out of the atomizer holes gently pushes back against it, thus keeping too much fuel from entering the venturi. The air essentially acts like a buffer, moderating the flow of fuel during acceleration.
Molto Verboso
Joined: 15 Nov 2008
Posts: 1896
Location: Seattle
Tue Oct 06, 2015 6:49 pm quote
a much better analogy is trying to drink soda pop with a straw that has a crack in it.

ever done that? suck a little, and you get mostly air. suck hard enough and you get mostly soda.

if you have a slightly bigger or smaller crack, the amount of soda and air that gets pulled thru for the same suction varies, but given full suck, you'll still get pretty much all soda.

now, in this analogy, you are the venturi, the soda is gasoline, and the crack is the holes in the atomizer.

the reason this works is because of the different amounts that the air and the fuel flow in response to the pressure difference. if you tease out alot of engineering regarding hole size and placement, you can affect this curve in all sorts of ways, and that is what the holes in the atomizer are doing. it's changing the fuel/air going into the venturi in the mid range of suction.

granted, the soda straw is a very, very simplified analogy. you also have the air corrector limiting the total air available to the atomizer and the main jet limiting the fuel, and there are effects that placement of holes and relative sizes have... but you get the idea.
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1476
Location: UK (South East)
Thu Oct 08, 2015 9:43 am quote
To try and understand this, I currently have a 55/160 idle and 160/BE3/132 in my 24/24e (Cosa top, 5.2mm needle 2.0mm drilled passage). I have been battling with pinking for some time (Pinasco 225 + BigBox). It occurs at 60/65+ MPH but I'm only at 1/2 to 3/4 throttle (mid range).

I have a BE4 atomiser. Will I benefit from using it? Sounds like this will richen the mid range, but weaken the low end. Did I get that right? Should I therefore go richer on the idle eg. 52/140?

Cheers.
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4429
Location: San Diego, CA
Thu Oct 08, 2015 11:43 am quote
If you follow the chart, you can't just look at the rich/lean numbers for each region of throttle. You also must consider that BE3 has surface area almost twice that of BE4. So overall it will be richer, but as you noted the idle will be leaner relative to the mid and high throttle. In other words - you may need a richer idle but you'll still have to try it .
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5331
Location: So Cal
Thu Oct 08, 2015 1:05 pm quote
Before you start messing with jets I'd make sure it's a lean fuel condition that's causing the pinking, and not bad timing, compression or squish.

One of the great things about SI carbs is that they have a built-in lean tester - the choke! It isn't really a choke, but a circuit for manually adding extra fuel to the carb. It works at any throttle position.

So run the scoot up to where it starts pinking and feed it a little more fuel via the choke. If that eliminates the pinking, you've at least narrowed down the issue.
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4429
Location: San Diego, CA
Thu Oct 08, 2015 4:27 pm quote
sdjohn wrote:
If you follow the chart, you can't just look at the rich/lean numbers for each region of throttle. You also must consider that BE3 has surface area almost twice that of BE4. So overall it will be richer, but as you noted the idle will be leaner relative to the mid and high throttle. In other words - you may need a richer idle but you'll still have to try it .
be4 will be richer due to less area, to be clear
bodgemaster
1963 GL, 1976 Super (x 2), 1974 Primavera (x 2), 2006 Fly 150
Joined: 26 Sep 2013
Posts: 5331
Location: So Cal
Thu Oct 08, 2015 5:40 pm quote
Hey John, does it make any sense to you how the BE2 - the atomizer with the most air holes - can be "super rich" at WOT? Or should we assume the chart is just wrong about that?
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1826

Thu Oct 08, 2015 6:47 pm quote
The way I understand it, referring to one mixer as "leaner" or "richer" than another is waaay too general a terminology since the size, number, and position of holes effects mixture at different throttle positions.

Just looking at the photos, I would say that the BE2 has...
1) More vent area than other tubes at the top end = leaner at low RPMs, and
2) Same vent area as many other tubes at the lower end = not much change at high RPMs.

Both charts below corroborate point 1), but why they would say that high RPMs are richer is beyond me.

emulsifiers 3.jpg

emulsifiers 2.jpg

Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1476
Location: UK (South East)
Thu Oct 08, 2015 11:17 pm quote
@SoCalGuy Think I tried everything else over the past year and still cannot get rid of the pinking. It was suggested that I raise my exhaust port by 2mm, but this is hit or miss with a nicasil lined cylinder.

Fuel mix is (I think) my last resort. I have tried going bigger on the main jet (up to 140) but I never seem to need more than 3/4 throttle, so the main jet size may be a red herring. The atomiser is something I have never considered through lack of real understanding.

With my gearing, 65 MPH = just under 6000 RPM, so it's by no means screaming. Fuel starvation should not be an issue as I have a fast flow tap and carb (BGM). I am just wondering how I can richen the mix at this point in the progression.
Land of 10,000 Scoots Rally   Vespa Wasp Pin Badges   Scooter Parts Company
Post Reply    Forum -> Not-So-Modern 12Next
[ Time: 0.1940s ][ Queries: 27 (0.0564s) ][ Debug on ]