Lets talk atomizers
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Molto Verboso
Black 2007 PX200, Dark green 1986 PX225 Pinasco, "1972"(yeah rite) Tangerine px200, several TRIUMPH TIGRESS SCOOTERS
Joined: 16 Jul 2010
Posts: 1082
Location: New Zealand
Fri Oct 09, 2015 12:07 am quote
id be going on retarding the timing to reduce pinking then advancing it from there to get the bang for buck etc
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: 7706
Location: Victoria, Australia
Fri Oct 09, 2015 12:16 am quote
swa45 wrote:
The atomiser is something I have never considered through lack of real understanding.
I'm glad you have it sorted out now!

I'm still seeing half say the atomiser holes make it richer and the other half the opposite.
Molto Verboso
One or two fun scoots....nothing too precious
Joined: 17 Jul 2013
Posts: 1476
Location: UK (South East)
Fri Oct 09, 2015 1:35 am quote
@Trumpy I am using a Kytonik on curve 7 with stator at 24 degrees. Therefore I am (in theory) back to 18 degrees round about 6000 RPM. I have tried more aggressive curves which bring it back to 16 and even 14 degrees at 6000+ RPM. Still pinks.

I am convinced that fuel is the problem, rather than timing. This atomiser thread has got me thinking.
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4430
Location: San Diego, CA
Fri Oct 09, 2015 5:03 am quote
@socal I think what comes into play is that the main jet size matters, and some of these are probably intended for bigger mains. Also the air jet size definitely matters and some of these are probably intended for different air jet sizes.

I may be off base but I'm still taking it as overall hole area determines overall rich/lean level, and the numbers in the chart show the relative rich/lean for low/med/high throttle/revs within that atomizer.

Disclaimer: I am not an expert on vespa atomizers. School me if my thoughts are incorrect. But I did think the chart makes sense if interpreted in this fashion.
Rally 200, SS 180, ET3
Joined: 21 Feb 2010
Posts: 683
Location: UK
Fri Oct 09, 2015 11:43 am quote
I maybe thinking too much but i don't 'quite' agree with those charts. I mean they can be a little misleading

Again, to make it easier to compare, omit the BE6

At low rpm the different atomizers each have a very different effect
Hole size and position of the hole determines what effect that has,,,whether it makes it leaner/richer/switches idle and main circuits earlier or later
The idle jet will affect this range too to a certain degree

At mid range its not so pronounced but the size of the holes will still have an affect. More holes = more air stopping the fuel reaching the venturi. So leaner

Imagine there was just one hole in the bottom of the mixer. At WOT the mix going into the venturi would still be sucking all the fuel it could, being regulated by the size of the main jet. Having less holes in the atomizer would mean there would be less air to stop the fuel from flowing.,so more fuel would flow. So it would be richer

At full rpm the main thing you need to concentrate on is the main jet and AC

The pretty good ' sucking through a straw' analogy posted earlier is probably the best/simpliest way of looking at it i think.

Swa, i didnt realise you still had pinking problems Damn!
You have a BE4? stick that in. It will be richer
Along with the biggest MJ you can use
Premium petrol and 2% quality oil
Try another brand of fuel?
Ive had mixed reviews of Shell 97 recently and try to use Esso wherever possible

Do check your fuel line from the tank hasnt got any bubbles in it.
Lean ya scoot way over to the left for 10 seconds to make sure.

Does it pink on a full tank of gas?

Changing your atomizer to a be4, will alter where you circuits change and you may get the stutters where you did not before, so you may have to tweak the idle.

Basically id go rich rich rich as you can on everything to see if that cures it.
BE4, you could even try a 140AC too but that will no doubt alter everything so youll have a bit of tweaking to do.
Richen the idle too, why not!? Even its only undoing the mix screw a tad

Keep the head well torqued down

On my 224, i had two very similar heads made (modded px200)
There is less than 1cc difference in terms of combustionable volume
Head No.1 has a squish band 0.5cc larger than head No.2
Head No.2 has a C.C. bowl of 18cc
Head No.1 CC bowl is 16.5cc
The squish gaps are within 0.1mm of each other

Head no.2 pinks (in around the same rpm as yours) and runs slightly hotter!
Both heads are very similar to the one you are using now i believe but the slight difference is enough it seems!

How about the head itself? Is it properly centred on the barrel?
I made some cylinder stud sleeves out of some copper sheet to centre my head. Pretty easy to do. They last longer than the pen top lids i used to use!!

If youre running the kytronic i would have thought it would have made a considerable difference!?
How about trying stock ignition @ 18 and ditch the kytronic. Just for fun.
'81 and '80 p200e
Joined: 14 Aug 2013
Posts: 78
Location: California
Sat Oct 24, 2015 4:10 pm quote
I wanted to try some alternate jets after reading this discussion. I pulled apart my jet stack into 3 parts and noticed something. The stack is not hollow. My BE3 is sealed at the bottom. I assumed that the fuel is drawn through the main jet (130 for a new malossi) into the mixer tube before exiting the stack. But now it would seem that the fuel exits the main jet holes and the air exits the mixer tube and they don't blend until outside the stack.

Is this what was meant before that the si jet is not an emulsion tube?
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1827

Sat Oct 24, 2015 4:20 pm quote
That is normal. The mixer tube only calibrates where and how much air is "leaked" into the fuel well. The fuel jet is separate.
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1827

Sun Sep 17, 2017 5:15 pm quote
I know this thread is getting a little old, but something keeps bugging me that I'm hoping someone can eventually explain to me.

I understand how increasing the holes in the upper part of an atomizer will lean out the lower end of the throttle, but what I don't understand is how this richens the upper end of the throttle? I've seen this mentioned a couple of times and don't get that at all since the holes in the lower end haven't been changed and overall there's even more hole area than before. How does that make anything richer?
Molto Verboso
2005 PX150 In a Love-Hate Relationship with a 2-Stroke Vespa Since 2007
Joined: 01 Sep 2007
Posts: 1827

Sun Sep 17, 2017 5:38 pm quote
I just found an answer to my own question...

"Holes up high on the tube cause a leaner mixture at all engine speeds and the more holes there are the further up the RPM range the weakening will reach."

Old douglas ..smallie with polini 115..super with nasco 177
Joined: 07 Nov 2013
Posts: 606
Location: New Zealand
Sun Sep 17, 2017 6:02 pm quote
throttle slide effects mid range too
...you can also alter the mid range ratio by swapping out the throttle slide. Check out the SIP site where they explain how the cutaway section on the base of the new slide is bigger or smaller depending on the need.

ie on the underside of the throttle slide is a small area that has been cut away. By altering the size of this cutaway..the mix is altered. SIP sell a few different slides to suit different tunes (god bless em)

swa45..I had a very similar prob to yours with Pinasco tune that was solved by changing BOTH the atomiser ( to richer) and also the throttle slide to richer

Game changer
2007 Stella 150
Joined: 02 Nov 2019
Posts: 761
Location: Rochester, Minnesota
Thu Jul 30, 2020 9:04 am quote
oopsclunkthud wrote:
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.
Did you ever try adding a power jet?
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