aftermarket air filter = less speed?

with a leadair kit this isnt an issue, but the reverse shockwave caused by the reeds closing forces mix back out through an open filter, whereas with an airbox it generates a pressure which holds more of the mix in the manifold and carb, therefore wasting less of the charge. the correct dimensions in an airbox will give more power than an open or no filter as a high percentage of that charge is being lost directly to the atmosphere, hence having to upjet to compensate.

I have looked into this in depth and all the results i've come back with have pointed in favor of the airbox.

plus the fact with the great british weather, its gonna suck in more water than air anyway.

 

I agree with you in part, the wet weather is not good with a open filter
Lucky i live in sunny surfers paradise where it rains about 10 times a year and on those days i take my car

Im not sure what you mean by leadair ??


What your talking about is called standoff and it when you get a fine mist of fuel out the intake of the carb
Its caused by the ram effect that happens when you get a coloum of air moving and then you stop it , it rams against the reeds and then the pressure of the air overcomes it momentium and blows back out the carb,rotary valve engines are the worst for it
This happens at certain speeds and it effected byh length of your inlet and rpm as well as the diameter of your intake
A restrictive filter will help stop the ram effect as it cant get the coloum moving
However there is a benefit to a still air box style as the stock airbox employs
ie less turbulent air so carb settings dont vary wiht road/air speed over the filter
, the best situation would be the intake in the helmet box so it can still flow correctly but has a large still air system to draw off

Im not interested in fuel economy its not like these things use much fuel anyway

 

the ledair is a sheild for the atomiser which only allows fuel from the mainjet to go towards the engine (leadair is a trade name).

 

All i can say is if you think that if a scooter needs the back flow like you are saying to run correctly then it goes against all the two strokes i have tuned ,jetskis , gp125 bikes , and karts
I think what you are really finding is there is less turbulent air off a still air box and this is making more stable carburation for you
I personally will stick with an open airfilter for my max power and not try to restrict it down with the stock airbox to
Every two stroke has the reverse air pulse it not just something caused by the airbox its a two stroke trait

Im going to have to google that ledair thing it sounds good
The racing carbs we use on some of the skis have that very thing built into them to stop them flowing on the back pulse it called a d-booster on the novi carbs

 

I have never really understood jetting. The stuff that has been posted in this thread is interesting to read though :nerd:

 

well here is a basic on it for you
the carb does not know what size your engine it
all it knows is pressure drop across the jet
think of the fuel bowl as running at 14.7 psi
thats atmo pressure absolute
when we speed air through a venturi the increased speed of the air drops the pressure in the ventur
so lets assume we drop the pressure in the venturi to 10psi absolute then we have a 5psi difference in pressure between the fuel bowl and the venturi
Since air want to equal out the pressure it forces fuel through the jet and into the carb

This is why when people say put a bigger jet in for a 70 on a given size carb over a 50 its flawed
The 70 will pull more through the carb and thus have a greater pressure difference between the venturi and the fuel bowl so hence more fuel flow through the same jet
The beausty of this system is that the jet can flow more or less depending on the speed of the air and is to a point self compensating
So if you give it full throttle at a low rpm( low air speed) the fuel flow through the jet is reduced to the lesser pressure difference as you increase the air speed so does the pressure difference and the jet flows more

Wow its like being back at the tech colloege we i used to teach this stuff
If you not sure about what i just worte then please ask and i will explain it differently

 

Read it all 2 times carefully out load. Didnt really make any sense to me at all :emb:

 

ok
here we go
drive down the road in a car at 50 mph
wind down the window and hold a chip packet in your hand what happens
the packet tries to head out the window , why ? because ther eis a low pressure out side and a high pressure inside the air inside is trying to force the packet out


drive at 100mph and do the same thing what happens the packet want to fuck off harder , this is because there is more pressure drop out side the car and the same pressue in the car , greater difference does this make sense

answer me and i will continue on , sorry i am not trying to sound condersending with how i am writing it just trying to get you to understand

 

Cheers for being understanding haha. Yeah that makes sense to me

 

Good stuff and its all ok , the more you understand the better the decision you can make

well think of the air outside your car as the air inside the throat of your carb
and the petrol in the fuel bowl as you in the car
as the air rushes pass it creates a low pressure in the throat of the carb it creates a low pressure
Since the fuel is sitting at atmospere pressure and the air in the throat is now at a lower pressure it forces the fuel up the jet in the same way as the air movement tried to force the chip packet out of your hand

To understand carbs correctly you have to stop thinking about the carb as a sucking device but more as a pressure device it just spends its life trying to balance pressures and the rate that it does this is determined by the size of the jet

Is this clearer , if not then i will try and explain some more

 

So based on that information then you can understand that a larger engine on the same carb can actually sometimes require a smaller main jet
Not always but sometimes ,however if you listen to waht your engine is telling you it will let you know what it wants
This is why no one that has a decent idea will give you the maic settings for your scooter , they may give you an idea of where to start but the fine tuning is up to you

 

This is what I have gathered so far.
The slower your going in the less pressure there is in the carb, and the faster your going the more there is. And bigger Jets allow more pressure in the carb?? Probably totally wrong but hey :emb:

 

the slower the air speed in the carb throat the less pressure difference between the float bowl and the throat so less fuel flow
The higher the air speed the greater the pressure difference and hence more fuel flow through the same jet size

 

So the faster the air speed the greater the pressure causing the jets let in more fuel to balance?

 

ok have alook at this picture and forget about the fact it trying to show vapour lock
but as you can see with out that blockage in that pipe the fuel bowl is vented to atmo
as the pressure in the carb drops and the pressure in the float bowl stays constant fuel has to flow
its in fact forced up the tube by the weight of the air in the float bowl
the greater the difference of pressure the more fuel can flow

 

your getting there now

 

So the greater the difference in pressure the more fuel is being used??

 

the more fuel flows yes
Used? well if your engine can drag the more air to allow the fuel to flow then yes it will use it

At this stage we are just talking carbs

its a case of getting the right size jet to flow with yourpressure drop to get the correct amount of fuel
This is why one setting cant work for all engines as even slightly different porting will have a different airspeed in the carb let along other cylinders , pipes , and different carbs

 

Flow is the word that I was looking for. Cheers for you time