Question concerning CV carbs

[rogson]

Please correct me if I'm wrong, but my understanding is that on CV carbs the slide is raised by the low pressure created by air flowing at (high velocity) underneath the slide. This pressure is transferred to the chamber above the slide by the air hole in the slide creating low pressure in the chamber which overcomes the spring load.

But air is drawn beneath the slide only during the intake stroke when the intake valve is open (once every 4 strokes). During the other strokes there is no air flowing and the pressure underneath the slide is the same as the airbox pressure (close to atmospheric). So, does the slide drop back down under the load of the spring as air bleeds into the chamber (through the transfer hole) as the pressure equalises during these strokes?

The reason I ask this is because I have heard/read drilling the air hole bigger allows the slide to move faster due to quicker transfer of the low pressure to the chamber, which improves throttle response. But if the slide is "fluttering" up-and-down, enlarging the air-hole will increase this fluttering because there will be less damping effect due to a lower restriction to air movement through the (larger) hole. The impacts of this as I can see it would be higher wear of the slide and the needle (which moves with the slide), and also a richer running condition in the midrange because the slide would move higher and draw the needle higher.

[Motu]

''Proper'' CV carbs like the SU which was practicaly the first one,have a damper,and a spring.You can use different strength springs,and adjust the viscosity of the oil in the damper.The damper is your accelerator pump - the more resistance and the less the piston (and needle) rises....thus the air speed is increased over the jet and more fuel drawn for a given amount of air.On a bike if you enlarge the hole the piston will slam to the top quicker,the air speed drops,lifting less fuel - you'll get a spit back.So if you want better throttle response you need the slide to lift slower,giving more fuel....more air by itself is no good,it has to take more fuel with it.

CV = Constant Velocity.That's air speed across the jet.I hate motorcycle CV carbs,give me a good old SU anytime.

CD = Constant Depression,is another name for them,perhaps a better descriptive name.They maintanin a constant depression over the jet,the piston rising to allow more air,the taperd needle allowing more fuel.

[Sketchy_Racer]

Not quite.

Most CV carbs have a air inlet at the top of the intake (where the air filter goes on)

Blow in it. The slide rises. Above the slide is a big round thing. Insde that there is a plastic ballon almost. When the air starts moving fast a small portion of the air is used to blow the ballon up. When the ballon inflates it pulls the slide up. The slide does not jump up and down with each stroke.

If you think that at 4000RPM it sucks air every 1000rmp it is sucking air in 16 times a second. The slide does not have time to fall back down.

its hard to explain.

pull one apart and you will see how. Also be careful drilling stuff. The little asians that think this stuff usually only put the hole a certain size other wise all the airflow dimentions can get screwed up and it might not work at all

[Motu]

Um...not quite.

That means the carb is compensated - working off air pressure in the airbox and not atmoshereic.The top of the diaphram chamber is connected to the throttle plate side of the slide,the lower part to airbox air pressure.When you open the throttle plate a negative pressure is created on the top of the diaphram,the positive pressure in the airbox pushes the slide up.It's not sucked up,it's pushed up by atmoshereic pressure...but the pressure in the air box can be different from the outside world,so airbox pressure is used.This is called compensated.

[Sketchy_Racer]

ohhhh its all clear to me now.

But how come when you blow into the air 'inlets' the slide still rises. although there is not air pressure difference ?

[Motu]

Well, when you blow,are you not raising the pressure?

[Sketchy_Racer]

Yes.

Its ok. I understand. (i think)

I get what you mean about low and high pressures now.

When air flows into the ilet of the carb it goes into the little holes which goes into the diapghram. The higher air pressure under the diapghram pushes it up. and because it is connected to the slide it also raises the slide?

Do i get it now? :spudwhat:

[Motu]

No - there is no high pressure,just atmospheric.A low pressure (vacuum to laymen) above the diaphram allows the higher presure to push it up (or it gets sucked up to those who believe in vacuum).