pod filters and jets

[SlowOldGuy]

My opinion, and everybody has one... is that because you'll have more air entering across the board, that is throughout the whole rpm range, then you'll have to compensate by adding more fuel across the whole range as well -

I've ranted on this too many times to count.  The air filter DOES NOT control the amount of air the motor gets!  The THROTTLE PLATE does. 

The type of air filter, or airbox, or no airbox changes the restriction to airflow through the carbs and, thus, changes the VELOCITY of the airflow.  Velocity is what's important to a CV carb, not the amount of air.  Filters clean and restrict air, they don'ty control the amount, unless your filter is totally clogged up.

DavidR.

[Dads_FJ]

My opinion, and everybody has one... is that because you'll have more air entering across the board, that is throughout the whole rpm range, then you'll have to compensate by adding more fuel across the whole range as well -

I've ranted on this too many times to count.  The air filter DOES NOT control the amount of air the motor gets!  The THROTTLE PLATE does. 

The type of air filter, or airbox, or no airbox changes the restriction to airflow through the carbs and, thus, changes the VELOCITY of the airflow.  Velocity is what's important to a CV carb, not the amount of air.  Filters clean and restrict air, they don'ty control the amount, unless your filter is totally clogged up.

DavidR.

Would you agree that running an engine sans air filter, be it with CV carb or anything else, runs leaner? 

[Pat Conlon]

Hello John, yes the bike will run leaner but with the CV carbs it's due to the slower air velocity thru the carbs. The slower air velocity results in the vacuum slide not lifting as high as it would with a higher velocity. Attached to the slide is the jet needle,  therefore the needle is in a lower position in the jet nozzle resulting in a leaner mixture. That's why we suggest shimming the needle to a higher position.
80/90 percent of our street riding is on the pilot jet and needle, only at WFO are we on the main jet.

A good primer on the subject can be found in the Files section under carbs.

Take a deep breath David. There is a reason we saved your discussion in the Files.

John, it took me years to figure out what David was saying.....

[andyb]

...slower air velocity thru the carbs...

Higher air velocity = lower pressure, if memory serves.

The harder part to consider is that the volume of air entering and exiting a motor doesn't change with throttle position nor with load, only with rpm.  The density of that air is what changes, and that's what matters.  Air's damned compressable stuff!

David knows this bit also, the tuned airbox function, which at some resonant frequency will cause the motor to be slightly supercharged and flow more air than typical, but that's not the filter's fault, it's the airbox.  Moral of the story, you don't need to know exactly how everything works at a physics level if you can see cause and effect of jet changes.

And while i'm nit-picking.  The main jet does have some influence over fueling at any time that the motor's getting fuel from someplace other than the pilots.  If the main jet is sized appropriately, it won't be the restriction so it won't matter too hugely though.  It'll mostly be important when you're at WOT and above about 6krpm.  WOT at 2k won't give you a good idea on the mains effect, as it's minimal down there.

[Dads_FJ]

It's all good, I'm just trying to get a better understanding of the how's and why's, sort of educate myself... I don't like spewing misconseptions anymore than I like hearing them.  I'd read up in the files section.  Now what chain lube is best?

[racerrad8]

...Now what chain lube is best?

Did somebody say Chain Lube?

Randy - RPM

[SlowOldGuy]

Sorry, didn't mean to come across so negatively.

K&N's "flow more air" advertising is mostly responsible for all the misconception.  Yes, their filters are capable of flowing more air (at the significant expense of not being a very good filter, but they don't mention that).  What they don't tell you is that engines don't need more flow from the air filter.  Unless the air filter itself is the choke point on airflow, it doesn't matter how much air it CAN flow if the engine is not asking/needing more air.  Usually the ultimate restriction to air flow is the carb size, but the only happens at WOT approaching redline (a place not visited often unless you're a drag racer).

If you take the air filter off and the engine speeds up, it's because the mixture leaned out, not because it's suddenly getting more air.  It's getting the same amount of air because the throttle plate is controlling airflow, not the filter.

Most auto carbs work on the venturi (velocity) effect also so when you remove the air filter on your car, it will lean out because the air filter restriction is now gone and the air flow has slowed down and is not pulling in as much fuel (lean!).

I could go on (and on, and on), but I'll stop for now.  :-)

DavidR.

[moonrunnah]

If you take the air filter off and the engine speeds up, it's because the mixture leaned out, not because it's suddenly getting more air.  It's getting the same amount of air because the throttle plate is controlling airflow, not the filter.

im not trying to be an ass but for my knowledge: how would it lean out if the filters do not control the flow.
i get that the throttle plates limit the air so what does it matter how much is air is getting to the throttle plates 

[Pat Conlon]

No worries Matt, that's what we are here for....

You know that our BS36 CV(Constant Velocity)Mikuni carbs meter fuel to the motor based the various vacuum levels induced by the opening or closing of the throttle plates, right?

In other words, the opening and closing of the throttle plates do not mechanically add or subtract fuel.
The opening and closing of the throttle plates affect the air velocity (or speed of the air) and as the air whistles thru the carb it produces a vacuum which raises and lowers the slides which actually meter the fuel.
High speed air produces high vacuum (slide raised= more fuel) Low speed air produces low vacuum (slide lower=less fuel)

Unipods are less restrictive. (I think we all get that)
1) Less restrictive means that the air flow (velocity) is slower thru the carbs. (this point was hard for me to get)
2) Slower air flow results in lower vacuum levels.
3) Lower vacuum levels result in a lower slide position (which is run on vacuum)
4) Lower slide position results in a lower needle position and thus a leaner mixture.
http://www.fjowners.com/index.php?topic=4714.0

[FJmonkey]

No worries Matt, that's what we are here for....

You know that our BS36 CV(Constant Velocity)Mikuni carbs meter fuel to the motor based the various vacuum levels induced by the opening or closing of the throttle plates, right?

In other words, the opening and closing of the throttle plates do not mechanically add or subtract fuel.
The opening and closing of the throttle plates affect the air velocity (or speed of the air) and as the air whistles thru the carb it produces a vacuum which raises and lowers the slides which actually meter the fuel.
High speed air produces high vacuum (slide raised= more fuel) Low speed air produces low vacuum (slide lower=less fuel)

Unipods are less restrictive. (I think we all get that)
1) Less restrictive means that the air flow (velocity) is slower thru the carbs. (this point was hard for me to get)
2) Slower air flow results in lower vacuum levels.
3) Lower vacuum levels result in a lower slide position (which is run on vacuum)
4) Lower slide position results in a lower needle position and thus a leaner mixture.
http://www.fjowners.com/index.php?topic=4714.0

Pat, I think that was well put, even a 5th grader should understand that.

[Pat Conlon]

Thanks Mark, I 'think' I described it right.

[dcole]

With all this said, would a cheap trick to make your bike ride better while riding through high allevation be to remove your air filter?  It may not help with idle or start up, but if you could get your bike running in theory this may help while driving, right? 

[SlowOldGuy]

The key to the CV carb is the vacuum controlled slide.  It responds to the velocity (and vacuum) produced by the air flowing through the throat of the carb.  CV stands for CONSTANT VELOCITY, and that's what the slide seeks to maintain.  At a steady state throttle position, the slide is held at a position that results in a certain air flow velocity.  During acceleration, as the throttle (and thus the throttle plate) is increased, more air is allowed to flow through the carb. 

For more air to flow, it has to speed up.  When it speeds up, it creates a higher vacuum which initially picks up more fuel (kind of like an acceleration pump on a Holley).  The slide diaphragm also sees this increase in vacuum and reacts by pulling the slide further up, exposing more throat area to the air flow.  The larger throat area causes the air flow to slow back down to the steady state (i.e. constant) value.  The slide tries to keep the air flow velocity through the carb at a constant value under all conditions. 

This is where Andy's reference to RPM comes in.  Pinning the throttle allows the maximum air flow for a given RPM.  Because you can't instantly accelerate from 1000 RPM to redline (at least not while riding, not counting missed shafts), the engine doesn't need a bunch of fuel when you stomp on it.  As the engine RPM increases, more air is drawn through the intake, and the slide rises to maintain the constant velocity throught he carb.  As the slide comes up, the needle exposes more of the needle jet and allows a proportional increase in the amount of fuel pulled into the flow.


If you do anything to an intake component which disturbs the speed of the air, the carb needs to be adjusted to compensate.  The best example is air filters.  A "less restrictive" filter will allow air to flow "easier" which results in a lower velocity.  The slide reacts to a lower velocity by dropping to a lower position which reduces the carb throat area and thus speeds the flow back up to the desired "constant" value.  You're now too lean because the jet needle is lower in the needle jet and not flowing as much fuel for the given amount of air passing through the carb.

davidR.

[andyb]

The key to the CV carb is the vacuum controlled slide.  It responds to the velocity (and vacuum) produced by the air flowing through the throat of the carb. 

In truth, it's relative vacuum, not absolute vacuum, so it's really the pressure differential.  Doesn't really matter unless you're setting up a blow-through turbo system though.

[Dads_FJ]

The key to the CV carb is the vacuum controlled slide.  It responds to the velocity (and vacuum) produced by the air flowing through the throat of the carb.  CV stands for CONSTANT VELOCITY, and that's what the slide seeks to maintain.  At a steady state throttle position, the slide is held at a position that results in a certain air flow velocity.  During acceleration, as the throttle (and thus the throttle plate) is increased, more air is allowed to flow through the carb. 

For more air to flow, it has to speed up.  When it speeds up, it creates a higher vacuum which initially picks up more fuel (kind of like an acceleration pump on a Holley).  The slide diaphragm also sees this increase in vacuum and reacts by pulling the slide further up, exposing more throat area to the air flow.  The larger throat area causes the air flow to slow back down to the steady state (i.e. constant) value.  The slide tries to keep the air flow velocity through the carb at a constant value under all conditions. 

This is where Andy's reference to RPM comes in.  Pinning the throttle allows the maximum air flow for a given RPM.  Because you can't instantly accelerate from 1000 RPM to redline (at least not while riding, not counting missed shafts), the engine doesn't need a bunch of fuel when you stomp on it.  As the engine RPM increases, more air is drawn through the intake, and the slide rises to maintain the constant velocity throught he carb.  As the slide comes up, the needle exposes more of the needle jet and allows a proportional increase in the amount of fuel pulled into the flow.


If you do anything to an intake component which disturbs the speed of the air, the carb needs to be adjusted to compensate.  The best example is air filters.  A "less restrictive" filter will allow air to flow "easier" which results in a lower velocity.  The slide reacts to a lower velocity by dropping to a lower position which reduces the carb throat area and thus speeds the flow back up to the desired "constant" value.  You're now too lean because the jet needle is lower in the needle jet and not flowing as much fuel for the given amount of air passing through the carb.

davidR.

This I actually get... but say we we're looking at a non-CV carb.  Does removing the air filter also slow down the velocity of the air creating a lean condition?  Or does it just induce dirt and wear out your rings faster?!