Re: *Why* aftermarket air filters (UniPods) affect fuel mixtures in CV carbs.

[cm08]

No, no, no! It's not about added air volume causing the lean condition, it's all about the *lower* air velocity thru the carbs, as David Raforth reminds us:

I wonder with more air flow in will it increase my fuel usage... I thought more air would mean it runs leaner but its seems that it sucks in more fuel to compensate?

Popular misconception about air flow (my favorite subject).  The ONLY time those filters are "flowing more air" is at Wide Open Throttle (WOT).  At EVERY other throttle setting (99.99999% of typical riding), the throttle plate is controlling the amount of air entering the engine, NOT the filter.

Better example.  At 50mph, the motor needs a certain amount of air (and fuel) to maintain that speed.  This is solely dependent on the throttle plate angle and has absolutley NOTHING to do with what kind of air filter is installed.  The stock filter is plenty capable of flowing more than enough air for all throttle conditions up to WOT.

What those filters will do, however, is make the mixture lean.  This is NOT because they are flowing more air!  The pod filters breathe air "easier" than the stock filter/airbox arrangement.  As such, they are less restrictive to air flow.  The easier flowing air actually flows more slowly through the carb throat.  Try this demonstration.  Open your mouth wide and inhale.  Next close your lips like you're going to whistle and inhale.  Notice the difference?  When it is harder to intake air (more restriction) the air must speed up to compensate.

By installing the pod filters, you slowed the intake air speed down.  In a CV carb, it's all about air VELOCITY rather than the AMOUNT of air.  A lower velocity will cause the slide to move to a lower position for a given throttle angle.  The net result is the jet needle is now sitting in a LEANER position.  VELOCITY is the reason why you're now lean, the "more airflow" capability of the air filter has nothing to do with it.

DavidR.

Hey wow thanks David...

I am very curious about fuel consumption

I am a little curious about exactly what the mixture screws do inside the carb as well...
if the adjustment screws are the pilot air adjustments (changing the rate of air available in the mix), if I now alter the amount of air available in the carbs (restricting air to richen the mixture) doesnt that then kind of do the opposite of what I am trying to do by adding filter pods?

I understand about the "speed" of the air flowing into the carbs and slowing it down by using the pod filters... I undertsand the reasoning but not sure now what the effect in the carb / engine of slower moving air..

Since here in Brisbane the weather can get a rather hot, I am concerned now about running too lean and thus the engine gettign too hot, the FJ already gets hot in summer as it is...

Im beginning to get a little paranoid about running the bike too lean and thus too hot, blowing out my fuel consumption, and messing with adjustment screws that are said to be notoriously easy to damage... and the bike runs ok already...

Carb tuning is a very dependent activity.  I cannot provide exact answers since it depends on equipment installed, state of other tuning variables, location, etc.  I can hopefully provide information to help you understand what's going on so you can make better choices.

In "theory" the pod filters should not affect fuel mileage.  At everything less than WOT, the engine is breathing the same amount of air as it did with the OEM airbox/filter.  However, since the air velocity has decreased, the jet needle  is now lean and needs to be shimmed higher to "recover" the original air/fuel mixture setting.  Thus fuel mileage (and performance!) "should" not change.

Where the pods "could" help performance is at WOT.  For this condition, the main jet should be bumped up at least one size (maybe more, depends).  A bigger main jet should not affect mileage at less than WOT throttle settings since the needle is controlling the majority of fuel flow (thus shimming the needle is WAY more important than changing a main jet!).  

So, changing to the pod filters should not kill your mileage.  What typically kills it is greedy jetting when people think more fuel = more performance so they grab the largest jets in the kit and install those.

If you spend lots of time on the highway and the FJ is a commuter bike, then go for an 18/38 sprocket combo.  Otherwise, just watch the angle of your right wrist as it has far more influence on mileage than jetting.

For reference, I'm running stock exhaust, airbox with UNI replacement filter and a DJ kit with 110 mains, 144 air pilot, and needle on groove 1.5.  I typically get high 40s mileage but I don't bother tracking it since that's the price of fun.

DavidR

Very good explanation.  I am personally very happy with the standard airbox and filter arrangement except for when I need to remove carbs which should be rare. That's when pod filters would be practical. But never installed because I really do not want to rejet.

Based on your explanation one should be able to install cleanable/reusable pod filters and restrict them, somehow, to increase the velocity back to oem spec.and without rejecting carbs.

Does this make sense and if so has this been attempted?

[oldktmdude]

Very good explanation.  I am personally very happy with the standard airbox and filter arrangement except for when I need to remove carbs which should be rare. That's when pod filters would be practical. But never installed because I really do not want to rejet.

Based on your explanation one should be able to install cleanable/reusable pod filters and restrict them, somehow, to increase the velocity back to oem spec.and without rejecting carbs.

Does this make sense and if so has this been attempted?

It would be alot easier and quicker just to re-jet your carbs than trying to get the correct amount of restriction required to make your standard jets to work at peak performance. Many people on this forum have already re-jetted to suit unipods so it's only a matter of asking which jets are suitable for your application. Elevation, type of exhaust system, type of riding and fuel octane ratings are a few of the variables that need to be taken into consideration when choosing alternate jets. And in any case you want to increase you air flow, not restrict it. That's my opinion anyway.   Pete.   

[fintip]

I have a friend that has been meaning to get an article to me (so I can read the specifics) but he tells me some big names, including Mr. Yoshimura himself, got together to properly tune a bike with pods and an exhaust, and discovered that maximum performance was actually attained by using a K&N filter and the stock airbox, to everyone's surprise.

I also read somewhere here (need to find it!) that pods decrease your midrange power, but increase your top end.

All of this is after jetting, of course.

That all leaves me wondering the same question as the PO. Sounds like what would be ideal would be a variable restriction system! Ironically, this is what the slides on the CV carbs are supposed to be, essentially, which then makes me feel like I am going in circles a little bit.

Still sorting all the info out myself, though. Discussion is welcome, I will glean away.

[racerrad8]

The biggest reason the airbox can make good power is the use of the rubber velocity stacks within the box.

If you could mount a velocity stack with a filter attached it would be a way to get better power.

The velocity stack could also be used as a tuning tool, the length will effect the power band range.

I am not saying that the air box is restrictive, but it does not provide a smooth flow of air to the carbs. The UNI pods allow the air to be drawn from any direction.

I would be interested in the reading of the mentioned article as well. I wonder if it was a unbiased test using all types of filters, O.E. paper, Fram, K&N, UNI, EMGO or if it was a article written specifically using only the K&N filter?

Now, in regard to jetting, Yamaha had to jet the US versions of the FJ with a leaner fuel mixture due to emissions controls. If you look at the other countries jetting they were all set up with a richer mixture. Even with an airbox, the US-FJ can benefit from being enriched.

I have found the fuel mileage will stay the same of actually improve slightly as the engine is now running more efficiently and you are now using less twist of the grip to maintain a speed.

I can tell you this, when Frank (Flynt) had his bike being tuned on the dyno, they said his bike ran the best without any filter on it. They wanted to sell him K&N filters, "because they are better than the UNI's". After they tested the K&N filters compared to the UNI's they found no difference. The Biggest effect on the engine without the air filter was at the top end, above 9500 RPM and well north of 100 mph.

The lower ranges were not affected by the filters as much as the WOT, top end pulls.

Ultimately, when you mount a filter directly to the carb, the biggest issue you should be concerned about is the filtering quality before the performance.

I can tell you of all of the world championships, national & state titles, my sons or myself or any of my customers have won were achieved with the UNI dual pod foam filter, they beat the K&N filtered engines every time. Those are the only two choices of air filter that can be used in the series. Since the performance it not an issue, the filtering is the biggest concern.

As I have said many times on this forum, as well as many others. I build close to 100 sets of carbs annually. I have three sets here that have arrived since last Wednesday. I can tell you what filters the carbs are using just by the cleanliness of the carbs. UNI win every time, hands down.

Randy - RPM

[fintip]

Oh, I didn't meant to do a K&N vs. Uni comparison, I think they just used K&N when the article was written as a high performance filter, and were basically saying that a good filter and the airbox gave higher performance numbers than pods.

I'll get ahold of that article and get back to you on specifics.

When you say velocity stacks, are you referring to the length of rubber pipe? Couldn't one mount the pods on lengths of rubber pipe?

Also, what jetting is an ideal bump for just a stock system (I'll be putting an uni filter in my stock airbox soon, but one doesn't really need to jet for that I guess, so it should be the same number I presume?).

I am not saying that the air box is restrictive, but it does not provide a smooth flow of air to the carbs. The UNI pods allow the air to be drawn from any direction.

I had always heard the opposite; shouldn't the carbs provide more turbulent airflow since it doesn't have to 'straighten/smoothen out' through the length of the system via an airbox? I had always thought air turbulence was one of the factors that made pods problematic for CV carbs.

Educate me!

Interesting that flynt's bike ran best without any filters; what conclusion can we derive from that about restriction?

I wonder what the origins of the 'air box' are? Who came up with this idea? Was the purpose to make it easier to just replace one filter instead of several, or was it engineered for performance? Probably the former, no?

[fj11.5]

I think airboxes were origanly fitted to keep noise levels down, filters dry,  stop water getting into carbs and make tuning easier

[Arnie]

Airboxes do a number of things.  They cut noise, keep filters dry, and they tune the resonances of the incoming air so that you don't have the "holes" in the hp/torque dyno curve.
Most engines have a dip or hole in their response at certain rpms.  A properly engineered airbox can minimize or eliminate this drop off in response.
The FJ is no exception.  When I removed my airbox and substituted the UNI dualpods I could feel a slight lessening of engine response in the mid-range, but the FJ has HEAPS of power so it was an acceptable (to me) tradeoff for the high end (over 6K rpm) power and the new intake honk.

An "IDEAL" jetting requires testing of individual engines, but if the only change is the intake system - from airbox with filter to dualpods, then raising the needle 1 groove (1mm) and upping the mains by 1 or 2 steps should be very close.

Arnie

[danv93]

So if UniPods decrease the velocity of air going into the carbs, couldn't you modify them to increase the velocity? I tried putting rubber washers inside mine to decrease the hole size and therefore increase the air velocity. I don't bog down in the mid range anymore. My only concern is that at higher speeds it may not be getting enough air and overheat the engine? Anyone think the washers are a bad or good idea? Thoughts? Advice?

[Pat Conlon]

Bad idea.
Setting aside the ease of carb maintence issue (no airbox to r/r) UniPods shine in the decreased restriction of air in the upper rpms...the kookaloo zone. Washers, or any other type of air restrictions would defeat this benefit.
In the low to mid range rpm zone, there is no appreciable performance difference with UniPods. There is a potential of your engine running leaner from the lower air velocity (lower slides) but you can compensate for that by running #40 pilots (which you should anyway) and shimming your needle 1/2 step.
You can go up 1 or 2 steps on your main jets if you also have a aftermarket (more efficient) header system.

Jets are easy, you only have to do it once....On the other hand....

Removing and replacing that oem air box for carb Maintence is hard. If you plan on keeping your FJ and you wish to do the work yourself, (no stupid $$$ dealer) with the eathnol in our fuel, it will be a matter of time until you will need to clean your carbs.

[JPaganel]

Removing and replacing that oem air box for carb Maintence is hard. If you plan on keeping your FJ and you wish to do the work yourself, (no stupid $$$ dealer) with the eathnol in our fuel, it will be a matter of time until you will need to clean your carbs.

What is it that you guys are doing that makes it hard? I've had a half-dozen other bikes and the FJ1200 airbox is the easiest I have dealt with by far. Takes me about 15 minutes to pull it, including some dawdling. Given that you don't need to pull it all that often (once a season, if that), ease of maintenance is about the silliest reason to switch to pods.

It looks cool and makes you feel like a racer, that's fine. Those are as valid a reason as any. Maybe it gets you some top end power gains you never use due to speed limits. That's fine too.  But the claim that there is some huge difference in ease of maintenance just makes me laugh. There is absolutely nothing hard about removing the FJ airbox, unless you are talking FJ600, and on the 600 you don't need to remove it to pull the carbs.

[Bones]

Yeah, mightn't be that hard if you know the easy way by undoing the subframe and lowering the frame, but I bet you wouldn't be laughing at the extra labor costs if for some reason a mechanic that didn't know the easy way had to remove and refit the standard air box compared to the unipods. Better airflow, cool induction noise, plus more room to fit "extras" where the airbox was, pluses all round in my book.

[X-Ray]

Mmmm, what's my reason for changing to pods? Have a look here and look at 3rd photo down
http://www.fjowners.com/index.php?topic=10458.msg101434#msg101434

Have never liked that upside down filter arrangement.  

[JPaganel]

I bet you wouldn't be laughing at the extra labor costs if for some reason a mechanic that didn't know the easy way had to remove and refit the standard air box compared to the unipods.

Who is this "mechanic" you speak of? 

Better airflow

Didn't we already establish at the top of this thread that "The ONLY time those filters are "flowing more air" is at Wide Open Throttle (WOT).  At EVERY other throttle setting (99.99999% of typical riding), the throttle plate is controlling the amount of air entering the engine, NOT the filter."?

more room to fit "extras" where the airbox was

I've heard this one a bunch too. What are you stuffing in there?

[Bones]

I bet you wouldn't be laughing at the extra labor costs if for some reason a mechanic that didn't know the easy way had to remove and refit the standard air box compared to the unipods.

Who is this "mechanic" you speak of?  

Believe me, their out there. I had a mate that was one.

Better airflow

Didn't we already establish at the top of this thread that "The ONLY time those filters are "flowing more air" is at Wide Open Throttle (WOT).  At EVERY other throttle setting (99.99999% of typical riding), the throttle plate is controlling the amount of air entering the engine, NOT the filter."?

I didn't mention a specific throttle setting, I just said better airflow

more room to fit "extras" where the airbox was

I've heard this one a bunch too. What are you stuffing in there?

Personally, nothing yet, but there's enough room there to let your imagination run wild.

[MOTOMYSZOR]

Nobody mentioned vacuum / underpressure in inlet system.....

"sport" air filters are less restrictive for air flow - well know fact.
That mean when engine sucks in air, it is easier to do that.

With OEM filter while air is sucked in to engine, MORE vacuum is created in air box.
More restrictive air filter (or dirty) mean more vacuum. So engine must use more energy to take in enough air in to cylinder and because air is "thin" cylinder will be not full in 100%.
And vacuum works in carburettors as well. More petrol will be suck out from jets due to difference in air pressure.
After installation "sport" air filter we have less vacuum in air box. Engine loose less energy, can take more air with every cycle, air velocity is reduced and that mean better cylinder filling.
Vacuum is the reason why we need adjust carburetors after fit new filter. No extra fuel will be sucked out from jets - we got leaner mixture.

Also that is the reason why many "normal" users describe engine work as ,,smooth" after filter replacement.
Quick change in throttle position don't create big difference in vacuum in air box. Those changes due easier airflow are smoother..