installing Uni Pod filters - assistance please

[Tengu]

Well, got my 2 twin pods from Randy at RPM racing... no hassles with Randy or his service at all

I oiled the filters - messy but ok
pulled all the stuff off the FJ including the airbox - I hate doing that - only thing I hate worse is putting it all back on!

now to the problem
one pod went on fine - to the two left hand carbs
however on the right hand carbs I am now nothing but screaming, cursing, pulling my hair out frustrated after trying everything I can think of... the boots just wont tighten up without pushing off the carbs

when I tighten the circle clamps around the pod boots they just push off... I cant get them even close to tight before they push off the carbs
if I get them as tight as possible just before they do.. then they come off with less than finger pressure anyways (and will certainly come off from the vibration of the bike)
I know that I first tried to get the other pod (the one now on the left carbs) onto the right but they did the same thing (popped off as soon as tightened). The pod now on the left is tight and on and wont come off with any reasonable pressure - its fine

so please help... why does the pod keep popping off the carbs?
nothing is visible wrong with the carbs or the pod

[Tengu]

Well I tried some advice from another local group and really cleaned up the seats and the boots

It got the filter on tight enough that it wasnt just popping off the seat at a sneeze or a dirty look...

I will take it for a quick spin in the morning and keep my eye on it

It was quite odd to hear the carbs working, sucking in the air and fuel...

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?
I had 20 kms worth of fuel left in the tank and after a minute of running with the pod filters the fuel light lit up!

[andyb]

In theory you'll use more fuel because you'll have to jet to compensate, but you'll also use less throttle to maintain the same speed.  Depends more on the angle of your wrist than the tune in the bike, and with the addictive sound of the intake and more power available...

[SlowOldGuy]

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 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.

[E Double]

David,

Cheers for the very good explanation about air flow and the pod filters.  I currently am running the stock air box and filter but have been contemplating putting the pods on and rejetting the carbs to match.  I currently have the V&H Supersport exhaust and stage 2 jetting. The only thing I worry about is taking a big hit on my MPG.  I unfortunately have to hit the slab more often than not, and as of right now I'm getting between 32-34mpg on the slab.  This is at 75-90mph (New Jersey traffic is either standing still or trying to win a race) with either my Givi side cases on or just my top case.  In your experience, have you seen a big MPG hit, or is it negligible (less than 1mpg) or even better did you see improvements in mileage?  Cheers for the info.

[rktmanfj]

David,

Cheers for the very good explanation about air flow and the pod filters.

+1

The more David talks about carbs, the less magical they seem.   

[Tengu]

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...

[SlowOldGuy]

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.

[Tengu]

Wow... carbs are obviously way above my pay grade at this point...

Original airbox back in place... carbs NOT fiddled with...

Lesson learned... and I must say I have learned helluva lot in 2 days

years ago I took apart my carbs on my kwaka GPZ750... cleaned em up and stuff... and that worked fine... but was way easier to get to than the FJ and I recall it was messy and really needed a workshop space...

this project is officially now in the "one day" category!
when I have workshop space and the time (and another bike) to play with the rejetting etc (or the $ to pay someone else to do it for me but where's the fun in that!)

thanks everyone for the assistance...

[WestOzXJR]

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..

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...

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...

I ran pods on mine in Perth over the summer so often saw temps of 40 degrees C +... In traffic too as a commuter as well as open highway.

The engine will be fine.. Just make sure the heat shield between engine and carbs is in good nick so the carbs are drawing the coolest air possible given the conditions.

And if you change to pods focus on getting the jetting right to suit this and other mods... Manbe you've got 4 into 1 exhaust as well?

I found the mixture screws needed to be turned out to the point they became insensitive so required to go up one pilot jet size from the standard 37.5. I drilled mine out .002" which equates to about size #40 which meant the screws came back into the lower end of adjustment range. If I'd got a bit fussier probably would have opened out the air bleed to the pilot circuit too and I now understand why some jet kits come with a larger air pilot. (to halve the effect of one size larger fuel pilot).

Although I'm not suggesting you SHOULD put pods on, if you decide to do it then as jet changes/working on the carbs becomes so much easier therefore you're more inclined to actually make the effort to tune the carbs...

You won't be disappointed by fuel consumption, as Andy pointed out it's all about you're right wrist... I got up to 21kms per liter with pods, 4 into 1 and 5 degrees ignition advance... So no complaints about that aspect.

Just one more thing about pods... I've never seen back to back dyno testing/sheets for FJ1200 air box vs pods... BUT for every other engine I have seen such testing results for, pods gain in the top end WOT but at an approximately proportional expense of mid rpm range torque WOT and this is a function of the momentum of air mass contained within air box/plenum volume.

[SlowOldGuy]

Whoa!  Don't let me discourage you.  Pods are not a bad thing, they make getting the carbs on and off infinitely easier.  Just don't expect a massive performance gain and they will require some carb tweaking.  Shimming the needles should get you 95% of what you need.  

I have a set of pods that I'll probably install the next time my fuel petcock quits working.  When that happens I'll finish the electric solenoid shutoff that I'm investigating.

Oz, that's a very good point about the midrange loss.  I would assume the higher velocity air flow would provide a slight overfilling of intake charge that you could lose with the pods.

DavidR.

[E Double]

Yeah, the midrange loss would kill my MPG right there, as I would be lighting more of the wick every time I wanted to hustle a little bit.  The midrange is one of my favorite things about this bike;  It means I don't have go all WFO on the throttle every time I need to move out a little.  Eh, I'm just gonna keep the airbox on there and put in a replacement uni airbox filter come the late fall.  Its running like a top right now, and I don't want to upset whatever balance I've achieved.  Besides, it has 78K miles on it and I have no means to add a second bike to my stable anytime soon. This joker's gotta last for quite a while, and I'm not easy on it in the least.

[WestOzXJR]

Oz, that's a very good point about the midrange loss.  I would assume the higher velocity air flow would provide a slight overfilling of intake charge that you could lose with the pods.

DavidR.

That's my take on it too David... That the volume of air contained in the air-box has mass and as it's constantly moving through also has momentum, and so at the point in the induction phase where the intake valve is about to close and the cylinder is no longer actually sucking (because the piston has just begun it's upwards travel), that mass of air is actually ramming, or as you point out, overfilling. Hence why ram tubes on ITB's are called "ram tubes" and are to create an additional volume/column of air to simulate this effect...

And incidentally, according to all current induction tract design theory the overall tract length on an FJ ends up a little short to be considered tuned for the engine rpm in which we operate, so suggesting a bit of extra length of ram tube would have a marked noticeable improvement of mid range torque on our engines...

[Harvy]

David, I have been enjoying your carb post for a few years now, thanks for the education.

I couple of thoughts:

The OEM airbox is supplying a cooler air charge, therefore more dense. My thoughts here......the intake "horn/s" is/are further back from the engine than pods are. Temp difference may be small,  but would still be of significance.
The volume of air that can free flow into the airbox is less because the total area of the intake horn/s is less than the combined total of 4 carb throats. So the velocity of air into the carbs must be greater with OEM airbox and intakes.

So with the Euro DJ kit, with the drill bit to enlarge the vacuum port in the slide, this is to do what? I am thinking the negative pressure will be slightly greater because of the slightly enlarged hole, and so the slide will be drawn slightly higher against the spring which is trying to seat the slide, thus allowing that little bit more fuel into the mixture.....Am I thinking this correctly, or do I have it arse about face?


Cheers
Harvy

[WestOzXJR]

Yeah, the midrange loss would kill my MPG right there, as I would be lighting more of the wick every time I wanted to hustle a little bit.  The midrange is one of my favorite things about this bike;  It means I don't have go all WFO on the throttle every time I need to move out a little.

Actually what we're talking about here is a minor loss of mid rpm WOT torque loss, not loss of part throttle throttle response... And in fact part throttle response with pods actually improves for the exact same reason... A smaller mass of air to get moving...