Carbs w/ Pod Filters: Air Starvation Problems?

Discussion in 'The Perfect Line and Other Riding Myths' started by nicholastanguma, Jan 30, 2020.

  1. nicholastanguma

    nicholastanguma nicholastanguma Supporter

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    As far as I know the following only applies to motos without side covers or air boxes, an aesthetic theme usually found only on custom machines, not factory stuff.

    I've been told that CV carburetors equipped with pod filters suffer from air starvation when the bike starts reaching higher speeds, sometimes starting as low as 25mph, this due to the fact that the rush of air created by the bike's passage creates a vacuum behind the pod filter itself. I've heard that to combat this problem back in the day some race tuners would simply replace the pod filter with an unusually long velocity stack (the extra length creating a continually present column of air inside for the carb to drink from) or place the pod filter on a tube that extended all the way out to the frame rails (where the vacuum was not present).

    Starting at 7:05 you can hear this old school builder and racer explain in this Bike Shed video.




    My question: is this air starvation problem only present on CV carbs when equipped with pod filters because a CV "pulses" as it draws air? Or would something like a flat slide carby experience the same issue?

    Vintage, air cooled, carbed singles are my favorite engines of all time, so I pay especial attention to all their build details when I'm observing race bikes and similarly performance oriented machines of the thumper ilk--and I don't remember ever having seen one equipped with anything but a simple pod filter, even the builds from tuners who specialize in nothing but high performance thumpers.
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  2. scott123007

    scott123007 Been here awhile

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    Fuel starvation was the problem, not air.
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  3. foxtrapper

    foxtrapper Long timer

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    That's the first time I've heard the excuse of vacuum behind the engine of a moving bike.

    Pod filters did often create a lean fuel problem on bikes (not usually cars though, even with the same carburetors). Not an air starve, but a gasoline starve.

    Pod filters vary tremendously in their flowability. Good ones really act like they aren't there. Really bad ones can strangle an engine. Caveat emptor! And then there's the oiling of the filters.

    Since you've a picture of a Suzuki there, it's worth noting that way back in the day, Suzuki's in particular were always hyper sensitive to the likes of pod filters. In fact Suzuki's didn't do well with any change to the stock filtration system.
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  4. anotherguy

    anotherguy Long timer

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    Airboxes today are a simple solution to a very complex issue. Removing them from a modern motorcycle no matter how simple it may be mechanically allows the problems they eliminated to return. If you would like an understanding of it take a look here. Generally it's a bad idea to remove the airbox no matter how "uncool" it may appear.

    Vacuum behind the pods!!!??? :imaposer
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  5. Plaka

    Plaka Brevis illi vita est

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    Total bullshit. As soon as you see words like "custom" and "brat" you know you are dealing with a styling job, not a performance job. The performance will be murdered to get that cool look. Who cares? It'll make it to Starbucks and back and you'll feel good about yourself.

    Intake and exhaust systems are carefully tuned to get maximum scavenging at the proper RPMs. Mfgs. spend big bucks on this, they need to get all the power out of the motor they can to offset emissions deficits. The tracts can be re-tuned with some math and a dyno to provide a different torque curve. This is the province of competition bikes, not "custom" bikes.

    I have a funny setup on my bike. At the back of the bike the exhaust pipes end at the same place, that is they are matched. I know this is for styling, it would look funny to have staggered pipes (except on a Harley or similar). But the jugs are staggered so the exhaust tracts are different lengths. The intkes are also staggered but not as much. I thinking of matching everything.

    Pod filters flow a lot and filter poorly. Yhe OEM filter on my BMW flows a lot, filters well and costs a lot to replace. But power output is not about about flow, it's about timing.

    BTW, CV carbs do not "pulse". The entire intake tract has pulses in it resonating back and forth. The CV carb has a vacuum operated throttle and this makes it 'laggy'. Not the crisp snappy response of the slide throttle carb. Those pressure pulses are opening and closing the throttle (slightly) and this in turn works the jet needle so the gas delivery is always right for the vacuum (airflow) through the carb. So they get good mileage and are clean burning.

    If you want more performance out of the motor make it bigger or increase the compression, (for the easy stuff) do not screw with the intake and exhaust tract tuning without a whole bunch of dyno time. if you just want to give it that racer look, do anything and don't worry, it will likely still run.
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  6. nicholastanguma

    nicholastanguma nicholastanguma Supporter

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    :thumb Thanks for all your explanations, chaps! :thumb
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  7. Mattbastard

    Mattbastard Lazy ass

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    Back when I put pods and GPZ550 CV style carbs onto a CB650 I did some extensive wideband tuning. Some interesting, and some obvious things happened:

    The low speed mixture needed to be leaner. The lack of airbox restriction allowed air thru the carbs easier (duh) but on the CV carb it meant a lower vacuum was taking place which drew less fuel in with the air so typical jetting needed to lighten up. I tightened up the idle mixture screw half a turn and went down one size on the pilot.

    Everywhere else was leaner. Again, duh... My needles got a shim and the main went up about four sizes. Granted I built a high flowing 4-1 too so that helped.

    I like pods, you just need to know how to tune the bike to use them.
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  8. st3ryder

    st3ryder Been here awhile

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    I had a 1987 FJ1200 with air box removed and four K&N filter pods on side mounted, rear of the engine, behind body panels, CV carbs, and the bike ran great at all speeds. AFAIK, the "holy grail" for supplying air to naturally aspirated bikes, is to draw from a supply of "still air". AFAIK that's never achievable as air has to enter an induction zone boxed or open so there's always some movement. Seems to me there may be more still air found in bikes with induction zone behind engines and side panels than otherwise.
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  9. MATTY

    MATTY BORDER RAIDER

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    With typical slide carbs, its basically down to just working out the jetting and it is good to go on pods.
    With CV carbs its another thing altogether.
    Way a CV carb works is down to the function between the Butterfly the venturi and the slide movement, to function properly they need a air box so the vacuum is reliable and the lift rate is comparable to the requirements of the engines set up / state of tune.
    This void/ neutral air/ chamber needs to have enough air for the carbs to draw air as required at any time with just the right amount of flow & hold back restriction if you like to allow the slide movement to be at an ideal spot to let the needle jet and idle/ pilot circuits do their jobs efficiently.
    CV slide activity is dictated by spring rate diaphragm and damping and jetting .
    If you had a limitless amount of springs jets etc, and lots and lots of free time, i dare say it is possible to rejet a set of pods on any motorcycle, and get it very close to factory set up.
    But the way CV carbs work wont give you an easy time of this. and as such think long and hard before you go down this route.
    We could become embroiled an days of debate here on this brand/ type of CV carb Vs that brand, and the pros and cons of each. And some are very user friendly some not so much.
    Motorcycle manufacturers spend a lot of money calculating airbox size shape and flow, to not only make the bike ridable but meet Emissions laws in the target country they intend selling that bike in.
    Now flow is good . RIGHT? Well not necessarily so.!.
    As a rough guide you need about three times the chamber capacity of the CC of the engine to give the CV carbs a decent chance of functioning unhindered with neutral / clean/ undesturbed air. So a 250cc with CV carbs will most probably need around a 1 litre volume airbox, Shape and baffling or restrictions internal / external snorkelling, all play a part, and manufacturers test with various options for best results, to say overcome a lean area in the fuel delivery at X RPM etc.
    So where does that leave pods, Well lets say you have no airbox, your pile of parts was minus its air box, and you can not practically replicate some form of box to try to copy the factory box, for whatever reason/ can not be bothered / or no room in the butchered frame or modified parts.
    Whatever scenario, give it a go, see how it rolls, you might get lucky/ or you might not.
    Chances are it will start ok idle ok but fall flat on its face / stumble around a 1/4 / 1/3rd throttle before spluttering its way up the rev range and delivering reasonable but lean condition upper rev range situation. This is not guaranteed but its pretty typical of what you come across.
    Let me clarify, IT WONT BE PERFECT, there will be some compromise somewhere at some point in the engines performance , that is when directly compared to the stock airbox set up on any given bike.
    Changes to main jet pilot jet needle and spring rate/ length/ strength. & basic adjustments and jet swapping, might get you close and close enough so you can tolerate live with the idiosyncrasies of the pod set up. But even if you are sure you got it right, every now and then that stumble in that overtaking manouvere or even on that windy day.( No joke i have witnessed this that a GS550 suzuki got to splutter and bog down in high winds, on pods).
    It is the very way CV carbs work make them fussy, a conventional slide carb overcomes much of the airbox dependences compared to CV carbs, but thats beyond the scope of this thread to all intents and purposes. Good luck whatever you chose to do.
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  10. Tripped1

    Tripped1 Smoove, Smoove like velvet.

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    I will add I HATE Mikuni CVs. POS needle killing always clogged, and just generally annoying.
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  11. Sal Pairadice

    Sal Pairadice Captain Obvious

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    Old bikes had cable slide carbs and they did not seem to be affected very much. I cafe'd an old Suzuki GS 400 one time, did the pod filter thing and also the intake runner. It ran well until it hit 4k rpms and then it started to go lean. It was like a huge flat spot. The slide wasn't lifting as much as it did in its stock configuration, without an air box to suck against and it lost a lot of low end torque. I rejectted it, and I cut down the slide springs to get it decent and sold it. To really get it proper I would have had to " drill the slides" which means opening the air passage that draws the slides up. That seemed risky and irreversible. There was no guidance out there for doing it.Oh how I wished I hadn't removed the airbox.

    If you follow in the footsteps of other people who have done the pod thing on the exact same bike as you you can spare yourself a lot of aggravation. Like I said, I couldn't find anything on drilling those slides. Later on , I got over the cafe thing and when I picked up old bikes to tinker with I kept the airbox for all the practical reasons. And those old bikes will only gain a couple of HP anyway and never really be fast. I ddn't do a great job on my cafe project but I did sell it and get every penny I spent on it back and it was fun.

    #11
  12. nicholastanguma

    nicholastanguma nicholastanguma Supporter

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    Great explanation, thanks for taking the time to post!
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  13. anotherguy

    anotherguy Long timer

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    I jet a CV opposite of how I do a slide needle VM. Get the mian right and then pilot/needle. You will likely never get rid of the rich spot even with ample supplies and dyno time only make it tolerable. That includes playing with intake tract length. Personal experience.
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  14. Sal Pairadice

    Sal Pairadice Captain Obvious

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    Makes sense. The pilot shouldn't be any mystery but the slide is tricky.
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  15. MATTY

    MATTY BORDER RAIDER

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    If you get the fall flat at 1/4 3/3rd throttle:lol2 and more on the pilot circuit is still not getting it done, i have found drilling the slide vent hole a shade bigger can fudge a bit more to get the slide up a little quicker. It does not solve the issue but it can on some set ups get it more Liveable in nature.
    If you find yourself, in a kind of no win situation where you are struggling to get smooth all range carburation.
    Revert back to some form of airbox, if the stock plastic monster is not an option. A Alloy or mild steel box or tube can help with controlling the CV carbs function.
    My opinion on this is at least try to match somewhere near the stock airbox volume, if you practically can.
    The dyno jet idea with a soft needle a template and sandpaper is supposed to work, but no experience, and as much as i want to believe it will work as claimed, i can not help but think its going to be a compromise somewhere.
    I have returned to a home made airbox, on one bike i did a Kawasaki, and it made my life so much easier and, i did acctualy regain as far as i could tell factory airbox performance, but even then i had to fit a baffle plate with the stock foam element and rubber bellmouth to get full performance.
    My advice is think long and hard before cutting off that big old airbox, remove it carefully. And if you want a cooler looking option (perfectly understandable0 then try and make something up. and get it running the stock settings if you can on your creation rather than trying to swap out jets and re invent the wheel so to speak.
    Agree its a complicated way of doing things perhaps. but restoring performance/ economy without the stock airbox or very similar is often equally if not more complex, than basic airbox fabrication and trial and error. YOMV but its how i see it.:lol2
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  16. MATTY

    MATTY BORDER RAIDER

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    I agree with that they tend to be less than perfect, The keikhin seem to be better all round.
    Yamaha XS650s a good swap is ditch the Mikunis and Get some Keihin s off a Kawasaki 544 or Ninja 500 etc. transforms the bike, for the pice of a a pair of used carbs and a few moments time and couplings .
    I will stick up for mikunis on some other bikes though, i have at this very moment in time, a set of Mikuni flat ABS type slide CVs on my triumph 900 triple 1996, and i must admit it runs great on them but they are cheap horrible abs tops and ABS slides they look horrible, but they function superb so i supose they have some saving grace in saome applications, but in general i do agree.
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  17. Sal Pairadice

    Sal Pairadice Captain Obvious

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    That is probably a good approach for a cafe bike builder. Find some slide carbs and adapt them. I'm not sure a Suzuki GS400 is worth the effort and expense but I think as time goes by and they get more rare this may make some sense. All of the parts you get to customize these bikes easily triples what you spend on them. I was lucky to sell mine to a hipster from Brooklyn and get all my money back.
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  18. anotherguy

    anotherguy Long timer

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    Kehin FCR.........the way to go.

    DSC_0808_090tag5.JPG
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  19. MATTY

    MATTY BORDER RAIDER

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    :lol2. If pods are an absolute must for the look/ practicality. then, with CVs you might have some compromising to accept.
    Slide carbs is IMO a better option. Do not need Expensive mikunis etc, you can but round slides and Flat slides on the super cheap fro china Taiwan Russia and India and Japan.
    Pecar OKO kosso KO and others all make replicas, some are poor quality some just poor quality control, but all offer a chance at low money to get any carbless engine up and running on anybodys budget. And often at less than used Original fitment carburetors. Research the carns then the supliers as much as you practicaly cam, and then, pre fitting check everything properly.
    If your frightened by the cheap carbs, look for A set of KOs used off an early CB750 four K1 to K6 and probably others too. but these carbs might be laid about, and are a nice tunable old carb, plus you got four to go at so you got spares or you can move the spares on to perhaps finance the whole purchase. .
    I have two OKO kosos on a ER500 kawasaki engined sprint bike, it works great on these carbs.
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  20. baldman1

    baldman1 Long timer

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    Can you elaborate on the needle getting clogged.
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