SIMPLE MOD MAY SOLVE POD TUNING HARDSHIPS!

Ok let me try to understand this as it seems that you guys are sort of over complicating this. Im not that active of a member(at least not post wise, I Troll a lot though lol) so dont go flipping out that I said that. Im almost to the point where Im about to go out to my freezing garage to rip apart my carbs so that I can experiment with them a little.
First let me try to wrap my head around the issues with the PODS:
Pod Issue 1) Lean condition?
Pod Issue 2) Lean condition is caused by a vacuum being formed by the pod at the kidney shaped diaphragm vent?
Pod issue 3) Said lean condition is caused by the main jet needle not being able to lift high enough?

Now from what i understand running a stock airbox means that said issues do not exist. Right?

Because:
1) The air box boots are designed in a way as to act as velocity stacks?
2) No clue since in my mind a velocity stack would more then compound the problem.

Theory 1 as to why an airbox works better than pods.
In an airbox all 4 carbs are connected via the airbox. The carbs draw air through a filter with the combined surface area of maybe 2 pods thus making the airbox more restrictive(in a perfect world comparison with filter media types being the same with equal void volume and micron rating). There is however more "static volume of air"(using that term loosely at the moment because the volume is not technically static) in the airbox at a constant rpm. What does that mean to us? Inside the pod itself you probably have a few CC's of air, as to where the airbox probably has a CL if not more.

Again, what does that mean to us (bear in mind this is just my theory).

With the pods your intake charge going through each pod will be more turbulent and have (theoretically speaking) a higher velocity due to each pod on each carb having more surface area than the original factory filter. That can theoretically speaking cause a vacuum to form at the diaphragm vent. Want proof that theory is sound? Next time your taking off on an airplane in the middle of a snow storm look out your window and observe the cone of air being sucked into the turbine. I bet you that not a single snow flake will touch the inside of the ducting that leads to the turbine fan. Thats because the velocity of air moving through the ducting will cause a pressure front of sorts to form on the inside of the ducting. Think of it in a way as water going down the drain (I know its different with its own set of physics but just stick with me on it lol) This is also why it seems to me that a velocity stack would compound our "vacuum at the vent" problem more. Because even though the carb throat isnt a perfect circle the pressure front formed at the inside of the carb throat would draw air away from the diaphragm vent. Also not to mention that at low RPM you are drawing air near the bottom of the carb throat and continue to do so until mid to high RPM and the main needle plunger begins to rise.


With the air box we do not see said issue because:
As stated before the filter on the airbox has less surface area and is thus more restrictive causing a reduction in velocity. That also helps us because as you crank the throttle, the vacuum on the engine side of the carb drops because you are now getting more air through the carb and into the cylinder. But at the same time the static volume(again used loosely) is being depleted, causing a pressure drop at the back of the filter, but just as the filter begins to draw air, another carb starts sucking, thus causing a slight velocity increase, then the next carb and so on and so forth. My theory is that the vacuum on the engine side of the carbs will be higher at say 3500RPM with an airbox as opposed to running pods dude to the fact that
1) Pods are less restrictive.
2) An airbox is.

What that means to us is that there is more potential energy to force the carb slide up with an airbox due to the increased vacuum than with pods due to their less restrictive design. But how could that be Mr. Madness?! Specially with the carb throat velocity being lower and us theoretically speaking not getting as much CFM at the carb throat. Simple! The increase in vacuum at the engine side of the carb causes more vacuum at the top side of the diaphragm due to the little passage thats at the top of the carb on the inside (cant remember the technical name lol)

With that said. Theoretically speaking it is the drop in engine vacuum that causes issues with pods as opposed to anything a velocity stack can do. So then why are people with velocity stacks having awesome results and going 88 miles per hour and causing their flux capacitor to engage thus sending them back to the future?! Well my theory is less than offensive. My guess is that just as the first ape picked up a stick and wacked another ape thus sparking an arms race. The folks with velocity stacks got just as lucky. More than likely The velocity stacks cause the tail end of the above mentioned intake air charge cone to be slightly offset, or just more turbulent causing it to feed into the diaphragm vent, and offsetting the reduced vacuum on the engine side. After all, there is only so far you can take a pulsing air charge before it starts to loose velocity and the piping of the stack starts to cause a suction drop.

So. In a way I may have to agree with Rick that his idea will work. Though it was tested with a shop vac which causes a constant draw of air and not a pulsed draw. But that is not necessarily a bad thing since at high RPM the engine of our bikes has an almost constant pull of air going through each carb, and after all it is in those high RPM's that we would get in trouble with a lean condition.


With all the crap i mentioned above i have a proposition. (Rick Im sorry if this seems like a thread jack I really dont mean it as such. But reading your original post and sort of thinking out loud as i type this up gave me an idea)

Instead of capping off the vent of the diaphragm, and getting atmospheric air for it through a hole drilled in the body. Why not just get a small brass tube from a hobby shop. Fill it with sand and solder both ends shut. Shape said brass tube so that it fits in the kidney shaped hole. Angle it so that when the ends are cut one of the open ends fits snugly into the kidney shaped opening and at the same time the other open end faces the pod while being as close to the center of the carb throat as possible.

If my above mentioned theories are correct that will yield a much more responsive carb, and a much more pod/tune friendly one as well. What do you boys and girls think?

 

Ok so a lot was said as I was typing my post up. If the kidney shaped port does in fact have a double function as an intake and exhaust vent for the diaphragm( which it more then likely does). Then installing a brass tube in said port will in fact yield a more responsive carb, as the tube velocity will drop and reverse as the carb intake velocity decreases.

 

The underlying problem with pods is that they provide an unrestricted volume of zero velocity air;

and CV carbs require a FINITE volume of air at a certain velocity to work properly. The stock airbox limits the volume and rate of intake; and along with the stock boots provide the velocity.

As delivered, the bikes will run from 2K to 10K without a stumble, and just a nice boost in the powerband above 6K.

Fit pods, and the motor begins falling on its face, especially in the mid- to upper mid-range.

Thus begins the challenge to compensate for the various factors that cause this. In the long run, I agree with chacal; it's all a "system" and if you go messing with part of it, you'll upset the functionality.

Which is also true of not just the carbs, but the entire original intake/exhaust systems; mis-placed attempts to "improve" things by fiddling with one part of the system often end up not improving anything at all.

Keep in mind that the prevailing reason for fitting pods is NOT to improve performance; it's because they LOOK COOL. Then it's just a matter of how much you want to give up for appearance over function.

 

Rickey, don't give me that "God bless you" jazz, i didn't sneeze and i don't need or want your forgiveness.
if you've got a lot to say to me and others are urging you to confront me, you should, just keep in mind i speak from my experiences not third party hearsay and untested theory, or is that what Technical Writers do?
so what we have here is a personal attack on me for pointing out where your theory's and procedures are flawed, i don't think i ever personally slammed you but if that's what you've stooped to.......bring it

 

Idk big fitz. We are going to have to agree to disagree on a few points. If velocity is a function of vaccum then a decrease in vacuum will decrease velocity(unless you are running forced induction). If you decrease the amount of work the engine needs to do while drawing in its fuel/air mix then you are theoretically increasin that engines potential, efficiency, and potential power output. All that however is based on how close to stoich you can get with the a/f ratio that is. I disagree that pods will only improve apperance because you are in effect reducing the amount of work the engine has to do to "feed" its self. Also with propper tuning you can get very favorable results with any air filter/ exhaust combo. But its important to remember that you will almost never get a stock feel on anything if you are tuning for performance. If you want a stock feel stick with a stock setup. I thimg further proof of that would be one of my first bikes. A fz600 that was a 86 model year. It was bone stock when I bought it and when I sold it it was rummimg just fine with a set of pods and a kerker exhaust. Yes I had almost no power below 3500rpm, but then again yamaha never built its engines with the intention of them producing power at low rpm. That claim I feel is more then backed up by years of yamaha engines that produce gobs of power at high rpm and fall on their face at anything below 3500rpm.

 

tskaz, where do you buy your jets ? I've only suggested members DRILL the existing jets LARGER, cost a couple of bucks & I've only said it's a starting point.

 

Actually, I haven't had the need to buy jets, but have seen them from $8 to $12 each.
So average that at $10 each, $40 for a set of 4, two sets for fuel if they don't get lucky and hit the right ones when they buy the first set is $80, plus at least one set of air is another $40. So that's $120 for the low side.

I haven't done the restorations you have, and don't have the knowledge you do, so I would not be comfortable giving someone the advise of drilling the jets, as I have never had the experience doing that.

My limited knowledge of carbs is why I limit my posts to "you will have to re-jet for pods".

I would never give someone advice on something like this unless I had first-hand knowledge.

When I said in my previous post "spend a couple hundred bucks on jets and good luck", I was referring to the only advice that I could personally give someone in regards to pods. It was not meant about anyone on this forum other than myself. I would never knock another forum member, even if I disagreed with someone.

I stayed out of this thread until a member admonished another member about putting a theory out there for us to discuss, even though Rick did go about it the wrong way. But he fixed the title of the post to read more accurately that is only a possibility.

We should not discourage our members, especially one such as Rick that has helped numerous XJers, from putting ideas out there. But they should also be labeled AS ideas, not proven facts, until they are proved.

I will be following this thread because I'd like to see the theory discussed, but I have nothing further to add to the discussion, so this will be my last post on this matter.

 

Those of us who have been around here a lot longer, can remember this Mod' has 'form' for stating things as proven, when, in fact, they were not. Who could forget the 'starter clutch quick fix' fiasco, or 'fluid doesn't flow back into the M/C reservoir, the guy should be censured, but he won't be.
& if he is losing an argument he will get personal.

 

Censorship isnt the way Wizard. If someone is too lazy to do their own research and takes some other guys's advice then the failure they experience is entirely their fault. I tok Ricks post for what it is, an out of the box thought for what seems to be a common problem. I see this kind of thing on some of my local automotive forums that tend to be populated by youger kids. They take someone's advice blindly and melt their engine. Then they get mad at that person. There shouldn't be a sticker on a knife that tells you that it can cut or kill you. Those are common knowledge points to a knife's function. Yet those stickers are there none the less. Survuival of the fittest is a common law in the wild. Why shouldn't it apply to humanity? The last bit might be hars but if you think about it you will find it to be true. Ill take anyone's oppinion. But before I act on it I would do some homework.

 

The Boot shapes the Intake Air.
With the Boot in place the stream of air expands enough to have Low PRESSURE available at the widest area where the Boot affixes to the Horn.

Some AIR MUST be present there; because a Vacuum cannot exist in nature.
The Boot allows enough PRESSURE to have a Volume fill the Diaphragm Cavity.

Without the Boot; the Air rushes right past the opening.
Instead of there be a minimal pressure; there is a DECREASE in pressure as the Air Curls and Swirls into the Air Horn.

There's NO Pressure what-so-ever.
Instead of Low Pressure; there is reduced (Negative) Pressure before the opening.

Therefore: Alleviating the condition that totally eliminated the formation of low pressure with a condition that causes NEGATIVE Pressure ... is the problem.

It OK to have AIR Flowing at it.
Its NOT OK for Air to Flow across it.

 

Greetings from the Honda Nighthawk forums! Sorry to jump in as a newbie on the forums with a question. I was doing some searching because us folks over on our forums seem to have the same issue. If you even mention pods over there a million people will scream don't!!! in a heartbeat. Are your guyses carbs the exact same as ours? http://www.youtube.com/watch?v=7r-m23wJ91A There's what mine are like in the video.

I'm working on a cafe build and just removed my airbox and am waiting for my pods to come in. When they do, assuming they're the same as yours- I'd be interested in trying this mod to actually run a set of pods on a NH. I am experienced with jetting carbs but I have a vibe I'm going to spend a heck of alot of money to get these ones right and this mod seems much more cost effective.

 

There is no great mystery to the kidney port. When the diaphragm goes up, the volume beneath it expands, like a lung when you breathe in. Pressure drops, so air comes in through the port. When the diaphragm falls, the air volume beneath it decreases, air flows out. In every device I have ever used which has air actuated pistons or diaphragms (thousands over 20 years in the industrial air compression field), there is always a breather port to allow proper movement of air in and out of any changing air volume to prevent the mechanism locking up. If you block this kidney port and drill a much smaller hole, you will be reducing the responsiveness of the diaphragm and piston as you will be restricting the movement of the air in and out of volume under the diaphragm. You will have to provide a hole which can move as much air as the original kidney port, and you will have to replicate the pressure conditions found there, neither radically increasing or decreasing vacuum or pressure. If you want the piston to rise more quickly, you drill the holes in the piston to increase the rate at which vacuum is produced above the diaphragm, which allows the piston to rise more quickly. This is a common modification in many jet kits and the correct sized drill bit is often included, or a reference to it is made in the kit instructions.

The whole idea of a "constant velocity" is that the intake air velocity is controlled by the movement of the piston in the carb in response to engine vacuum. It meters the air as much as it meters the fuel. This is why you can whack open CV carbs and not kill the engine. Ideally, the carbs will only allow as much air into the motor as the engine vacuum calls for. Older carbs which were directly controlled by the throttle cable could easily be opened too far and too fast, killing intake velocity, resulting in no fuel take up and a bogging or stalled motor. They can be more responsive than CV carbs if you control your throttle input, which is why they are sometimes more desirable in high perf applications.

EDIT: Which is guess what Darkfibre said a few posts ago, heh heh.

 

I am a lucky ape dude. The velocity stack is designed, created, tested, and produced by yamaha. All I did was attatch a different type of filter to what yamaha had designed to work.

 

Based on my experience, I am going to disagree with some of your basic assumptions which appear to form the basis of your argument against pods. I don't believe any air filter, pod or otherwise provide any particular velocity of air. The filter may or may not limit the volume of air depending on the volume requirement of that particular motor at that particular RPM. If a given filter flows more air than the motor requires at a given time, then how much more is irrelevant. The velocity is determined by the amount of air required and the cross section and shape of the restriction to that flow.
To state that a CV carb requires a FINITE volume of air is far too simplistic to define what the needs actually are. A CV carb needs to have a certain amount of volume based on the needs of the engine at that RPM and throttle opening. That volume just needs to be there and the carb will take what it needs based on the slide position and the butterfly position.
The major problem with pods is laminar flow and turbulance, not velocity and volume. My Seca 550 carbutates well and cleanly throughout the RPM range with no airfilters on it whatsoever. It has no flatspots and dips. No midrange stumble and no holed pistons. It pulls cleanly from midrange all the way to the revlimiter. It does have the wide open original rubber carb connector velocity stacks on it and it has been carefully rejetted. Before anybody gets all crazy about no air filters, this is a dedicated race bike that spends time only on the track. It also has a custom 4-1 pipe on it and a properly built stock engine. The bike in this configuration is significantly faster than with the stock airbox and pipes. With more air going in and out of the engine it requires more fuel, ie larger jets, to maintain the proper air to fuel ratio. More fuel in that is completely burned will provide more power.

Obviously I don't run bikes with no filters on the street nor do I recommend that anybody does. All of my street bikes, in fact, have stock airboxes on them with either OEM or K&N filters in them and the appropriate jets for their state of tune and exhaust system. However, if I had the need to use pod filters, I wouldn't hesitate to do so as long as I put them on the end of the stock velocity stacks.


Ken

BTW, this also holds true for the Honda CV carbs I have used on racebikes, too.

 

tskaz i didnt mean to admonish anyone, i was just answering fitz's question with a hint of humor and a faint smile

saying rick should have tested before theorizing was harsh and misstated, i also welcome theorizing and i agree with your view about posting them.

i dont have deep understanding of carbs so i have nothing to contribute to this thread, just that with my knowledge and skillset i would like to see the theory in practice with loads of pictures.

hence my post.

with a smile.

 

-I was trying to "simplify" it; you are correct of course. There's a lot more to it, as you said, based on the "needs" of the engine and it's ability to move air.

-The turbulence thing we've pretty much figured out...

-and again this one simple detail (the original carb rubbers as velocity stacks) with or without pod filters on them, seems to be the KEY to the whole thing; along with the appropriate rejetting of course.

 

We're all in agreement that this modification will present "atmospheric" pressure to the Kidney-Area, and the theoretical tubing and filter will not cause any restriction, and the result will be that the diaphragm will lift the piston/needle a little higher, allowing more fuel.

But what happens in the emulsion tube?? Does the presence of more air pressure (even slightly more) cause less fuel to be pulled into the emulsion system, and a higher slider piston (slightly) will cause less velocity, which would pull less fuel past the Main jet??

Don't tell me the float bowl vents into the air horn - - it would need to vent into this system also ( in theory 8O )

 

Hitachi and Mikuni carb bowls are "vented" into either the airbox boot "horn" (inside the boot) or to atmosphere via an external fitting.

The main air jet (which feeds the emulsion tube) is either under the diaphram (Hitachi) or in the carb throat horn (Mikuni). Mikuni carbs (some models) also place the pilot air jet in the carb horn.

It is my understanding that the air pressure is atmospheric from the airbox forward until the air column passes either into or thru (?) the venturi portion of the carb throat......only then does the pressure drop occur. So at any point "behind" the vac piston----which includes the main air jet/emulsion tube pick-up points, be they round jet holes or the kidney-shaped port----as well as the bowl vent holes, are (already) seeing atmospheric pressure.

 

The Emulsiom Tube behaves as it is designed too.
Without a Negative Pressure robbing the air below the Diaphragm Piston Rubber; the Piston rises along with the Needle.

With the pressure below the Diaphragm not negatively effected a supply of AIR, present to satisfy the Siphonal Flow through the AIR Jet, its passage to surrounding the Emulsion Tube, and it's subsequent pull through the Emulsion Tube's Air Metering Ports ... the design function returns to a more normal condition.



Swift intake AIR lowers the Pressure at the E-Tube Opening and Main Jet Fuel starts being supplied in an increased volume plus atomized more thoroughly to aid in better performance.

 

So much anger. I have pods and pipes and have rejetted. My bike when I find time to keep it in tune runs really hard across the RPM range with a distinct and aggressive pull at 6k to 10k. But I do have a idle issue that I can't seem to nail down. It will rev up at a stop to 3k and not idle down so I believe it to be an intake leak, but if you let out the clutch and load the engine down it will settle back to it's set idle speed and stay there. I digress, I am going to tune it up this spring and build some sort of velocity stack to test as well. So I have a tapered type of pod but have seen some that are cylindrical in shape. The taper is supposed to alleviate a problem with air intake pulse between carbs. It seems to me that stacks would also help to control this pulse problem by separating the carbs from the point of air intake. Has anyone built a forced air intake system for their bike?? I have done this on a car with great success and they are put on sport bikes. Can anyone see any potential problems with a small amount of air pressure being applied to the intake of the carbs? This should be a different thread huh.

 

Pree -- sice -- lee !!

With the Boot in place there is Atmosphere present and the process to delivering Air to the Vent and beyond is not disturbed.

Remove the Boot and Install a Pod.
Air cascades from all directions.
There is a flow across the vent where there should not be.

Ultimately, any measure to assure that some air is allowed to reach the space below the diaphragm to provide the Main Air requirement will aid performance.

Naked Pods: Seal and Aux Vent as you please.
Boots, Stacks: Might not require relief.

 

Fuel injected bikes have the ram-air systems, but this would most likely mess up an XJ (in theory - search POD threads for the truth)

I measured 2 1/2 inches of water pressure at 85 MPH in my truck, having a tube out the window, and a tube in the cab. (I was bored).

This would be about 1/10 PSI of "boost", and since 15 PSI gives you 50% more horsepower, you would gain less than 1 HP.

 

Fundamentals if required:

http://www.motorcycleproject.com/motorc ... ork_v3.swf

 

Len, Rick, et. al.

I do not understand how "...the air pressure is atmospheric from the airbox forward *until* the air column passes either into or thru (?) the venturi portion of the carb throat......"

With the Boot in place there is Atmosphere present and the process to delivering Air to the Vent and beyond is not disturbed.


If this were the case, then running no filter or running, say, a K&N would not require rejetting? No? IF the stock filer is more restrictive than none at all or a K&N -- Then there MUST be *some* level of less pressure INSIDE THE BOOT (i.e. from the filter to the throat) -- The filter provides some restriction...No?

And therefore the kidney-breather's size, etc. is balanced within the expected relative pressure difference between the tightest-part of the throat, and the outer-diamter of the "boot" where the kidney breather is located...

No?

 

DM..........pressure and volume are not the same thing. With each engine revolution (or intake stroke occurence), a certain VOLUME of air must be (or, more specifically, is CAPABLE of being) drawn into the cylinder....this can be easily calculated via math by the physical dimensions of bore x stroke of the cylinder. This VOLUME of air must be available thru the intake "system", i.e. the cylinder head passages, the carb throat passage, airbox boot volume, airbox, etc.

There are restrictions (think of them as the airflow equivalent of fuel jets) that limit the amount of airflow available. Look at the air vent opening on the airbox, compare that to the air volume in the rest of the "system". That opening is pretty small and, along with the restrictive nature of the paper element, limits the amount of airflow thru the intake systems (there are other factors, also).

Although the AMOUNT of airflow may be restricted, until a venturi effect is created in the system, the PRESSURE of that airflow should not vary in any meaningful manner. TIME measured it (thank goodness for those moments of boredom......!) to be about 1/10psi, and at 85mph, depending on the truck and its gearing, that's at (I suppose) a pretty good airflow rate in that system.

Although not insignificant, 1/10th psi of "boost" is negligible (my opinion; could be wrong) in regards to the situation being discussed here.

Factory engine systems and fuel injection rates (be it EFI or carb metering jets) are designed to meet the airflow requirements of the stock engine. Increase that air FLOW by any means results in a need for more fuel flow, which the stock metering jets may (or may not) be able to supply.

K&N air filters, pods, no filters, holes in the airbox, etc. provide their claimed "power increases" via increasing the amount of airflow available to the engine at a given vacuum strength (work). Otherwise, there can be no increased power output since more output means more energy input available to be converted into output (assuming all other engine design factors, i.e. compression ratios, port design, intake valve duration/timing/etc. remains the same).

Intake air turbulence (with associated disruptions of pressure in a random and unknown amount) may occur with the use of pod filters.......but, I don't think that is the main cause of the issues involved with their use (again: I DIDN'T stay at a Holiday Inn last night, I'm not a certified fluid dynamics engineer, and I could be completely wrong), and if it is an issue, I believe it is a minor issue relative to the increased airflow that low-restriction filters provide.

If using a K&N filter in a stock airbox (along with the boots), or a stock airbox with no filter, OR THE USE OF PODS all produce the same "tuning" difficulties, them it seems to me that the logical conclusion is that the main issue in play is not the pressure variance at the vac piston diaphram port (or elsewhere), it's the increased airflow that is the main "culprit". If the stock airbox boots prevent the turbulence/pressure changes, then why does the elimination of just the filter element (or use of a low-restriction element) create the same issues as when the boots are eliminated and pods used?

 

It seems that one should address the fundamentals of the internal combustion engine. Any internal combustion engine requires a certain amount (volume) of air/fuel mixture. However, it is always the same ratio. Same principal in say mixing paint. It takes the same rations of pigment whether you are mixing a pint, or a gallon. So If using pods, you most certainly will have to increase the
jetting to get the air/fuel ration back in check. Now in my opinion, it will not make a significant difference in power, because by design on the engine (combustion chamber size as Chacal pointed out) has not changed. You can modify the intake charge, and exhaust and see negligible difference, but unless the internals of the engine are changed, it will burn no more volume of air/fuel mixture than the factory designed it. The only was to increase power is to change every aspect of the engine design from the intake (pods) thru the exhaust. This includes the air intake, carb jetting, intake ports, valve, bore, stroke, exhaust valves, exhaust ports, headers, collector, pipes, and muffler.

Otherwise, tune the bike properly to factory specs to achieve excellent FULL RANGE performance, or completely modify everything on the bike to have pods and achieve more power and full range performance...

 

mook, I think the motive for a lot of people changing from the air box to pods, is the ease of pulling the rack & esthetics, not better performance.

 

Not always true. I can point to MANY examples of modifying one item and increasing engine power.
It is engine or vehicle specific but many vehicles have items that may be less than optimal for performance due to meeting emissions or noise laws.

A well designed free flowing exhaust for example.

Anything that can improve the volumetric efficiency will help.

 

I didn't say it wouldn't increase power, only that it would increase power negligibly. To get the same factory full range performance, you can't simple change exhaust or pods, and any other one part of the system and achieve optimal full range performance. Personally, I am not willing to sacrifice the solid full range performance for pods. If I just want the look of pods, I'll just cut one in half, cut off the carb boot, and glue it to the side of the factory airbox :lol:

 

I have not heard of problems with changing or eliminating filters in the airbox other than the need of increasing jet size. Engines are air pumps, the more efficiently you pump this air the more air you pump in the same period of time the more HP the engine creates. More air means more fuel. There are many restrictions to airflow through the engine those being the carb, the cylinder head and the exhaust. Although without those you wouldn't have a working engine. The purpose of high flow filters and exhaust is to increase throughput of air thus increasing power output. So these carbs seem to me to be finicky little pains in the ass. They seem to me to need a linear flow through them in order for everything to work correctly. Looking at my pods air is coming in from all directions attacking this nice linear flow and in my mind disrupting it across the port for the slide and the jets around the outside perimeter of the carb intake. Anyway as far as the forced air induction goes I do agree that the increase of air pressure would be of negligible gain but letting the engine breath some nice cool air not coming from the back of a hot ass engine would be a gain based on personal experience. I just worry about water finding a way into the engine being that weather (barring ice) really doesn't stop me from riding. Sorry I get windy sometimes.

 

I found a site which has a calculator to determine velocity stack length. Using the XJ750 cam duration of 276 degrees, 3 "induction waves," an ID of 3.2cm, and a opt rpm of 8000 I ended up with a length of 30cm.

Now given that a regular pod will have nowhere near that length, and that the stock carb setup is probably very close to that length, could that be the overall deciding factor?

Beats me!

Velocity Stack Length Calculator

 

The engine may be an air pump, but the carburetors work off one source of air to equalize vacuum on all 4 cylinders. Air pump or not, all 4 cylinders need to be synched together to run efficiently. The way these carbs works, having pods allows too many variables and fluctuations.


When everyone gripes about putting pods on, I am curious as to why everyone thinks they can outsmart Yamaha. If there was an issue with the air source, these bikes would not be able to do well over 100k miles with good maintenance. There are lots of people on here that have high mileage on a stock setup, so clearly the method used by factor is effective.

 

I don't gripe about my pods, my bike runs like a stallion. But in my opinion there can almost always be a little better overall performance available. You just have to decide if it worth your time and energy to find. I do agree with the increase in variables caused by pods it was many hours of tweaking to get my bike to where it is. I do think that there is some turbulence caused by the filters being so close to the intake of the carbs. so I am going to play around with stacks or something of the like and maybe find a nice clean way of routing an air intake to the front of the engine. Not sure how that will work but I love fabing stuff. But it does need to look like it might have been factory.

 

Some explanations here -- don't know the author - he may be a crack, but most of what he discusses seems "logical" -- dunno

http://bmwmotorcycletech.info/InExTuning.htm

Here's another good source
http://www.omnilex.com/public/bmw78/cvcarb.pdf

 

I don't gripe about pods either. Pretty sure the only ones who do gripe have never set them up, or are incapable in the first place. My bike with pods runs just as hard and well as the bike I had that is identical and has the factory airbox. I don't think it is a matter of outsmarting anybody. Stock is for some I know.

I just wonder why it is that the xvs1100 pod kit is dyno proven to increase hp to the wheel, and it uses the stock airbox to carb rubbers. I would love the opertunity to dyno my xj. I am making it my mission this summer to do so and share results. Good or bad.

 

Over the years, I think I have probably seen every kind of Mod used to get better performance from naked pods.

Such as;
Playing Cards attached to the sides of the Pods
Duct Tape
Nylon stockings added
Wrist sweat bands
And, ... Official NY Yankee Beer Cozy Slip-ons with air slots cut in them.
(Probably cost the guy more than the Pods)

 

Beer cozy??? Kinda similar to the foam sock on the paper filter element of a shop vac...Hmmm!

 

The reason for calculating velocity stack length is for optimum power, not carb tuning.

 

A Velocity Stack 30cm's in length would be ... about ... 1/5 of an Inch shy of 1-Foot.

 

This is intake tract tuning.

30cm from the back of the inlet valve.

 

Back to the topic. Has anyone tried it yet? I've been watching this thread to see if this theory has been put to the test. I would try it, but my bike is running great with my pods. Plugs look good, still need to do a couple chops and colortune. But for now, everything looks and feels good. So who's taking the plunge first?

 

There's a pretty simple "Work-around" you can try without drilling or too much fussing around at all.

Get yourself some Vinyl Hose the diameter of Battery Overflow tubing.
Cut eight lengths of Hose 5-1/2 Inches long.
Insert a pair of Hoses into the Atmosphere Vent -- side by side.
Seal the Hoses into the Vent Orifice with Silicone Caulk.

Slip a 3-1/2 Inch long section of Shrink-wrap over the pair of tubes.
Shrink the wrap to be snug, ... but NOT collapse the tubes.
Extend the Breathing Tubes toward the rear of the Pod.

Affix a Safety Wire at close to the open ends of the tubes.
Secure the wire to prevent the tubes from entering the Carb if the Caulk should come loose.

::: Alternatively, ... Drill a small hole in the center of the Pod-end and Fasten the wire such that the Open ends are suspended VERY CLOSE to the back of the Cap on the Pod.
Engineer the arrangement with enough slack to capture the tubes with a Strain-relief anchorage of the Breather Tubes to the I.D. of the Pod, ... lessening the chance of them being dislodged. :::

 

is the shrink wrap just to keep the tubes together? and is there non-turbulent air at the rear of the pod? or would the air smooth out as it travels down the tubes? Would it be the same effect or better if the tubes were extended through a hole drilled at the end of the pod and pulled out into the open air? Just for testing purposes I mean.

 

The Shrink keeps the tubes married together and provided some rigidity.

The Mission is to relocate the Atmosphere Opening away from the swift flow and turbulence from Air rushing across the Atmosphere Vent.

Your idea to extend the Hose through the Rear of the Pod is brilliant.
The Protruding Hose End could very easily be:
> Anchored. Preventing it from getting swallowed.
> Filtered. A Foam Filter could be affixed to the Protruding Hose End with an adhesive.
> Pimped. The Foam trimmed and shaped to provide a Custom appearance.

::: ( If the Hose is placed through the Back of the Pod and Anchored to prevent getting sucked-in, ... Fabricating where they enter the Sealed Vent with Short ends, ... and using Hose Unions to connect the Sealed Port to the External Atmosphere Vent Tubes seems like a practical solution to facilitate removal pf the Pods. ) :::

 

Watching this one with interest... if it works, then we'll all be chucking our standard airboxes and fitting (much better looking) pods or stacks.

Get on with it Rick.

 

Anybody who is running Pods, ... wants to give this a shot, ... lives close enough to me for a Day-trip ... can bring their Bike to my place and we'll do the job in a heated garage.

 

So, in theory. I should be able to clamp a "sock" around the carb intake and it should do the same thing. Since both ports won't be pulling air in and the top port will be open to atmospheric pressure. If this is the case, I'm heading to Walmart and buying a set of black socks!

 

No.
I think the sock theory is flawed.

If you slow-down the Volume of Air entering the Intake Horn, ... at some point there is going to be such a disruption ... that the function of the Venturi will be negated.

Intake Air will not be able to achieve enough SPEED to cause a Pressure Decrease atop the Emulsion Tube Supply Orifice to draw-out Fuel.
The Engine will Stall.

What needs to be achieved is:
Adding Speed (1)
Shape (2)
Preventing Air from being drawn-in over the edge of the Horn Opening.

1. Speed. [ Velocity Stack or Modified Boot ]
2. Have Intake Flow [( ⬅ "At" Intake Horn. Not from surrounding edges.

 

Hey Rick, for as much as I hate pods cuz of all the trouble---I just picked up another 650 that will be arriving in my driveway sometime this spring.

I have a set of brand new pod filters sitting on my shelf ( that way there's is at least one set that nobody can deface a bike with....at the moment).

I am willing to be a guinea pig.......is it possible to condense this procedure into an email for me? If so, pm me, and I'll give you my address.

Dave Fox

 

Masochist.

I'm only on board with this if you try the "airbox rubbers as velocity stacks" thing before any drilling; and compare.