Looking for a little more flow
 

I'd bought some Gates style thickwalled radiator hose a few weeks ago in Cebu and finally got around to swapping it in place of the slightly S shaped factory air box to carb boot in an attempt to straighten out that pathway out and pick up some CFM.

It was a bear to get in place and in the end I had to make second one that was angle-cut at each end for a better fit.

Going for a ride, the bike seemed different/down on power just a little. Felt like it was running rich. Since playing with main jetting with that tight fitting hose in place would have been impossible-I opted to just put the Kaw rubber piece back in place.

Test riding that; the bike was back to normal running crisp and pulling hard in the upper rpm range.

So what was happening with the modded section in place will have to be determined at a later date. Either when I use the Gates hose again-but shortened enough to not have it actually going over either end, instead I'll use short sections of rubber hose at each end going over both the carb/air box and Gates hose. That way i remove and replace it easily if I want to play with jetting. Or wait on the 2.25OD , .049 wall Al tubing on its way here.

The interesting part is it did make a change in the way the bike ran! So results were obtained. I don't see how straightening out the airflow to the carb would be a negative change, other than needing some rejetting that seemed to be necessary because of the mod. Maybe more flow was pulling harder on jetting that was previously working at less than 100% of max flow, and the mod now had the jets flowing closer to 100%?

Anyway since it did have a noticeable impact on performance, though initially down on power. It'll be interesting to see if a gain can be realized in the end by getting the jetting idealized once again.

It'll be easier to tell what's happening/right away after the a/f meter and exhaust bungs arrive and are installed. Then just change the jet to compensate :D

wouldnt it be easier to ditch the airbox completely and just mount a pod filter directly on the carb ? thats what i'm planning once i have a pumper.

It would, but even though my bike's mostly used on the street, I see any kind of clamp on filter as sucking up a lot of heat from the exhaust system; maybe the cooling system's hot backwash as well?


I'd thought about adding a clamp on K&N inside the air box to facilitate relocating a battery in there, but now I'd found a very small hi tech battery that I can attach to the bottom of the seat easily and will be in the upper air box opening and I can keep my current K&N Kaw air filter at the same time.

The battery pack is only 2" thick-I could also mount it inside the air box against the back wall..


I'm just fooling around with little mods to see what happens while waiting another 2mos for the 331 kit to arrive

This is very interesting. Another example of improving airflow that results in less than optimal results. Port matching the header was a performance failure until I saw that the A/F went way lean(From the dyno). After correcting that with the DJ132 jet, the engines' top end came alive with gains of right at 1hp.
Improving airflow can only cause the A/F to go lean - not rich. The only way more fuel will be pulled from the main jet at WOT is by a restriction increase in the intake tract upstream from the main jet.

I don't know about that. Guess it depends if you feel engines generate vacuum or if it's atmospheric pressure rushing to fill the void that acts on jetting. I know a vacuum gauge seems to answer half that statement but I've heard the argument that what's really happening is atmospheric pressure.

Remember that engine vacuum is just air pressure lower than atmospheric pressure. The starting point to evaluate engine vacuum is the intake manifold. When you connect a gauge to a tap on the intake, you're measuring manifold vacuum. (from MOTOR Magazine)

So let's see..if intake mass flow or velocity was increased because the Gates straight hose was less of a restriction in the intake tract, then the venturi effect in the middle of the carb/over the jets would be increased. Or so it seems. If it was, otherwise perfect jetting could now be too rich if a jet can be said to flow within a range.

:confused:

I just enjoy talking out loud about it, and am open to any idea being a bad one, but I don't see why straightening out part of the intake tract would be a negative after jetting to compensate if necessary. That would almost have to be a first wouldn't it?

Main jet will operate at maximum flow at WOT. Increase the air flow and the jet can no longer provide enough fuel - the AFR goes lean - and you put in a bigger jet.

The increase in the venturi effect may cause some increase in fuel flow through the jet but it is not adequate compensation.

There is little to no vacuum at WOT in the intake tract.

Then there is the whole issue of inducing turbulence in the intake tract upstream of the carb, and all the crazy crap that happens when that occurs - the carb will be operating outside of its' design parameters and its' systems will fail to function properly.

I don't think your experiment would have induced turbulence.. Do you?

Guess we'll just have to agree to differ until I get some instrumentation on the bike and can know where I am at with jetting better.

I'd have to believe that each jet has high and low flow numbers-close as they may be, all dependent on the amount of air going over it.


Example:

Mikuni carb slides used to have an extra hole and a slot going from one hole to the other in the base of their slide to facilitate connecting the throttle cable to the slide. The cheaper round slide models may still have slides like this..

Decades ago the thought occurred to me cut a round gasket out of inner tube, poke one hole in it for the throttle cable, install it all in the slide and let the return spring hold the gasket in place over the bottom of the slide. Effectively sealing off the inner slide chamber from the intake tract. (think how a CV carb;s slide works)

Anyway, just as I thought it might, this increased throttle response; but I also had to reduce the pilot jet one size for any bike I did this on for it to start and/or run properly. Why then?

Because the same size pilot jet was now flowing more fuel than before in response to the increased airflow passing over it.


Thunder Products markets an add on 'space filler' for Mikuni round slide carbs that installs under the base of the round slide, smoothing out flow under the slide. Part of their installation tips include reducing pilot jet size by 50%!

Same principle? A jet that was previously sized perfectly is now too rich because of the increased flow over it.

As colleagues I am happy to agree to disagree. I hold out hope that whatever the A/F meter shows, the correction will lead to improvements. I am convinced this engines' systems and components were not designed for power production - but for compliance.
I wouldn't be surprised if the stock boot is restrictive.

The problem is not more air passing over it, rather velocity of the vacuum.

For instance if you put a fan in a room and let is blow air, increasing the size of the room doesn't cause it to blow more air. Putting an cone in front of the fan will increase air velocity.

The same is the the jets in a CV carb, air from the air box flows over the jets and pulls fuel. Opening up the airbox increases air flow which leans out the jet, more air doesn't mean more velocity. I'm not sure if I'm explaining it right, I'm pretty tired...

Air volume doesn't increase fuel from a jet, air velocity does. If anything it decreases velocity which leans out the motor. That's why with the 351 kit you can have the same jetting from a dyno jet kit on the 250 jug. The higher vacuum from the larger displacement pulls more fuel from the same size jet. We just jet up to take advantage of the larger displacement.

I can't forget that this CV carb and its' related intake components were matched up to provide 15-16 compliant horsepower. We are asking the same items to produce 20+ hp with our "uncorked" KLX's. Perhaps that is all wrong - Perhaps you are on the right track in modifying all the components from the intake port on the head to the airbox to get more flow. It's terrible that you don't have a Dyno available. My dyno testing confirms that my seat-o-pants dyno can detect a 1hp gain.

If you use a pod filter removing the airbox, jet up, and then port the intake of the head along with a larger diameter (or perhaps smoother i.e no bumps) intake boot on the head side of the carb, then hp can be acheived.

How come all you guys need all this horsepower?

I'm just doing it to give myself something to do, but the lack of a dyno or even someone to go through the gears regularly with can be frustrating. I may look into a couple of lap timer sending units to place along the road and a receiver unit/timer on the bike.

So while I am waiting for the box of mods to arrive from the USA I'll shift towards some bling for a change by adding CRF dual exhaust and try and find a 4.5 inch wide 17" back rim to replace the 4" wide rim that came on the bike as I convert it back to 17's front and rear..


I may get away with using the CRF header if I can heat it and take some of the bend out of the U section up front. There's a YT vid showing a guy at Yoshimura? bending exhaust sections with a torch and what I'd guess is a rosebud tip.. if a shop in town or the next town up has the equipment to do it, I'll give it a try. Only a few dollars to have them heat it up so..

The CRF headers seems to be the same bolt pattern and OD where it goes up against the head, though i think the header itself is a smaller OD than the 1.5" that's on my bike now. if I can't get the U bend opened up I'll just use what I have and shorten my header accordingly.

Changing the air box boot to something straighter seemed a natural thing to do and cost about $4 to try plus the time. Though I wasn't necessarily expecting to actually feel a gain over such a little thing, sometimes little things add up. I'll play with that idea later on this year

4.5" rim wit a bigger tire will for sure slow down the bike

I think you'll find that a single exhaust into a dual exhaust will not get any HP. But it will look cool. Then you'll need 2 mufflers, which will mean extra #'s.

Hey Richard, a 4.5 rear is going to be a big squeeze. I'm running a 4.25 now with a 140/70 and had a few boggles. I ended up spacing out my rear sprocket 6mm and my front 3mm for chain clearance, as well as offsetting wheel slightly to the right. All good now. Looks great from the rear with the big meat.:-D

I already have the complete CRF dual exhaust including mufflers from eBay

Could be, but it'd depend on the tire brand/size? From what I've seen there's quite a variation in tire widths even when the tire size is the same; 140/70 for example. I looked some brands up, or maybe found a post on a forum listing a bunch of them months ago and have it saved in an email to myself somewhere. Widths between brands but of the same size were often very different.

It was probably 140s from different manufacturers since I've read here that a 150 was all but impossible on a 2007

I was going to look for a 4.5 rim here in the Philippines or in China online, then go back and make sure I can find a tire with a narrower tread width or overall width before I bought the rim..can't remember how they are listed right now.

Ya too big tire might be slower. Can't fit too large a tire in there anyway.

Found the short list I started months ago. I think I measure a max of 6" for tire width based on the 18" knobby that's on the bike now, but I guess I'd gain a little wiggle room with a 17" rim

Tire size:150/60-17

Distanzia = 5.8" tire width on a 4" rim

Conti SM = 6.3" tire width on a 4" rim

Pilot Power = 6.46" tire width on a 4" rim

BT003 = 6.3" tire width on a 4" rim

I'm not having any luck finding a 4.5 rim so far, just 4.25-not much of a gain over the 4" I have now. There were some 5" rims in Thailand or China but that could really be asking for problems..

:D

Hard to say how much R&D Kawasaki put into the S version after so many years of building the 300 etc

On the one hand, I'm sure they'd love to one-up the competition by having a 35hp or more street legal 250 enduro, but they may save the R&D for other bikes.

Or they may want you to buy a KLX450

:D

You might have a look at the intake boot off of the motocross bikes. The design is for higher performance using 250cc. I'm sure the factories have been testing for years which one works best. Iirc Moto tessari came out with different intakes.

If they would jump through the hoops to have them factory street legal in North America I'd bite.

Depending on how it is installed it can. If it was truly straight from air box to carb throat that is best. If it is angled at all, that is a kink versus the smoother S turn of the stock tube. If you look at exhausts you seldom see a kink in the pipe, rather a smooth flowing curve. Same with two strokes, a high quality expansion chamber is either vacuum formed for smooth transitions or if it is made up of cones the cones are cut and rotated in very small slices in the bends. The less costly pipes will have more kinked turns.

So if the straight Gates tube is angled at either end it could easily be causing more turbulence. Fluid flow likes smooth flowing bends, not kinks.

In addition, there can be a self fulfilling prophesy too. There have been exhausts that sport riders have installed that they swear run better because of the big hit they feel. What they don't realize is the pipe hurt their lower mid range so bad that when it hits it makes the rider think the bike is stronger. Is this the case here? I don't know. Without something to prove actual increase it is purely subjective.

All very true of course.

So far, all I know is it ran different on the test run; seemingly worse - and then went back to normal after I switched it back to stock boot.

At least it did 'something', now I just need to try and find out what.


:D

I've seen some bikes around and online now and then with a bulge halfway between the carb and air box. An idea that could be sound. Otherwise I am not familiar with MXers, there aren't many to see typically though there are some around, and adapting a boot could be a big headache.

My XR200's boot had a round flat spot on top. Evidently to be used for an emissions connection on some model at one time or another. I froze it in water with the flat spot just below the surface so I could drill that out with a hole saw and add a small K&N filter with a 1" spigot. It seemed to not necessarily be a gain. Maybe it threw off the ram effect of the stock boot.

So mods are always a 50/50 proposition I guess

You are right, trying. That's half the battle, figuring out what and why is the other part. If there was a way to get the path totally straight that would be the trick. How much gain can be had might be questionable.

I did some calculations a few weeks ago on the snorkel. I measured the opening of the KDX snorkel and calculated the opening area. Then I measured and calculated the largest part of the bell opening on the carb. They were nearly equal. Only problem with the snorkel is the bending of the air flow. Same is actually true of the air box lid when the snorkel is pulled out completely. The sharp edge creates turbulence as air enters. I'm figuring the reason why taking the lid entirely off is better than just removing the snorkel is because the sheer size of the area opened is so much greater than the engine requirement that the turbulence around edges and such will not be significant enough to affect the performance.

One point on the intake tube. I believe it has a bit of taper up to the carb, that would cause the air flow to speed up as it flows into the carb. Same deal as the reason the carb has a big bell on the end going to a smaller throat.

I honestly think until the carb is significantly larger the air boot/air box won't affect the flow much. The lid and snorkel are another story. I think they may. I think Marcelino showed that on his dyno runs doing with and without air lid.

I also think anything much bigger than the 36 used by many here will be overkill until the engine is cammed and ported to suit. That will also make it peaky, because you are essentially duplicating the KX250F engine design. Harder to ride and less likely to run a lot of miles reliably.

There was a thread on the ADV rider site heralding the CCM 450 adventure bike. They seem to think it is going to be something special in performance. Fact is CCM cut down performance to the horsepower level of a DRz400 in search of reliability for long street riding. The big plus for the bike is the adventure type fairing, more comfortable seat and such. Kind of a smaller KLR650, which ain't all bad if the price is right. But it isn't going to have CCM supermoto/off road performance, because the engine will not last.

Makes me think of what on of the hot rod magazines pointed out. To make an engine get more horsepower out of the same displacement one has to raise the revs - considering the cam timing and head flow is optimized. I think cam timing optimization is the reason the Marcelino mod works for the 250 and not for the bigger bores. The bigger bores might need different cam lobe profiles to suit the increased displacement. With the OEM cam the Marcelino timing change is not any better at doing the job than the more advanced timing. But I'm not an expert in that area, just thinking about what I have learned to date.

And I also like "talking out loud" too. To make one think.

There are other concerns that come into play at times - like the shock or other parts. Air boxes sometimes make the intake boot have to "flow" around the shock. Problem is if it cannot maintain a round shape they will go to a larger D or oval shape (the flat of the D being in the inside of the bend, keeping up the intake velocity) They will look like a bulge, but usually the actual area of the cross section is designed to maintain the size of the boot. As the shape flattens it has to elongate in one plane to maintain cross sectional area, otherwise the flow is restricted.

In a side note, I think considering space available, the 250 intake tract is fairly well designed, as big an air box as possible and a good boot design round all the way through with minimal directional change.

There just isn't any way to get sport bike quality air box performance within the confines of a dual sport. It is a constant trade off.
More talking out loud....

Pleasant read klx678.
In the vein of thinking out loud. I hope we can research and test simple mods, (perhaps 1/2 hp at a time), in order to create a recipe that creates power levels close to those of a BB kit on a stock bike. And do so for far less money than the kit costs. This "recipe" would also have the benefit of preparing the engine for maximum benefit if/when a BB kit were to be installed.

So, in summary, if my a/f ratio did go rich, it's either because the kinks rather than bends in the Gates hose connections slowed flow/increased vacuum OR because flow increased drawing more fuel out of the jet(s)..

http://www.auburn.edu/~sje0003/scratch-head.gif

I imagine I can jet for it once I get around to playing with it again, but I'll still have to somehow measure performance afterwards

One poster here a few years ago swore his bike ran worse when he put the snorkel in backwards..(!) Go figure. I think he test rode it twice to verify and got the same results both times. The lower end of the snorkel is cut at an angle IIRC and ...maybe the angle with the backwards snorkel threw off flow enough to be noticed? If so then it'd seem the snorkel is right on the edge of providing enough air?

Note that KDX riders routinely take their snorkel out. You just don't see any hop ups on cars or motorcycles where they restrict the intake tract to gain power. Once in a while someone opts a forum that taking the lid off etc just requires a larger jet by itself and doesn't actually do anything-but that's counter to elementary hot rodding isn't it?

A carb larger than 36mm would be less likely to run a lot of miles reliably only if you were riding the bike harder? This 36mm pulls so well down low with my 14/48 gearing, I don't see the 40mm falling on its face. Worse will be starting from scratch with the jetting-I've ordered extra sizes in everything for the 40mm since it takes 2 mos to get here I can't effectively tune if I order jetting a little at a time.


The math may initially add up for snorkel area vs air box & carb size until you factor in demand. What about when the engine's asking for 20 liters a second at 10K rpm, and has only a very small fraction of a second to get it each cycle depending on cam duration?

How large an opening would you need to flow 20 liters of water per second at 14.7 psi? I suppose there's a formula for fluid flow somewhere online, but I'd bet you'd need an opening a wee but larger than the snorkel hole in the lid. :D

Previously I measured 2. 1875 OD at the air box spigot and 2.125 at the stock CV carb; the TM36 is listed at 2.125 OD as well. There may be taper built into the boot.

FMF is selling this doo-dad you insert into your boot for better flow-only $100 !