Airbox snorkel size

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Re: Airbox snorkel size

Postby John_S » 02:18 pm Oct 10 2018

I don't have the RB carb but messed around with jetting and air box changes a lot on my bike. Mainly because my back was too sore for a real ride. It's a 220, RB head, 35 air striker, V force reeds, and FMF woods. The actual jetting barely changes between having the snorkel in and no lid at all.

1) It has the best throttle response from idle to 1/4 throttle with the lid on and snorkel in. Its the snappiest off idle even when the other 2 below are dialed in. The midrange feels as strong as 2 and 3 but I think its softer on the top end. The air screw needs to be leaned out a 1/4 turn or so from the other settings below. The engine is surprisingly quiet like this.

2) Lid on / snorkel out I have no complaints on throttle response from idle on up. It sounds better to me having a little intake bark. It idles good and the air screw setting is consistent. This is how I usually run it

3) No air box lid, the results are always the same. Its much louder, feels stronger on the top end than with the snorkel, but the throttle response from 0-1/4 is not the same. The problem is that the air screw gets finicky every time. I'll come to a stop and it'll be idling at 2,000, a minute later it'll barely idle, blubbery rich and smoking, but it'll lean bog if you whack the throttle. Start it up to head out again and you don't know if it will idle high or barely idle. It made me think I had an air leak but it only does this with no lid. Its probably just something weird about my bike. I could ride all day and never get it set quite right.

If you have the lid and snorkel its worth messing around with. I think there's a trade off depending on what kind of riding you do. Snorkel in if I only rode technical, rocky single track. Lid off if I only rode wide open spaces.
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Re: Airbox snorkel size

Postby kdxdazz » 05:47 am Oct 11 2018

they are not theories,they are design parameters governed by the laws of physics that every airbox engineer follows,opening up the airbox will allow more air to be sucked in at the upper rpms but sacrifice low end air velocity and move the point of resonance frequency. like you say it would be interesting to see if people have tried and tested all combinations but i am starting to realize people are so hung up on horsepower and dynos that its almost like a form of brainwashing that sells aftermarket bolt on"s
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Re: Airbox snorkel size

Postby kdxdazz » 06:01 am Oct 11 2018

awesome advice thankyou john_s. i was having the same issue on my bike with the large cutout, i've just taped it up to a 2inch by 2 inch hole, only started it on the side stand but it idles really well and seems much more responsive, will take it for a ride tomorrow. as i was thinking a larger cutout makes it run richer at idle
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Re: Airbox snorkel size

Postby Greenbandit200 » 09:04 am Oct 11 2018

I am willing to test this issue when my 97 200 goes to the Dyno, as it was a question in my pocket as well. Unfortunately I won't have my build done till the spring of 2019. It will be a highly modded EG 225 running race fuel. I will post a thread on it when complete. I know that it's important to have the opening in the correct spot as well as size so the air is being brought into the cone equilly and not drawing the air into the top or side of the carb to create a swirl or wash interring the carb. I wish I could help now though. I'm definitely going to keep up on this thread too so keep us informed, thanks.
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Re: Airbox snorkel size

Postby ericr » 10:11 am Oct 11 2018

Since all I ride is single track, I'm mainly interested in low to mid torque and throttle response. I'm kind of sorry now that I modded my airbox lid and discarded the snorkel.

After doing some reading, I realize now that the snorkel is a crucial component of a helmholtz resonator - a device that functions much like an expansion chamber exhaust, but on the intake side.

For those of you running without lids, if you still have the snorkel and lid, would you be interested in selling?
'02 KDX 200, '02 KDX 220, '18 KLX 140G
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Re: Airbox snorkel size

Postby bufftester » 05:41 pm Oct 12 2018

I may still have mine from the hybrid conversion, I'll check the shop in the morning. If I do you're welcome to it, and I'm just down the road so I'll even ship it to you free.

As for the Helmholtz resonator, on 2T engines it is a design used to eliminate the need for reed valve induction which our later models have. More likely it has to do with reduction of intake noise and water ingress on these trail bikes. What John has said is reflective of the physics involved. A small opening will lead to smaller volume but at higher velocity. Coupled with the smaller carb throat size (as on the 220) you increase low end torque, but at the expense of some high end. That is why the 200 carb (a PWK35) gives the stock 220 a little more top end with no other changes. As you increase the opening size to the airbox, you will flow more volume at a lower velocity which decreases torque and increases fuel consumption. Somewhere in the middle is the optimum setup, but the stock settings are not that. Stock is designed to be quiet and reliable (rich) without respect to performance.
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Re: Airbox snorkel size

Postby kdxdazz » 09:13 pm Oct 13 2018

thought i would share my results of taping up the airbox. when i bought it the airbox cutout was roughly 5 inches by 3 inches which gives a surface area of 15 square inches,i taped it up to a 2 inch by 2 inch hole which gives a surface area of 4 square inches,this is twice the size of the original snorkel at 2 square inches. the results were amazing. the bike idled really well and now had throttle response, i took it for a ride and could instantly feel the difference,after it got up to temperature it had a really bad lean hanging idle, i adjusted the air screw and holy **** the air screw now actually works where as before it would not respond with the larger opening. using the 1173 needle and 35 pilot jet there was almost zero smoke at idle and no smoke at all when i was ridng around,there was no loss of top end and if anything there was a gain and now the pinging has stopped, the horrible gurgle/four stroking while riding along at 1/8th throttle has now gone. one thing to look out for with the airbox lid is it is secured by 2 rubbers on the seat,when i looked at mine the rubbers were not even touching the lid so the lid was not held down by anything, i had to make up my own rubbers to hold everything in place, that brings me to to 3 functions of the snorkel on the kdx,firstly it acts as a helmholts resonator which aids in the airbox hitting resonance frequency, secondly it raises the height of the airbox hole to allow for deep river crossing and thirdly and maybe unique to the kdx is the seat applies pressure on the snorkel to keep the airbox lid held securely in place.
throttle response and low end power is still not what i feel it should be but i am not going to try anymore jetting until the lid is repaired and i have a snorkel back in it, i have zero chance here in thailand of obtaining a snorkel so i will try make my own out of pvc pipe,i will make it roughly 20 percent bigger than the origianl snorkel which would give it a surface area of 2.4 inches
although i am happy at finally finding the cause of my awful running bike i feel bewildered that people know less now about the kdx than 17 years ago. when we put on after market pipes and rejet we gain roughly a 10 percent increase in power, why would anyone think we need a 500 percent increase in airflow to gain an increase of 10 percent power at the same RPM's ,after 17 years of this bike being released we should have designed the perfect sized snorkel NOT just say lets cut a big hole in the airbox and hope for the best,
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Re: Airbox snorkel size

Postby John_S » 08:11 am Oct 14 2018

Great to hear you're finally getting there after fighting with the bike for so long. Although our two bikes don't like "too much" air there are a lot of people that don't run the lid and have no complaints. When I got my bike I immediately took the snorkel out thinking I was going to make more power and then jetted it like that. Later I played around with the snorkel and no lid but one thing I've never done is main jet plug chops with the snorkel. Who knows, I may need to lean the main jet one size with the snorkel in and be perfectly happy with the top end power. I believe (and could be wrong) that regardless of the mods, if we started with the lid on, snorkel in, and followed the jetting guide from there our bikes would run excellent with no need to alter the original airbox design.

Mostly I've ran without the snorkel becuase I like the intake sound. I'm going to put the snorkel back in and give it a real chance with clip changes and main jet plug chops when the weather in Florida cools off.
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