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Discussion Starter · #1 ·
As I’ve become more familiar with my 2013 KLR over the past 8 months, I’ve thought several times about the intake screen downwind of the air filter. When I first removed my air filter to check and clean it, I noted there was some waxy crud accumulated in the screen and blocking it about 50%. Some carb cleaner and vigorous scrubbing with a parts brush washed it away. However that screen looks to me like a significant restriction in the intake tract.

I’d like to know who has removed theirs, and what effect it had, if any. I brought up this question on another thread last summer, but the responders either didn’t know or didn’t think it made much difference.

I hope Eagle Mike is reading this, because I know he cut the screen out of his. Mike, did you do any measurements before and after? Do you think it makes a difference in WOT flow? Did you cut out the screen with the air box on the bike, or did you have to remove the air box first? What’s the easiest way to do it?

TIA
 

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I will never be one to say "I told you so", but if you cut your screen and suck shit in I will say "I advised you thusly".

How effective at doing anything removing the screen is I cannot say, but I offer this:

As to the waxy crud, perhaps there has been too much sticky oil on the filter.
 
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Discussion Starter · #3 · (Edited)
Tom, thanks for the comments. I’m not real concerned about the screen stopping anything from getting in the engine. It would have to get through the air filter first, and if that happens I’ve got far bigger problems. Besides, as I understand it, the screen is for backfire spark suppression, to prevent forest fires. No other bike I’ve owned had one. And, if a backfire can get out though the air filter, through the air box, under the seat, and past my ass—I’ve got far bigger problems!

Thanks for the link to your previous posting of your calculations of the square areas in the air box. Clearly, pulling the rubber snorkel and holesawing several holes in the top of the box creates far more flow area than the screen or the carb throat. And your tests of AFR show that additional holes don’t make a difference.

But to my engineer’s eye, that screen causes much more flow resistance than a clear duct of equal air cross section. All those little wires cause turbulence and flow resistance individually. At intake flow velocities, I’d be surprised if that screen passes half the air that a clear duct of equal air cross section would pass. Remember, aerodynamic drag increases roughly as a square of velocity.

Just for curiosity, let’s try to calculate the air velocity on the intake side: Assume the middle of the RPM range, 4000RPM. The engine displacement is 652cc. The carb throat is roughly 4cm, so at WOT it is 3.14x2cm^2=12.56cm^2. The intake event lasts about 1/2 of a Revolution. So, at 4000 RPM (67 RPS), 652cc of air crams through the 12.6cm^2 carb throat in about 1/134 second, or .0075 sec. 652cc/12.6Cm^2=51.7cm. 51.7cm/.0075sec=6900cm/sec, or 69m/sec.
But, we’re not done yet because the flow is not constant, instead it roughly corresponds to the lift profile of the cam. Eyeballing the “area under the curve” of the lift profiles of most cams (approximate it as a triangle), I’ll estimate that the peak flow velocity is about 3 times higher than average flow velocity. This we arrive at a peak flow velocity through the carb of over 200m/sec (not counting compressibility, which is negligible at these velocities). Considering the speed of sound is about 330m/sec, that’s zoomin’ along purty quick. And that’s at 4000 RPM. At redline, it’s pushing the sonic velocity! Which gets into the regime of “choked flow” which I won’t get into now...

Now, further upstream at the screen and air filter, the velocity is lower, in proportion to the air cross section of those parts compared to the carb throat, so their effect on flow impedance will be lower by the ratio squared, but I think still significant. By the way, although these numbers are approximations based on some simplifying assumptions, they are in line with other calculations I’ve done for other engines.

Anyway, too much arithmetic for a Saturday, but you should take away the idea that, at those velocities, the spark screen contributes significant impedance to air flow. Whether removing the screen makes a real difference in the operation of a KLR engine, I don’t know (yet), and determining such will require actual testing. So I’ll defer to anyone who has done that testing.
 

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Discussion Starter · #4 ·
Oh yeah—that waxy crud, I’m almost certain it’s from dried air filter oil. I don’t know what else it could be. So anyone else reading this thread, check the screen the next time you take out the air filter.
 

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Oh yeah—that waxy crud, I’m almost certain it’s from dried air filter oil. I don’t know what else it could be. So anyone else reading this thread, check the screen the next time you take out the air filter.
I think "that waxy crud" may actually have been some 'talcum powder' fine dust stuck to the slightly oily screen! So it kept that much out of your engine already!

I also 'Know' that you need to read this. Click on all bolded or High-lighted words or phrases.
 

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I don't recommend doing the screen removal. I got an airbox from a guy that had already removed most of it, so gave it a try. I don't think the difference is notable for the street riding KLR. If you are going for land speed stuff, sure. Otherwise, no. Don't do it.
I agree with Tom. I only know of one guy for sure that lost the nut for the filter screw into the engine. It did get through the carb, and did damage. I've had another couple of heads come through the shop with damage, and i suspect it was from the same thing.
I also had a couple of guy back east that just could not solve a carb problem. We talked several times on the phone, but we were all stumped. Then it turned out some of the little rubber drain seals on the bottom of the airbox were missing. That was enough to cause the problem. everything perfect after they were replaced.
BTW, don't let your aftermarket muffler setup leak gas against the airbox either. It can cut a hole, leak exhaust gasses into the airbox, and cause damage. This has happened many times. (I was going to type "several", but "many" is more accurate)
While we're on the subject, let's not remove the airbox, ok? Maybe I might do some tests without the airbox at some point, but IMHO it's not something I recommend. One guy with a meter did some testing with just the door off the airbox. How he held his leg made a difference. I don't recall if the side covers were in place. Another guy did try just running the clamp on filter, and ended up holing his piston. I'll admit I have not done a good cycle of testing for this yet. It's way way down the list, as preliminary indications are it's just not a good idea. Maybe a good closed loop EFI system could compensate enough for it to work.
 

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Discussion Starter · #7 · (Edited)
Paul, thanks for the link to KLR Chris. My air box already has 4 holes drilled in the top and snorkel removed. I have the Uni filter too. Curiously, Chris recommends removing the screen, and his table of flow numbers shows a small improvement from removing the screen. However, small gains on a flow bench often do not translate to real gains on an actual engine.

Mike, thanks for your info too. I appreciate your experience that removing the screen doesn’t make a noticeable difference. Based on that and your warnings, I will leave it alone. I ain’t goin’ for any speed records! Good throttle response and torque at low to mid RPM are more important on an ADV bike.
 

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Paul, thanks for the link to KLR Chris. My air box already has 4 holes drilled in the top and snorkel removed. I have the Uni filter too. Curiously, Chris recommends removing the screen.

Mike, thanks for your info too. I appreciate your experience that removing the screen doesn’t make a noticeable difference. Based on that and your warnings, I will leave it alone.
I'm a little bit conservative.. I'll always try to give you what I think is the best overall advice. For instance, I'm a big fan of KLR engines that run really good in the midrange, as opposed to searching for maximum peak power at high RPM. The engines live longer that way, and have good drivability. That's my usual approach. I never want to give advice that might lead someone into a problematic situation.
I appreciate your comments and support provided to the forum.
(general note - these days I have a lot of insomnia after all the chemo and the covid. Please forgive me if I make a mistake or 20 when posting from time to time :) )
 

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Discussion Starter · #9 ·
...especially after midnight! And that goes for me too.
 

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Maybe its just me, but. I think my 2018 runs, sounds perfect for what it is. It seems to get enough air, with the 17 front sprocket, going against a 25mph headwind, holding 70-75mph, real mph, not the off speedo, at a solid wide open throughtle, to keep speed, full helmet, metel paniers, so not very airodinamic. I don't find side winds to be a problem also, no death wobble. With the 17, bike really don't need to go over 5,000 rpms.
So, air no problem, kinked pet cock vacume line, problem gitting fuel with headwind. ( not a problem anymore )
I am glad I am a little older now, as younger days, I would be throwing lots of money at it to get 2 more HP, for no reason. Take the average,,, normal, sailboat. There is a calculation that one can do, to tell hull speed, boat isn't made to go faster than that speed, if it did, it would be ugly.
The money, I totally waisted on many O Harleys, to get a little more out of it/them was not smart, a pig can only run so fast. Anyone can do the burnout pit, in first gear, and pretend to go through all gears, but keep it in first. ( seen I many a time )
 

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Curiously, Chris recommends removing the screen, and his table of flow numbers shows a small improvement from removing the screen.
The air box screen removal was his absolute last air box modification, to show us all just what it was worth, when trying to get the absolute Maximum air flow thru a standard KLR air box.
It is really only justifiable IMO, with a FULLY ported head and aftermarket cams, which he did use in his quest for his 60HP normally aspirated, EFI'ed KLR650.

With only 4 holes in the top of a standard air box, screen removal would not help anything, IMO.
 
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Paul, thanks for the link to KLR Chris. My air box already has 4 holes drilled in the top and snorkel removed. I have the Uni filter too. Curiously, Chris recommends removing the screen, and his table of flow numbers shows a small improvement from removing the screen. However, small gains on a flow bench often do not translate to real gains on an actual engine.

Mike, thanks for your info too. I appreciate your experience that removing the screen doesn’t make a noticeable difference. Based on that and your warnings, I will leave it alone. I ain’t goin’ for any speed records! Good throttle response and torque at low to mid RPM are more important on an ADV bike.
Sorry, I'm late to the party on this one but airbox airflow has been an interest of mine. The bulk of my motorcycle wrenching experience has been on offroad race bikes, the vast majority of which did not have such a screen; the ones that did (CRF's) were removeable and the one on my CRF250X was noted as being restrictive.......so I tried to remove the screen on my KLR and it was a GIGANTIC PITA......I ended up throwing the airbox in the trash and getting a new one as I couldn't remove 100% of the wires and I was worried that I'd end up with some debris inside my engine.

During my airbox research, I came across some testing info from KLRCary (Cary Aspy, RIP) and this is my regurgitated post on the subject;
A lot of missunderstanding with airbox issues.



First, the screen. Airflow gains are not linear. Removing the screen with an otherwise stock KLR airbox will only gain you 2 cfm. The engine won't know the differance. However, with a heavily modified airbox, the gain from removing the screen is 8 cfm. Depending on what else is done to the engine, you may make more power. I doubt that you'll feel it, but a dyno will show it. Butt-dyno's can detect very small low rpm changes, but don't detect higher rpm changes very well at all. You'll likely not detect it, but that does not mean it's not there.



Comparing screen removal to other models is wrong, particularly with the newer sportbikes. With them, the screen often serves another purpose as well. Air distribution in a 4 cyl sportbike can be a real problem, particularly with "ram air". Removing the screen on them can cause very real losses, depending on the model. That does NOT apply to the KLR.



Same thing with air filters. There is only about 2 cfm differance from the best filter (UNI), to the worst (K&N). With a modified airbox, that differance grows to 9 cfm.



Here is the flow chart:



Completely stock - 64.8cfm

Same - Remove snorkle - 74cfm

Same - With UNI filter - 76.2cfm

Same - Remove screen - 78.6cfm

Same - Small "L" cut - 85.1cfm

Same - Large "L" cut, open snorkle area further - 92.4

Same - Remove door - 103.2



Alternate - UNI filter, No snorkle, With screen, No door, No "L" cut - 95.4



All at 2" of water, tested at 1 1/2" and 3" and averaged to 2"



To answer the larger question, how much air can the KLR really use?

....................

A stock KLR about 70-80cfm. With a good pipe about 75-90cfm. A modified motor about 90-100cfm. Having a bit more capacity than you you need will not hurt anything. The effects are not linear though. Going from 65cfm to 75cfm you will likely notice, but going from 75 to 85 cfm you likely won't.



Part of the confusion might be due to the effects of the carb shimming. Since that mod is for a stock needle, the snorkle removal serves not just to add air, but to lean out the top end. The stock KLR till 07 is rich on the top. ( The 08 has a smaller main jet.) Airbox mods have much more effect on top end mixture, so, with the stock needle it also helps straighten out the fuel curve.



Finally the airbox door. You've got to carefull there. You don't have clean air there. It's flowing past that door when riding. You can easily create a partial low pressure area there, depending on wind direction, and where you place your leg!



Cary"





I do not recommend removing the backfire screen (PITA) or running without the airbox door for reasons that should be obvious.



Though a KLR isn't going to have a bunch more power no matter what you do, a KLX kit combined with the snorkle-ectomy and L mod or 4 - 1" holes will have it running properly. don't bother with the little 1" foam filters if you drill the holes - they are useless.



Cheers,

Dave
 

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As I’ve become more familiar with my 2013 KLR over the past 8 months, I’ve thought several times about the intake screen downwind of the air filter. When I first removed my air filter to check and clean it, I noted there was some waxy crud accumulated in the screen and blocking it about 50%. Some carb cleaner and vigorous scrubbing with a parts brush washed it away. However that screen looks to me like a significant restriction in the intake tract.

I’d like to know who has removed theirs, and what effect it had, if any. I brought up this question on another thread last summer, but the responders either didn’t know or didn’t think it made much difference.

I hope Eagle Mike is reading this, because I know he cut the screen out of his. Mike, did you do any measurements before and after? Do you think it makes a difference in WOT flow? Did you cut out the screen with the air box on the bike, or did you have to remove the air box first? What’s the easiest way to do it?

TIA

I read lots of good responses to your question so far on this thread from some of the best wrenches on this site so ditto. I would only add that the excess of waxy/oily build up on the screen is probably from the previous owner putting their freshly oil soaked air filter in and the riding it without squeezing out the excess and letting it drip dry before putting it back in. I've always soaked my air filters with the proper oil then wring it out and hang it over a drip catcher overnight then put it back in the bike the next day. I have only experienced minimal screen buildup and clean it with CRC or any evaporating carb/air intake cleaner each time I clean my filter. Just my 2cents.
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I've seen a few bikes with some build up on the screen. It was typically remnants of a deteriorating air filter. Using the wrong filter oil can result in a filter shedding foam, and sticking to the screen. Always use the correct oil for the filter you are using. Some oils are universal, some are not.
 

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I've seen a few bikes with some build up on the screen. It was typically remnants of a deteriorating air filter. Using the wrong filter oil can result in a filter shedding foam, and sticking to the screen. Always use the correct oil for the filter you are using. Some oils are universal, some are not.
Great point! But I hope we haven't opened up the Pandora's Box of another air filter oil thread? :cool:
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As Dave P eluded to, removal of the air box screen is only reversible by replacement of the entire air box. They are pricey!
 
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