Blow Back Screen/'09 KLX250S
 

Yesterday I removed the blow back screen between the carb and air filter, might be a little lean but noticeably stronger from 4.5K through red line.

So far; Cam mod (removed ACR completely) rejetted with Keihin jets (N1TC needle jet, #128 main and #40 pilot), DG can, stock header, now the screen removal.

Ain't no power wheelies, I'm a big guy, but for what it is, it pulls very strong and 50-70 mph and I'm getting 55-60 mpg.

Great bike! I have 6 others all much bigger, but I really enjoy the KLX, reminds me why I started riding 45 years ago. Light on it's feet, zippy, comfortable.

I really need to do this mod.. Don't I ?

No idea what specific power increase it produces, but every 4-stroke dirt bike I've owned since an '84 XR250R...I think about 10 up to now...have required a one-step richer main jet when removing the backfire screen...everything else being equal, of course. With an aftermarket pipe/muffler and everything jetted optimally, as soon as I removed the backfire screen and made a WOT run, I could detect a small but discernable detonation from a lean condition. One step up on the main jet, and all was well. Of course this says nothing about a specific power increase, and it says nothing about how this relates to every other bike out there. However, increasing airflow that requires a richer main jet almost always translates to better performance. Obviously a dyno, very consistent 1/4 mile test times, or other such monitoring would answer the question, but I think it's pretty conclusive just taking the cue from the detonation and jetting. Since it's mainly a WOT area of performance, it's not one of those seat-of-the-pants deals that you can feel...like a pumper carb, proper pipe, proper jetting, and other such tinkering. It costs practically nothing but just a little time to do, so what the heck...give it a go.

Removing the screen is not exactly a walk in the park getting the screws out of the inside of the airbox was a bit of a scream, I cut the actual screen out with a utility knife, and then used a small drum sander to smooth the edges of what was left.

I replaced the screws that held the screen in the air box with some screws that had hex heads which made it a lot easier to screw them back in when I was ready to reassemble.

Well, since I'm the only one that rides it now days.. guess I'd better remove the screen and see what I get.. Current carb setup could use a little more airflow and is set for max increase off the screen removal.. I was afraid my son would wreck with some chick on back, catch fire, and I'd be sued for removing the screen... You know if it gives another HP up on top, that'd be close to 24hp and I'll be DONE !

Your mention of the possibility of fire was, as you know, the reason for the backfire screen in the first place. However, over many decades of owning bikes with the screen removed and riding with many other owners of bikes with the screen removed, I've never seen a fire occur in those bikes. I've heard and seen internet stories and pics of black smudges from backfires on the inside of foam filters in the worst cases but not an actual fire. In cases where you have a backfire in a modern motorcycle engine bad enough to cause a fire, it's probably accompanied by some serious fuel and/or ignition issues. Most bikes don't have adjustable timing for a goober to screw up, so severe ignition ignited fuel fires aren't at all common like the days of adjustable distributor controlled ignition. It's good to be safe, but the backfire screen may be more of a lawyer holdover.

Yep, I agree..

So guess I'll fire up the cutting torch and warm up the saws-all.. Actually I think ARL said he pulled the rear suspension n swingarm to get the airbox out..

I did the job just taking the seat off and removing the air filter.

Yeah, 1/4" drive swivel socket set allowed me to remove that screen mount without much heartburn. The bulk of the work is making a clean job of the screen removal, which also, wasn't that hard, just a little tedious.

I removed the screen and had the same results as others. It felt a little off and needed a bump in the main jet. It does flow more air, but I did see a yout*be video of KTM on fire due to this mod. I run a K&N so I feel a little better as it has a built in screen, might be a false sense of security. I pulled the airbox out and disasembled, made things easy.

I cut mine out and gained 50 hp! http://voices.nationalgeographic.com...picture-81.jpg

Crap!...I only got 47hp out of my screen removal.:mad:

No big deal... they were just itty bitty ones :rolleyes:

I was about to do it, but it seemed like the risk of dropping screws down the intake manifold was very high.

I guess they would hang up against the carb slide and could be fished out with a magnet.

When I talked to DynoJet recently they did not even recommend taking the airbox cover off. Made no difference on their dyno tests. Snorkle, yes. Cover, no. Some air resistance is needed for the intake system to function.

Go figure. Lot of variables in the equation.

Removing the entire airbox is not that difficult. Makes removing the screen much easier.

Yep, KawaBiker, that is the knowledge they have and the data they have to go off of - and for a very good reason - call em back and ask what exhaust system they used on the "mule" they created the kit with.. I can tell you - It was dead stock! Now I'm not gonna knock DJ at all - I have nuthin-but-love for em.. The fact is that using their kits on bikes that are modded beyond the test mule they used to create their kit/data means dabbling in uncharted waters - and you do this by simply bolting on a good aftermarket silencer.

It is true that the MAJ and the PAJ (pilot air jet) are sized by Kawa for proper operation with the lid and snorkel - and they are not replaceable. So removing the lid means a large increase in airflow with an attending loss of velocity as well as altered vacuum gradients throughout the carbs' circuitry. This simply means the carb must be setup properly for being lidless. I've done this. The results are mid and top end power that I'm sure cannot be had with a lid.

If there is ever a dyno test with standard stage ll that has curves superior to mine, I'll be the first to put my lid back on.. The DJ specialist did admit, last week, that I'd probably loose top end HP and TRQ if I put my lid back on..

Ok, removed the screen.. I don't know guys, my 2012 had a single, open mesh, screen that was really free flowing.. Oh well, hopefully picked up HP or partial HP on top.. It's starting to warm up here in N Tx - in the 90's.. Probably should put back in the DJ140 main jet due to the higher altitude density..

Man, have we been spoiled by the mild spring temps up to this point in Texas, or what? And rain...what's up with that?:D

NeoTx specialties: Frog-Kabobs n Chicken Fried Turtle n Gravy?

Read through this study on mesh screen aerodynamics and see. http://www.inia.es/gcontrec/pub/273-...5240539140.pdf

There must be some increase in pressure and density at the carb inlet with the screen removed logic dictates, but is it enough to warrant the effort? Could there be negative consequences? Are they in the bike's design due to forrest service regs as in exhaust systems? Or just due to lawyers?

It is entirely possible that the size of the opening with the snorkel out provides enough flow for the engine. You can't jam more air in unless you set up a sealed ram air system, so any bigger an opening does nothing.

Of course I did the KDX snorkel with the larger opening because I don't want to risk water in the air box should I end up splashing about a bit when off roading... creek crossings. I measured up the snorkel opening to figure the area - pretty much on par with the largest part of the carb bell mouth at the very tip.

So it may not be resistance, it may just be exceeding the limits of air flow requirements for the engine.


One thought on the backfire screen -

With a wet filter, wetted with oil that would take a serious amount of time exposed to flames to light, I doubt it really would do much that the air filter couldn't do in the "lightning strike" chance of a true backfire through the carburetor.

On the other hand, anything in the way of flow is blockage. Fact is most screening used in any sort of automotive work is rated for how open the screen is. I'd bet that backfire screen could have up to 40% blockage. So removing it will go from 60% open cross section to nearly 100% disregarding the bridging that holds the fastening nut.

That screen might have had value when intakes were shorter and dry paper filters were used, but not so much now. For the exhaust is if the screen was required by departments of forestry it would likely have some sort of stamping, like exhausts do. The spark arrestor is likely more for stopping hot bits of carbon from blowing out as it is for the actual flame.

Mentioned before, but Dirt Bike Magazine has dyno'd many a bike pre- and post-screen with no power gain. Riders notice improved response.

Scott, if I recall those tests were on "real" dirt bikes, weren't they?...like KTM, Honda CRF250/450R, Kawasaki KX series, etc. I don't remember seeing dual sports in those tests. The dual sports have to meet sound/db restrictions, and I think the larger, more open intakes on race bikes are designed to provide max airflow in a competition bike with a screen. I may have missed it if any of them covered dual sports.

It's Dirt Bike Magazine. I'm sure you're correct. But why would you expect pre- vs post-screen horsepower increase in a dual sport that's already had the air flow opened up with lid removed and with aftermarket filter? I've not owned a race bike. Is the screen itself that much different between the two?

Perhaps klxster's testing will show us something?

No, the screen material/design isn't different, but the size is larger from the ones I've seen. Seems like they take into account their intake size, airbox volume, and screen opening size to meet the requirements of the engine's breathing needs. Just doesn't seem to be as important on most dual sport designs where intake noise is a consideration, and tons of power aren't present. If I recall, isn't the air filter on your KLR about the size of a softball? Pretty small for a 650...even it does only have about 34hp.:D

Yes, I think the KLR air filter is just about the same size as the KLX's.

I removed the backfire screen on my KLX250/300 and can't say that it had any performance effect. This is seat of the pants testing.
When I had the Keihn carb with modified jetting I tried the KDX snorkel and had a slight boost in response.
Then I removed the lid and felt even more response so that's the way I ran it
All the dirt bikes I've owned over the years (mostly 2 strokes) did not have a backfire screen and I've never had a back fire on the KLX so I saw no need for the screen.
I've since upgraded to a Mikuni TM36-68 and that again gave me a really good boost in throttle response.
I don't have any scientific proof but I believe the less resistance you have for air going into the carburetor the more air fuel you'll have to the engine.

Here's some information on removing the backfire screen Backfire Screen Removal

Doug