IMO the screen is there mainly to keep larger pieces of dirt from falling into the carb when air filter service is being done. As mentioned, having a fire start in the air box is very unlikely. Mine is staying where it is.

 

Has anyone ever pulled a trailer wih a 4 cylinder truck carrying a trailer with a tall ramp that is made of expanded metal or blown through a porch screen? Notice any difference in pulling power or air flow? YES! That's the same thing that happens with this screen. I will try and do mine this weekend and report my findings. I've seen other threads on here addressing this issue and screen removal does improve air flow. Do a search. Thanks for the great write up YYY.

 

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cool, cant wait to see that.

exactly what i thought. thanks for the info.

 

Backfire isn't only flame, it's thought that the pressure wave from it can tear or dislodge the basically unsupported foam. I think the aftermarket "flame proof" foam is a just a marketing gimmick to get you to keep buying their filter after you paid big bucks for the screen- less cage (sold for many mx bikes).

I know crf250 hondas don't gain anything removing the screen, the 450s's very little. But many racers do take it out because it make the filter easier to get in the tight fit box. Every brand of filter is used and none catch fire.

Never seen a foam catch fire but have seen bike violently backfire through the carb (usually when the intake valves are too tight) and puff dust out from under the seat.

The engine's basically an air pump. Making the intake or outlet of a pump larger can help to a point, any more than needed isn't going to help. Whats the area of the two small intake valves when they are open? vs. the screen openings?

 

Rimbender quote:The engine's basically an air pump. Making the intake or outlet of a pump larger can help to a point, any more than needed isn't going to help. Whats the area of the two small intake valves when they are open? vs. the screen openings?

While the valve openings are small, there is the velocity to consider which can also affect intake volume. Intake/airbox design is a fascinating issue, and it's one that I surely don't have all the knowledge to comprehend all the possible elements. The shape of all the elements in an intake design from the airbox shape, boot shape, and intake manifold can all play important parts. Even that "dead air" space shape and volume is critical in some 4-stroke designs.

Even though the KLX250 doesn't have the intake requirements of say a ZX14, it certainly responds to some intake tweaking as evidenced by the removal of the ridiculously tiny intake opening on the OEM airbox lid. The aibox screen may also have a velocity blockage impact rather than just a volume reduction issue on some bikes. It's interesting stuff for sure.

 

I agree.

I think we already have sufficient air volume. Probably a more critical factor is air velocity with the minimum amount of turbulence. I think the factories have done everything they can to make this happen. It's probably not an easy task, considering the limited air box volume available.

The conventional wisdom (or urban myth?) is that air boxes must be opened up because they can't provide the air the engine needs. It would seem to me that if an engine easily reaches red line in the lower gears, then enough air is present.

To find out for sure, I decided to do some tests. I was already fairly close on my main jet, but needed to finalize the size, so that gave me an opportunity to test the snorkel thing at the same time.

Here are my bike stats:
2006
351
KLX300 muffler
KLX300 carb specs
Unifilter
Screen intact
14/39 final ratio

Not having a dyno, I decided to test main jet/snorkels by recording highest average speed with each combination. I went to a lonely desert road near my house and did two WFO high speed runs, one in each direction for each of the combinations. I went both up and down in main jet sizes. With each main jet I made runs with both the oem snorkel and the KDX snorkel.

In every single case the oem snorkel out performed the KDX snorkel - the average top speeds were higher. Additionally, throttle response and acceleration were much smoother throughout the rpm range.

Disclaimer: I know this flies in the face of internet wisdom and most likely would not apply to any other bikes...

Ron

 

Ranger Ron, that certainly was an eye opener! I think your thoughts on air velocity must be the key, as I can not see any other logical explanation.

When I first read your post I missed that you have the 351 kit installed and was thinking the lower cc'c might help explain/justify the results, but once I saw you have the 351 I was really shocked!

I will soon be installing the 351 kit, pumper carb, Megabomb and Q4 and will likely remove the screen. After that I may also try the two different snorks to see if I get the same results as you did. Although none of this will be before 2 - 3 weeks from now.

 

Went to pull my screen off and, ......................... mine is an 09 and has f$%&ing phillips screws that go through the screen housing and into the airbox. As YYY said, the lower one is a B@&%$. I tried to put a short phillips bit on the end of the long noze vise grips and decided that I will have to think on this for a while. I can just see a cheap *** screw head being rounded off. Will see if there is a better way when I have more time. Maybe the 06 and 07's have studs with nuts. The air box on the 09 looks to be two parts, making attachment of the screen housing easier durring assembly but not so on disassembly. Will let you know what I do.

 

Yeah, I got my 2010 model's screen removed by the guy who did my rejetting. He didn't bother removing the screen, he cut the mesh out with the box in-place while the carb was off, then cleaned it up before putting the carb back on. Might be the way to go.

 

Looking at this photo, I decided to try inserting a metric allen wrench into the drive end of the socket and it worked! Loosened up easy as you please.



Getting these nuts back on the studs could be tricky. I'll find out tomorrow, then I'll add my new K&N