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