OK, i thought I had my 2009 klx250s all dialed in, 2000 miles with my full Muzzy set up, jetted, Uni filter, snorkle gone, all broken nicely... then on sunday i took my seat off to clean the filter ,and thought i should try riding it with the airbox lid off... what a huge difference! like i added 5-10% more power on both ends. other than the water getting in issue, and more freq. cleaning of the filter anyone know of downsides???

 

You main jet is even close to correct EITHER with lid on OR with lid off.

In other words, the MJ was too big before and now it better, or it was correct before and now you're too lean.
The air box configuration makes huge difference in WOT jetting. The good thing is that jetting can be quite far from correct and it still "feels great".

Many people go way large on the MJ with the stock carb and then find great improvement in removing the lid or snorkel. Well.... that's because the overly big main now works properly with the reduced venturi vacuum.

--
Mikko

 

What main jet did you have in there? As Finn suggests, it sounds like your main jet is now more compatible with the air volume. There are 2 main components in the "pump" that is your engine. The air/fuel coming in and the exhaust going out. You just let more air in to mix with your fuel, and you already had a bigger exhaust outlet with your Muzzy. Your main jet must have been on the slightly rich side. In WOT runs just make sure you don't hear any detonation, and you should be good.

 

Kawasaki recommends removing the lid when you do THEIR "race" mods for the KLX. I've ridden plenty of water with the lid off without any problems. Speeds up filter changes too as the lid doesn't have to be removed.

 

Nice pic "dirt diggin **** gansta of pwi"

time to get wet...

 

Yep, my lid has been of since the day I got the bike jetted and the Muzzy installed. The mechanic that did my bike told me to put it with the other stuff we took off. LOL

 

I took my lid off this afternoon (never had snorkel ) and took it for a spin. I didnt notice any difference at all.

Running 130 main, thick plastic spacer on needle from jet kit and 2 3/4 out on the mixture screw. (slide not drilled)

Going to pull my plug tomorrow to see whats up.

 

You gentlemen with much more knowledge than me on this subject, please help me understand this jetting thing. My understanding is that a very restrictive snorkle/box top on, creates a greater "vacuum pressure" across the main jet (at a given rpm) than compared to the vacuum pressure when using a much less restrictive intake system...therefore the main jet acts a bit richer than one would expect with the stock system. So, when these restrictions are removed, the vacuum pressure is not as negative, and a slightly higher jump in jet size (than simple math might lead one to expect), is required in order to get enough fuel in the combustion chamber. This is also the reason that fuel economy doesn't take as huge a hit as one might expect by the relatively large jump up in jet size....the much larger jet isn't adding all that much more fuel to the system than was seen with the normal jet with intake restrictions in place.

Am I thinking along the correct lines on this?

Thanks for helping me understand the physics....

 

I don't know. The ultimate physics and engineering are over my head. One reason I say that is because there are more factors at work in a fuel delivery system than just the ones you mention, aren't there? At least with most carbs, air flow and velocity across the venturi are the base elements in pulling fuel up through the fuel circuits at given slide positions...pumper carbs and other manipulations aside. This air flow does indeed cause a vacuum. But if it was all so simple, we wouldn't have ram-air systems, some designs that benefit or almost require a huge dead air space, and other various oddball designs that have either worked, failed, or only achieved mediocre success. The devil is in the details, as they say, and I'm not smart enough to discern all of them.

That said, there's been enough time and experience with the KLX250/300 engine over the years to pretty much get a decent handle on what works with its CV carb. Therefore there's probably little left that's a mystery. Even though the finite science and physics may have some unknowns to all of us, there's been enough trial and error, success and failure, to nail down quite a few "absolutes" for what works well on this bike and this carb.

 

Thanks. Certainly know the if's and's and but's have been figured out...even if it was by trial and error. Just have a curiosity about the physics. Interesting you should mention postive pressure intake systems...most are in Fuel injection systems, I'd think. With venturi setups, it seems it could screw with the flow (and therefore, pressure gradients) across the jet, leading to a whole 'nother set of complications. Obviously...I just don't know very much about it. Thanks again!