DirtRider . Net MX, SX, Arena Cross, Off-Road Community

This is the text version of DirtRider.Net
Click Here for the Full Version

Pages: 1

Air Box, on or off?
(Click here to view the original thread with full colors/images)

Posted by: Asrith---------------------

Hello all, I must say this is a great forum. Ok, so can anyone tell me what is up with having the air box lid or off? I hear off is good, but not sure. Thanks!

Posted by: smb_racing---------------------

taking it off allows more air into the motor due to the larger opening. It will lean your jetting though so you'll need to go a tad richer. More air = more horsepower :D

Posted by: OLD-N-SLOW---------------------

Try this link to KDX heaven. http://justkdx.dirtrider.net
All will be explained. Use the force Luke
And yes loose the lid or drill large holes in it.
I would not richen the jetting though, if your jetting is stock.
From the factory they come very rich already.
Well maybe I'm getting ahead of myself. I'm talking about the 95-01 models.
Steve

Posted by: Mac---------------------

If you remove the lid you will have to relocate the "black box". Just drill some holes or remove the rear section. Check this other thread. Airbox Mod

Posted by: Asrith---------------------

thank you! I am glad to have seen the picture, this looks to be a little better than leaving it off I supose. :silly:

Posted by: canyncarvr---------------------

Keep the lid on. Drill it instead.

Running without the lid gets you a 'honk' noise every time you twist the throttle.

Some KDX'rs actually LIKE that ignominious racket...

.....go figure....

Posted by: freydog---------------------

I have a 00 kdx200, and I removed the airbox lid, only to put it back on about 6 months later. I'm running a totally stock bike, no pipe mod and stock jetting. With the airbox lid removed, the bike did have more midrange and upper rpm punch, but the drawback was a nasty bog right off the bottom if I tristed the throttle too quickly. This made slow, tight trails with steep sections much harder to ride, but the bike ran better in all other terrain. I tried jetting the bike richer to get rid of the bog, but then the engine would get loaded up in the slow sections. I eventually got tired of messing with it and decided to put the lid back on. It's a shame because there seemed to be a lot of potential there, even with the stock pipe.

Someone help me out here, but I think the airbox lid removal trick only works if you open up the exhaust with an aftermarket pipe.

Posted by: FlashBurn---------------------

I tried running with lid off on my 98KDX220 and my 01KLX300 and found that I liked both bikes better with the lids on.Both bikes have exhaust mods.On the KLX I run the Stroker headpipe and muffler, stock jets.On the KDX I run a K35 pipe, 48 pilot,155 main,needle on second clip.Both bikes run smooth from top to bottom.With the lid off neither bike would run at a steady speed without sputtering a bit.If there was an increase in power anyware I couldn't tell because I hated how the bikes ran.It's more important to me to have good running bikes than it is to try to get every last drop of horsepower.Good Luck!:D

Posted by: TexKDX---------------------

Keep the lid, pull the snorkel, and drill 1/2" holes around the back raised part. Also use duct tape behind the tank and along the sides of the subframe rails so that front wheel splash does not come up under the seat and dump into the airbox.

Pulling the lid allows too much water to get in the box from my conservative perspective.

Posted by: OLD-N-SLOW---------------------

Texkdx,
Good idea! I had heard that mentioned before but did not understand what was meant, its all clear now.
FlashBurn,
Have you had your carb bored? If it is still stock 33mm i think you may be way
rich on your jetting. Might try 42 pilot and 145 main.
freydog,
Might try to loose the lid again and try a 155 main 45 pilot and adjust the clip position to get rid of any bog that may appear.Raise the needle(lower the clip) untill it disapears.

None of this is written in stone, just my opinion.
Steve

Posted by: canyncarvr---------------------

Quote:

. I tried jetting the bike richer to get rid of the bog, but then the engine would get loaded up in the slow sections.

Jetting what richer? Having a 'nasty bog' right off the bottom, you did what? Change the pilot?

If 'right off the bottom' you mean from idle, that's an air screw adjustment problem. If you are adjusting that circuit to 'hi idle speed' and leaving it there, that's a problem.

Adjust the airscrew to best THROTTLE response, which will almost ALways be IN (clockwise turnage..richer) from the 'hi idle speed' setting.

Make sure the bike is warmed up to operating temperature. Turn the air screw IN by 1/16 increments until a quick twist of the throttle shows NO hesitation at ALL...just snaps your neck!!

This is true even when you have the 'correct' pilot in the carb.

RE:

Quote:

..lots of potential there...

Oh yeah. My ride is an '00 200. There's all SORTS of potential in a stock trimmed bike.

DON'T waste your time riding a bike that fails only with the want of a turn of a screw!!

RE:

Quote:

..more air..larger opening

Well...in a word....no.

It's a matter of pressure differential..NOT 'more air'. A difference in pressure is what causes gasoline to be PUSHED from the fuel bowl into the carb. Removing the airbox cover DECREASES the pressure difference (it's easier to 'suck' air through a larger opening), which ends up with LESS gas being 'pushed' into the carb venturi. That's why you jet RICHER when you decrease the pressure differential..not because there is MORE air.

Posted by: Lutz---------------------

Quote:

It's a matter of pressure differential..NOT 'more air'. A difference in pressure is what causes gasoline to be PUSHED from the fuel bowl into the carb. Removing the airbox cover DECREASES the pressure difference (it's easier to 'suck' air through a larger opening), which ends up with LESS gas being 'pushed' into the carb venturi. That's why you jet RICHER when you decrease the pressure differential..not because there is MORE air.

No, that's not right. Taking the airbox off actually increases the pressure differential. Remember, the float bowl is vented to the outside atmosphere.

Quote:

It is easier to 'suck' more air through a larger opening

This means MORE AIR into the carburetor. More air into the carb means higher velocity through the venturi, meaning lower pressure in the venturi. This actually forces more fuel through the jets, and would require you to jet leaner...BUT the larger amount of air going into the engine offsets the richening effects (the increased air flow leans the mixture more than the increased gas flow richens it). That is why we jet richer when taking off the airbox lid.

The same principle explains why 220's with stock carbs are jetted leaner than 200's. Both engines flow approximately the same amount of air (actually the 220 does flow more, but that only concretes this example), but because the 220 carb is smaller, the velocity through the venturi is higher, resulting in lower pressure in the venturi. The lower pressure in the 220 carb means that the pressure differential is higher than with the 200 carb, and more fuel is forced through the jets. That's why a stock 220 has smaller jets than a stock 200.

If it were true that by jetting richer, we were just making up for lost fuel flow, it would mean that there would be no power gain from removing the airbox lid. Clearly that is not the case; the reason anyone removes the airbox lid is to increase power!

Short and simple, richer jetting is required when you remove the airbox lid because more air is flowing into the engine. More air allows more fuel to be burned, and this makes more power. Hope this helps to clear things up.

Note: All statements in this post assume that your jetting is optimal before a change is made. Any change in jet sizes is done to return to optimal jetting.

Posted by: OLD-N-SLOW---------------------

Except that with stock jetting they are already to rich. I cant think of an application where you would want to jet richer than stock on a kdx.
Maybe an artic winter.:confused: I think it would be wrong to tell someone
with a stock kdx to go richer when removing the air box lid.
Steve

Posted by: TexKDX---------------------

How about while riding the floor of the grand canyon while an arctic cold front passes ??

Posted by: Badgas---------------------

Tex,

Where you been? Wife and I are going to Skull on Saturday. Hope to see you there. Give us a call.

Greg S.

Posted by: Lutz---------------------

I agree that it is pretty much a given that stock jetting is way too rich. My saying that you jet richer when you remove the airbox lid assumes that your bike is already jetted optimally. My bike actually still has stock jetting with a Pro Circuit pipe & silencer, boyesen reeds, and the airbox lid modified, but it is still too rich! I'll get it jetted right some day, I just haven't had time; going away to college in the fall really puts a damper on playing with my toys!

Posted by: Mac---------------------

Quote:

Originally posted by Lutz

No, that's not right. Taking the airbox off actually increases the pressure differential. Remember, the float bowl is vented to the outside atmosphere. This means MORE AIR into the carburetor. More air into the carb means higher velocity through the venturi, meaning lower pressure in the venturi.

I'm not sure I agree with you Lutz. Some of what you are saying is true but think of it this way, If your air filter gets clogged your air flow decreases and the pressure in the carb drops because not enough air can flow through the filter to equalize the pressure in the carb causing the gas (which is at atmospheric pressure) to try to equalize the low pressure area richening the mixture.

The low pressure is created by the piston and the carb acts like an air restrictor which causes the pressure to drop, the venturi just helps drop the pressure even further. The gas in the bowl and air both at atmosphere see this low pressure and race to equalize it. Removing the airbox lid allows the pressure in the carb to equalize faster and as a result the pressure in the carb is closer to atmosphere and less gas is drawn up the jets.:think

So what I'm saying is that you DO NOT NEED air flow to create the pressure drop. To prove this just remove your airbox and seal the air inlet of your carb with your hand. Have a friend kick the bike (plug wire removed) and you will see that even with no air flow you have created a large vacuum. Do this a few times then look at your plug, I bet its fouled.

Posted by: Lutz---------------------

Oh yeah! Now I am beginning to see the whole picture. What Mac and canyncarvr said is partially right, but what I said is partially right too.

When you take off the airbox lid, the pressure at the inlet side of the carburetor is greater (remove the restrictor-airbox lid- and you eliminate the pressure drop in the airbox). Essentially, air and fuel are pushed into the carburetor to balance the very low pressure on the outlet side of the carb. With the airbox lid ON, the fuel is under comparitivly higher pressure than the air at the inlet side of the carburetor. Therefore, the fuel is pushed harder than the air, and baseline jetting is set to get the appropriate fuel air ratio in this case.
With the airbox lid OFF, the pressure on the fuel and air is comparitivly more equal. Therefore, after the airbox lid is removed, the fuel isn't pushed as hard and the air at the inlet side of the carb is pushed harder. This means two things: there is less fuel pushed through the jets, AND more air pushed through the carburetor; both lean out the air fuel mix.

BUT, there is more to consider. The pressure differential that pushes fuel through the jets is NOT independent of the airflow through the carburetor, because they enter the carburetor into the stream of moving air.
This is where things get complicated. We have established that more air flows into the engine after the airbox lid is removed. If there is more air flowing through the carb, then it must be flowing faster to get through the same opening. If the airflow through the carb is faster, the pressure in the carb is lower. If the pressure in the carb is lower, than the pressure differential on the fuel is higher, and more fuel will be pushed into the carb.
This would make the air/fuel mix richer, but not when other factors are taken into account.

Now, how the pressure differential on the fuel is made depends on throttle position. When the throttle is closed, there is a very large restriction to airflow, so very little air can be pushed into the carb. As a result the fuel is pushed very hard into the carb, through the pilot circuit, because the main circuit is blocked by the needle. That is why the pilot jet is much smaller than the main jet: less air getting in can burn less fuel, so the fuel needs to be restricted. As the throttle is opened, the fuel isn't pushed as hard because the air is being pushed harder, but because the air is being pushed harder, it goes faster, creates low pressure, and the fuel is then pushed harder at the same time. Damn complicated!

At wide open throttle, the fuel circuits are primarily exposed to the air stream moving into the cylinder and the low pressure it creates, and depend very little on the low pressure created by the piston. That is, at wide open throttle, the low pressure on the outlet side of the carb is balanced by the high pressure on the air at the inlet side of the carb, and the fuel circuits have very little to do with balancing the pressure. However, fuel is still pushed into the carb at wide open throttle because the air is moving through the carb (and over the fuel outlets) at high velocity, and therefore at low pressure.

Quote:

If your air filter gets clogged your air flow decreases and the pressure in the carb drops because not enough air can flow through the filter to equalize the pressure in the carb causing the gas (which is at atmospheric pressure) to try to equalize the low pressure area richening the mixture.

Right, but this brings us back to the baseline jetting idea. When the filter is clean, it flows some amount of air. This air travels through the carburetor at some speed, and interacts with the fuel circuit to balance the pressure on the outlet side of the cylinder, in the many complicated ways stated above. When the filter gets dirty, what happens is the opposite of when the airbox lid is removed. Even less air can flow, so the fuel circuits must make up for that, flowing more fuel, but at the same time, the velocity of the air through the carb drops, raising pressure, and letting less fuel in. And off we go down the same complicated path. If you totally cover the carb, then the only way to balance pressure is through the fuel system; so, no, you don't need airflow to create a pressure differential, but the real world situation is much more dynamic than that.

Anyway, I'm sorry if this isn't totally clear, but it isn't really possible to describe all the interactions of carburetion without writing a book, I guess. Mac, thanks for helping me to clear up my own understanding of what's happening, even if I didn't help anyone else. In the end, I still have to come to this conclusion: people remove the airbox lid to get more power; the only way to get more power is to burn more fuel; the only way you can burn more fuel is to have more air in the mix; this means that there is more air going into the engine after the airbox lid is removed, and that is ultimately why it is necessary to jet richer when the airbox lid is removed.

I love this forum! Keep up the great discussions, guys!

Posted by: Mac---------------------

so to sum up:

airbox lid off = more power =

Text Version Home

This is the text version of DirtRider.Net Click Here for the Full Version

Air Box, on or off?

This is the text version of DirtRider.Net
Click Here for the Full Version