Please help with air box mod


Mar 27, 2002
01 220R
stock air box
stock sprokets
stock silencer
stock air filter
FMF rev pipe
jetted to pull.

The installation of the rev pipe did certainly take away some bottom end over stock on..

OK,,,I decided to pull the snorker out and see how it would run..Improved mid and top end as expected but I just could not dial the pilot circuit in..I tried mirco adjustments from fully seated to better than 3 1/2 turns out.
It just seemed to have no bottom end and then would rev up fast in the mid.
Put the snorkel back in and,,,,,tada,,,,bottom end returned with a smooth transistion between pilot and needle circuits.

This really puts a damper in attempting a complete air box mod.

I am trying to get a great all-round tractor.
Love the mid and top end gains from the pipe but miss that bottom end grunt.
Torque ring is my next installation.Reeds to follow.( V-Force Delta II perhaps??heehe )
If I can't retain or regain the bottom end I just may have to install the Torque pipe sitting in the garage.
Boy will I miss the top end.
Oh well,,,maybe I just can't have everything.




Apr 30, 2002
I can't help you with snorkel/bottom end problem. Modifing the air box helped my 220, but I run KG-35 (torque) pipe. I didn't want to loose any bottom end. I have the DF II (high tension), it gave the bike more snap and a tad more top end. Just recently got a RB modified Air Stryker carb for my bike. I have to say that's the best performance mod I've done so far. No loss of bottom end, throttle is crisp, the bike pulls harder from bottom to top. Oh yeah, it has quite a bit more top end now. :)


Mar 27, 2002
Now that's what I'm looking for..

My initial thoughts were to go with the rev pipe and work on the bottom end.

Perhaps I should re-think it.

I've read alot about the RB carb mod and the V-Force Delta II reed set up.

Sounds to me like you have the bike I want.
I'm thinking I may install the torque pipe and try it out this weekend.
Our riding is split between tight/tech. woods and long double wide trails.

I do perfer torque over hp so I guess the gnarrly pipe it is...




Mar 27, 2002
Stock pilot.
a/s 1.5 to 2 turns out with snorkel on weather dependant.
stock needle 3rd clip from top.

with above settings the bike pulls,,,,well,,, pretty nice...
I can lug it up almost anything without the need to feathering the clutch.

no,,,not as much torque as stock but not bad..

of course however,,,we always want more....or at least the best performance as possible.

any recommendations??


John Harris

Apr 15, 2002
If you have spoose from the exaust, and the other symptoms described, I expect you need a leaner pilot/primary jet and a second from top needle clip. In short, sounds like a rich problem on the bottom to me and takes awhile on trottle to clean it out!


Jan 23, 2003
First, I would check your pilot jet and see if it is too rich. The stock jet on my 03 KDX 200 was very rich (and so was the main jet). To check, turn the air screw out about 1/2 turn so it will crank. Once you crank it, keep count and turn the air screw out 1/2 turn at a time. Your idle RPMs should increase as you turn it out. Keep a count of how many full turns you back it out before it does not increase the idle anymore. It should not increase RPMs beyond 2-3 (max) turns out. My stock 48 was super rich. If it continues to increase at 4-5 turns out, stop and go down about 3 points on the pilot jet and re-check it. This made a huge difference in my off idle power, so much that I could change my gearing to cut a little low end and get more high end.

Good Luck-


Mar 27, 2002
Please help me understand this.
Removing the snorkel will allow better breathing and increased air flow..Would this not lean the pilot circuit??

As mentioned,,,I have made small adjustments to a/s.Fully seated to better than 3 1/2 turns out with snorkel removed.
Yes I attempted to set using the idle speed method...No it did not help.
As mentioned....I have an aftermarket pipe...(FMF rev).... and thus the stock pilot works fine..

Snorkel in place gave best results with a/s set to 1.5-2 turns depending on weather conditions.
With snorkel in,,,,plug readings (and seat of the pants method, which I perfer ) on all circuits are correct.Transistion between circuits is clean and smooth.Bike pulls hard throughout.

I just can't figure out why I loose bottom end and why I can't dial in the present pilot jet with the easier flowing setup.Again,,,,would removing the snorkel not result in a lean condition on the pilot????
Setting a/s to 1 turn out was too rich and it seemed to not clear up even at 4 turns outs.

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Oct 14, 1999
re: 'I just can't figure out why I loose bottom end...'

1. The pipe you chose.
2. The jetting you are running.

re: 'why I can't dial in the present pilot jet with the easier flowing setup..'

1. Because your present pilot jet is probably wrong (see above).

If torque is what you're after, I don't know why you chose a -30 pipe.

The RB mod will improve both bottom-end response AND provide a more linear response throughout the lower throttle postions.

So will a good reed system.

So will a final drive ratio change.

BTW...'gnarly' used in reference to fmf pipes doesn't tell you a thing (regarding performance profile).

Removal of any restriction on the upstream side will reduce the pressure differential the carb works with resulting in an overall more lean condition throughout the throttle range. Not necessarily lean.....but moving in that direction. It will effect every circuit in the carb...and the way they relate to each other.
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Feb 6, 2001
You say you have a "REV" pipe and a stock silencer? Please excuse me, but that is the problem. A Rev pipe is for the top, and the bottom end is sacrificed. Also, get that stock muffler off of there. At least find a stock KX250 silencer, and make a middle bracket for the bolt hole to line up. The stock bike will run awesome with a MX silencer and FMF Gnarly Woods Pipe. Drop each jet one size leaner than stock, and the power will be there.
2001 KDX200
2002 YZ250F


Mar 27, 2002
Thanks but....I think everyone is focusing on the wrong thing. :ugg:

I like the overall performance with the rev pipe AND snorkel left in the airbox.Including the bottom end although slightly less than stock.
I know how/why/what the different pipes do and how they affect performance.
See HERE for some nice expansion chamber info.

It is jetted proper with the above set up.Well,,,,the way I like it anyway and hey,,,,that's what counts.

I'm not necessarily looking for huge bottom end improvements with my current setup.I know this is not possible.

I was merely playing around with the snorkel and was curious as to why there was such a HUGE loss of bottom end ( 0 -1/8 throttle ) with the snorkel removed as compared to having it installed ( with my current setup ). It was gargling even at the best a/s setting possible...

New pilots on there way. 1 richer and 1 leaner. I need them for seasonal weather changes at any rate.

Anyway,,,,,I'll figure it out and post my findings...

I do have a new torque pipe but will tinker with that at a later time..
I want to play with what's presently installed.

My final set up should be something like this.:
torque pipe.
RB carb mod
V-Force Delta II Reed Cage
Complete Air Box Mod ( perhaps I'll just toss the lid)
Final jet setting to match carb mod and my riding conditions.
00 XR400 front springs.
Rear race sag set to 100mm( done )

Buc :thumb:
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