my buddys got a Baja and I a 2000 DS I made a mod-endcap,no air box lid. he runs a FMF powercore 4 w/ k/n jet kit. I he noticed that his responce was slower with the air box lid off . we thought it was the jetting. we would drag down the road and he would run better with the lid on. to back this up while idleing off the quad he would hid the gas hard with out the air box lid it hit the rpms slower. with lid on faster. well after thinking the vacume caused in the air box with the lid on opend the slide in the carb faster in turn the harder hit.
I tride the same with mine and I got the same results. it hit harder. and seemed just as fast on top.

any of you that run with the lid off pu it back on and see if you get the same results

by the way I run a mod-end cap stock filter, clip on last position on the bottem , and a 170 main. I got diffrent end caps for noise, so it sounds sweet .
this set up hits harder of the bottem than a fmf 4 but it not as smooth

 

Take a look here DS jetting/mod info... to compare jetting with other machines. It's sounds like you might be way lean on the main jets. I would try stage 3 dynojet/k&n jetting maybe 195/190/...k&n jetsdepending on your elevation ,clip on 3rd notch, 185/180 mikuni main, clip on the last notch just to see if it's any better.

 

With a Dynojet kit or K&N jet kit, clip on third notch is the key - you need to follow stage 3 instructions with no lid. With stock needle - bottom groove. Dynojet mains are different than mikuni jets - #'s don't mean the same thing - take a peak at the conversion chart on Buttfungus' sight.
FYI - the vent tube from the carb that goes into the air duct is under the slide diaphram. The vaccuum above the slide diaphram comes from the two holes down the center of the slide.
I run lidfree and my entire power band is much stronger than stock, including bottom. Its just trickier to tune. Removing the lid increases air flow, and without jetting up the mixture is too lean and power is lost. Most guys I know of with modded end caps run a 180 mikuni jet at sea level, 190 Dynojet.

BTW - You interested in joining a few of us for a ride at Silver Lake Sept. 17-18? We're camping at Jellystone, riding the 17th, 18th, and coming back home the 19th.

 

I know it seems Im off my rocker.I wouldnt beleve it my self. but the Baja is running a fmf powercore 4 , 195 dyno jet main, k&n filter, clip on the 3 from top. and it runs dogger with the lid off. In nuteral he wackes the throtle hard and you can wach the rpms build slower with the lid off. with lid on it reves instantly and harder. so thats why I figured it was all in the vacume lifting that slide faster. if you put your hands over the air box and hit the gas the vacume caused by the engine reving is enormous.

get this my ds with mod end cap 170 mikuni main no lid, agenst his baja when it was all stock, I would smoke him off the line 4 -5 bike lengths and at top end he would slowly real me in an pass me.
he put the FMF powercore 4 on kn dyno kit no lid 195 main we would pull about the same off the line( he had to rev his harder ) but I would pull away from him on top. we put the lid back on and were about the same every wear, except mine still hits harder off the line.

for those of you with a tac try wacking the throtle hard in nutral, with the air box lid on and with it off. see if you can tell if thers a diffrance.
thank guys for repling, I know it sounds weird but I gess what ever works.

Ill look in to the silver lake thing . Ive been there twice once with a 250r and the other w/ 400ex. loved it both times.