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Found 25 results

  1. Hey guys, I just got a 2002 YZ426 and took it out to run up some hills. The thing has load of power once the RPMs get going but not so much on the lower end. I've expected this problem as it is a YZ and the cam is built for racing where you're always either on it or not, but I would like to make it more rider friendly for trails and have the ability to go slow. I already plan to go down a tooth on the counter sprocket but I was also looking into the FMF power bomb header and the Boysen(spelling) quickshot. My dad has both of these on his 05 CRF250X, I know, very different bike, and I was thinking they should both help me out in the slow, low end riding. While checking these items out I saw people talking about jetting and tuning. We never changed his tuning for these mods after what the shop did when he originally bought it and put on an FMF pipe and cut the air filter for better flow. All that said, my question is whether I should change the jetting and if so, what should I change them to? Along with air screw and stuff.
  2. What is this Red Top Jet Needle without any other markings on it? When I bought my 2012 YZ250 a few months back it wasn’t long before I took the carb apart and noticed it had a JN with no markings on it. The only thing I could ID on it was that the top/Head was painted red. I tried replacing the JN with others (EW, EJ, CW) for ID reasons but nothing seem to work as good as the Red Top JN. It just seems to be the magic JN! Need to know for future reference and recommend it to anyone!!!
  3. Rohr144

    Yz125 jetting for summer

    I have a 2006 Yz125 that had stock jetting with 93 pump gas mixed 32:1 and would run perfectly when it was 50-70 degrees outside, at 1000 feet. Little to no spooge at all and power from bottom to top. Now it's 90 degrees in Missouri with humidity as well, and there was more spooge and bike was running richer (obviously). So I decided to lower the pilot from 40 to 35 and start running 25/75 c12/93 pump to help with any detonation. Then the bike would bog down low and through the mid, it would slowly climb but when it hit the top, then it would rip. I had brand new packing in the silencer, and after just 30 minutes of riding, spooge was already running out of the silencer. So I took the clip and moved it up one, to the second position. Felt like it fixed the bog a little, but still a bog down low. Also, after I get done riding and go to pick the bike up to put it on the stand, when I tilt it, some gas runs out of the carb. The carb has all new gaskets and everything, plus I'm positive the carb had never been opened before when I took it apart the first time after buying the bike. I'd think float is off but could it be off if it's set to stock? Oil also leaks out of the power valve. The bike has to be running rich, and I tried going leaner, but then I hit a bog and more spooge so I'm really confused. Top end has 12 hours and will be getting a new ring on the piston in three hours. Checked the reeds about 2 riding hours ago and they were fine. Here's a list of all the info. Any help is very much appreciated. Temp - 90 degrees Humidity - around 40% 1000 ft above sea level Gas - 25/75 c12/93 pump mixed with maxima k2 at 32:1 Pilot - 35 (stock is 40) Needle - stock. Clip is on second from top (stock is third from top) Main jet - 410. (stock) Pipe - stock pipe with pro circuit silencer Vforce 3 reeds Type of riding - mx, novice rider All Im really looking for is if some other Yz125 guys have a good baseline for summer jetting, or if anyone can steer me in the right direction.
  4. What mainjet are you running and what's your altitude? Yeah, I know I shouldn't have bought it and did Dave's mod instead, but I already have it and would like to at least attempt to use it. The manual that came with the kit recommended a 160 main at sea level, which seems pig rich since a 160 dynojet is supposedly larger then a keihen 160. Maybe offset by the different needle shape? I've also found that the bike runs best with the pilot screw turned all the way in since I've installed the 160 main. Is this the kit's fault or did I break something during re-assembly.
  5. Supermotofool

    Stall At Hard Stop

    This is a battle I have fought ever since I got my bike. I have had the carburetor rebuilt, also re-jetted with a JD Jet kit. I have the 160 main jet, 25 pilot jet, and blue needle on the 3rd clip. 3X3 mod, yoshi exhaust, and this is on the BSR36 carb. When the bike gets warm, when I come to a very hard stop the bike slowly drops in revs and dies out. I am trying to get better at stoppies, and every time I put the back wheel down the engine is already dead. I cant progress any further unless I get this issue fixed. I have gone through all the vacuum lines, made sure there is no fueling issues up until the carburetor. I have pulled the bowl off many times thinking that the float is stuck and I have never found it stuck. If anyone has any ideas of what it could be, any ideas would be greatly appreciated!! Thanks everyone!
  6. So let me apologize in advance if this has already been covered. I tried to search and didn't come up with my exact issue. So I just picked up a beautiful 2015 yz250 2 smoker. Only 5 hours on it! Super tight and snappy. My issue is I just filled the tank with (possibly the first refuel since new) dragon fuels straight gas 95 octane and a mix of 32:1 klotz super techniplate. All of the sudden I cant get it to idle for any amount of time. Less than a minute. I started fiddling with the air screw and I get it set perfectly (imho) and just when I stand up and smile the idle starts to slow and I have to crack it a little to keep it running. If I adjust the idle higher than I think is necessary the outcome is the same. 30 seconds or less its fighting to idle again. I also have noticed more oil/spooge dripping out of the power valve drain. If I ride it it rides fine but once again it will not idle for more than 15-30 seconds. This was not an issue with whatever fuel was in it when I got it. It also smoked way less before. Before i go changing pilot jets and fiddling with (and messing up) a properly set up bike. Could I have the fuel messed up somehow? I used a cup and try to be very accurate with the mix. I try to buy the best gas I can. Like I said dragon fuels tarragon MX 95 octane no oil added. Any advice appreciated. I'm sure I'm doing something wrong. Jim
  7. Miles Holt

    YZ250X Bog/blubber

    So I picked up the YZ250X not that long ago. I live and ride between 4000-6000ft. I run Amsoil saber at 80:1 my current setting is 158 main, 48 pilot, 2nd clip, stock needle. It felt a little sloppy on bottom so I switched the pilot and it works great. It feels really good to me but last weekend at a race it bogged/cut out for a second a few times. And it’s doing similar things now. The first time it did it was 3-4 gear chop/small whoop section where I was just keeping the throttle steady 3/4-full throttle and in the middle of the section it cut out like it was running out of gas. But I pulled the clutch in and revved it and it was ok. It’s done it twice out of a corner, once in a long Sandy whoop section, and it did it twice on the faces of jumps. Each scenario the bike felt like it was running out of gas or fouling a plug in a way, it just sort of happened, and then went away. Obviously I wasn’t running out of gas, and not fouling a plug but it felt kinda like that. The race was at about 5000-5500ft, fairly fast GP/desert race, the course was pretty rough in spots It did it again on me today at 4000 feet, I tried switching the main jet to a 155 but it was too lean. Would switching the needle help? What about float height? Bike has maybe 20 hours on it
  8. Hi Guys, So I recently bought a 1997 XR400. I've done some research into getting more power through some basic mods and rejetting the carbs. Though all of the ones I've found have been for post 98 XR400s so the stock carb set up is a lot different. Usually people are opening up the airbox, exhaust ect. ect. and Jetting their carbs up to 160/162 for mains around 60 ish (I think) for the pilot. However for the 97 model this is close to stock for the jets. So essentially what Im asking (Just to cut to the chase ) Has anyone got a 96/97 (maybe 98, not sure) XR400 and has done these similar mods, and what sort of jetting have to done as a result. Thanks in advanced, also its my first post so apologies if I havent done everything correctly.
  9. Hi All, I did have another thread, specifically asking about Jetting changes whether I should go one step at a time...and I've moved on from that, so I've started this one. I welcome any assistance please Bike details: New to me (relativly speaking..I've had it since October, but only put 2 hours on it before I stripped it down. 99 YZ250 PWM38 Carb (tried 50 and 45 pilot. Main:172s. Needle: N3VF, clip 2nd from top). Only thing changed from stock is the pilot and needle position. See bottom of post for stuff I've done. 32:1 mixture. Compression around 180psi. Location: Altitude: Around sea level, problaby up to 100m (328ft) as I am only riding at one location (although I am fixing/tuning it at another...but again, not a huge elevation). My GPS says I'm at 80m when I did the below videos. Temperature around here ranges from 10-45 celcius (50-113f). Symptoms: It runs 'fine' in general. It doesn't stall easily when under load at slow speed up hill, it doesn't bog much, it has plenty of power. The only reason I am concerned is I read that you should jet the bike, and the process to follow...which isn't working for me. It's also now leaking oil out the powervalve overflow - I didn't notice this before I went to 32:1 (was at 40:1) I tried to do a leak down test, but I am getting a leak from the right side of powervalve, so I guess until that's fixed, I can't see any other leaks (if there are any) using soapy water sprayed around the engine. Videos: Please note with my videos, I have added subtitles so you know what I am adjusting. They should come on automatically, but if they don't this is how you turn them on https://www.wikihow.com/Turn-On-YouTube-Subtitles My biggest question is: Is my idle right, as this is what I am basing all my concerns on? I am struggling to find the right airscrew position, I am turning it out looking for higher revs as per the forum sticky, but it's more of a surge...is that what I'm looking for? See first video with 50 pilot - Someone assisted and suggested that I should start with a leaner pilot, and switch to a 45, so I did that, and I get this Now, if that surging is what I am looking for, does this actually mean the bike is lean, so I need to go richer? But, if it sounds like something is off, then what is my next step? Crank seals? This was suggested to me by @kxrob and @2 STROKE YZ DOC in another thread. The other thing that YZ Doc suggested, was that the bike should not run with the air screw all the way in...but it does. Does that tell me it must be an air leak? Or am I worried about nothing, and should just get on it and ride? Once I get this sorted I need to order new springs...I'm only about double the weight of the recommended weight for stock springs. Work/replacement that I've done: Carb Disassembled and Cleaned Replace Radvalve boot(had a crack) Set float Changed Pilot from 50 to 45 (KXROB suggestion) Replaced choke/idle screw as it was stripped (then since found out I should have just replaced the carb with a PWK38 instead) Exhaust Muffler packing replaced Cleaned exapansion chamber with oven cleaner Frame Etc Swing arm and linkage bearings etc Cleaned and regreased steering head stripped and painted frame Other Replaced wheel bearings Replaced air filter Thanks in advance!
  10. Br0ck

    CR250 wont start!

    Hey guys, I went on vacation all summer and let my bike sit (2002 CR250). When I got back I ripped it apart and cleaned carburetor and put it back together and it started fine and ran alright, but idled really high. I adjusted the idle screw and realized it was a ridiculous amount of turns out. I can't remember exactly how many but id say 6+ turns out. My killswitch was a POS and didn't work anymore so I replaced it with an oem replacement. My last ride I crashed the bike pretty bad and I was off the bike for 2 weeks. I can't get the bike to start ever since I got installed the new killswitch. I think I wired it right I just replaced one wire at a time and it looked pretty straight forward. I verified it by testing spark while holding it in I was getting no spark but with it in running position it would spark. Anyway I am just looking for some base settings for the Mikuni TMX to get my bike running again, my carburetor has a 35 Pilot and 380 Main jet which appear to be stock. I am located in Edmonton, AB, Canada which is about 2500ft and usually ride between 10-25 celcius. Thanks for any replies.
  11. So, after replacing exhaust and header on 2004 CRF450R with FMF Powercore 4 and FMF powerbomb there is strong smell from exhaust and smoke after shut off and still staining rear fender, it seemed at first I lost a little compression but restored compression, the bike seems to run fine from what I know but I'm doubting what I know. The bike has symptoms of both lean and rich bogging popping slightly on decel and accel occasional slight white smoke and black if that's a thing? Please help me, what am I doing wrong here with this bike, I'm lost and have no idea. Thanks
  12. idratherberiding

    Tuning YZ 250 on the Cheap

    I bought a 2006 YZ250, have ridden it 5 times. It needs some tuning, but I am not sure I'm keeping it so don't want to spend much on it. Has fmf gnarly expansion chamber and turbine core silencer. I ride about 1000 feet above sea level. I'm going to clean the carb and check the jetting. What is a good jetting for me? 85 main jet? Also check the reeds. Is there a big difference between getting aftermarket reed cage and reeds or just getting a cheap aftermarket set of reeds that fit the stock cage ie. Boyesen Power Reeds? The other things I'm going to do is permatex all the joints in the exhaust system and a compression test. It was 220psi 5 rides ago so I'm guessing itll be fine. Its bogging a bit but I'm thinking its carb and definitely exhaust joint related. Let me know if I'm overlooking anything that could make the bike run crisper and more bottom end without throwing much money on parts
  13. idratherberiding

    Tuning YZ 250 on the Cheap

    I bought a 2006 YZ250, have ridden it 5 times. It needs some tuning, but I am not sure I'm keeping it so don't want to spend much on it. Has fmf gnarly expansion chamber and turbine core silencer. I ride about 1000 feet above sea level. I'm going to clean the carb and check the jetting. What is a good jetting for me? 85 main jet? Also check the reeds. Is there a big difference between getting aftermarket reed cage and reeds or just getting a cheap aftermarket set of reeds that fit the stock cage ie. Boyesen Power Reeds? The other things I'm going to do is permatex all the joints in the exhaust system and a compression test. It was 220psi 5 rides ago so I'm guessing itll be fine. Its bogging a bit but I'm thinking its carb and definitely exhaust joint related. Let me know if I'm overlooking anything that could make the bike run crisper and more bottom end without throwing much money on parts
  14. Kylesweenz

    2000 trx300ex jetting help

    I have some issues with jetting i believe. I understand nobody can tell me exactly how it needs to be jetted but a ballpark would be nice. It has following mods, no air lid, uni filter, choke is removed, fmf header and white bro e series slip on, no baffle, 1000+ rpm cdi box, 20" rears, stock gearing. The atv runs great all the way through throttle until about full throttle then it gets a cut out. Like a slow studder if I let off throttle it improves. I've heard too big of a main jet before and it doesnt resemble that sound. It's the same with airbox lid on or off. It's not rev limiter either because a few months back it ran wide open and couldn't hit a limiter. Nothing has changed since mod wise. It has a 42 pilot. And 135 main currently. Needle in stock position and af screw idk exactly I've achieved best throttle response.and idle somewhere around 3 turns maybe. I like at about 450 feet elevation and it's hot out lol
  15. Carl Gipson

    96 xr400 carb/jetting help

    I have a 96 xr400, that is as far as I know stock. I just bought a white brothers e series exhaust and am trying to,find out if I need to reset the carb. Some threads say yes some say no. I am in louisville ky. and like to ride woods trail, no track riding. Can anyone tell me if I need to mess with the jets or leave them alone!!? Any help appreciated.
  16. So picked up an 81 cr450R replaced clutch among other things, went thru and cleaned and jetted the carb but for some reason once I drop the choke the idle wants to climb almost immediately, have tried adjusting idle screw to no avail. Not really sure what else to check any and all ideas would be greatly appreciated
  17. Kawirider04

    Klx 140 jetting

    Hello i have a 2009 klx 140L that i plan on modifying. I am installing big bore kit to 170cc's, modifying the airbox, high flow K&N air filter, and gutting stock muffler because other mufflers are just too expensive. Once i do this all i know it will WAAAAAAAAY lean it out probably to the point where it wont start. Can i run stock carb and just get new needles and jets? Thanks.
  18. I just got my 83 Honda XR350 rebuilt by a local shop as it was burning oil heavily. I was doing some research into my bike and have learned that these bikes are especially lean out of the factory. Would I be able to richen using the mixture screw on the carb? Or would i need to rejet completely? Has anyone had any horror stories from not rejetting the lean bike? Thanks
  19. I just cant get my jetting right ! Middle and top throttle position run excellent, but when I idle or chop the throttle in a corner she wants to load up . I’ve already got the smallest jet possible along with The JD kit . I’m at 800 feet above and ride in Northern Indiana / Michigan . I just did the top end and repacked Exhaust also installed new reeds . I bought this bike two months ago and haven’t been able to enjoy yet . . Any suggestions?
  20. Ok, this may be really dumb, but I’m stumped. ‘08 KTM 505 XCF was running great. I put a JD Jet Kit on it, with recommended settings (Blue needle, 3rd pos, 42/182, 1-3/4 turns on air/fuel). Post kit installation, including the accelerator pump diaphragm adaptor, and o-ring on the throttle linkage/screw, I cannot get it to idle correctly or get the hot start lever to do anything. It also backfires lightly when decelerating(like its lean). The blue needle is supposed to be the richer needle. It starts with the choke on, I can get the idle to basic normal idle with the idle setting, but when I rev it up, the idle accelerates to a very high idle, like it’s extremely lean. Then I can’t get it to come down without shutting it off, and it gets worse as it warms up. I pulled the carb, checked all the gaskets, checked the needle and hot start lever to confirm it wasn’t stuck. From what I understand, when you pull the hot start lever, it should increase the idle as it leans out the bike. Mine doesn’t do anything, and when the idle is revving, I pull it and it actually lowers the idle? I’m confused, as I thought the JD Jet Kit would be gold, as I’ve had it on other KTMs. Not sure if this is an air leak? Did the adapter I put on the accelerator pump diaphragm cause this? I’m about to pull the jet kit, and go back to a normal needle. Any suggestions or ideas would be much appreciated.....thank you....very frustrating.
  21. intimdatr

    2001 KX125 Jetting

    Been rebuilding an 01 KX125 over the past several months, Kustom Kraft replate and such. Im finally getting around to jetting the bike. Current jetting in the bike is a 50 pilot and 180 main and I'm pretty sure that is the standard jetting for the Pre 01 bikes. I'm showing stock jetting for the this bike is 50/360, is this correct? Does anyone have a good starting point for this bike? It'll be running in 500' elevation and 70-90*. I've seen most people are in the 45/350 area? Edit: Bike has FMF Gold series and Shorty exhaust, stock otherwise. Most recent picture of the turd after new fork seals and Tire/Tube.
  22. My dad and I have two XR650Ls, one is a 2005, the other is a 2008. We've been trying to get the carburetor tuned, but we've been having some trouble finding the right way to route all the hoses. Both of them have had Dave's mod done, and they've been uncorked and have an FMF Powercore 4 slip on exhaust. They're jetted at 55 and 158 at about 6,500 feet above sea level. Fairly often, when I'm cruising around 40 mph, the exhaust pops and the bike loses power for a split second. That's the main problem that I've noticed. Also, on my bike, if I let off the throttle without pulling in the clutch, it pops a ton. I know some popping is normal, but it seems like it's a bit excessive. If anyone has any tips on routing the carburetor hoses, or any other tips to get them running right, it'd be greatly appreciated. Thank you.
  23. First off, the title says it all. There is no guarantee that this is the problem your bike might be having, but i wanted to post my experience about my bike's issues that took me a few months to research, fix, and diagnose, in hope that it will help anyone with the same issues. Here were the issues on my bike.... It was a used 2005 450X that i picked up in October that had sat for about six months because the rider broke his back. The bike was well maintained (regular oil and filter changes, etc) and it looked pretty dang good for being 12 years old. I got a good deal on it especially because of all the extras on it (guards, exhaust, extra parts, etc). However, when I ran it, it idled good but bogged like heck unless i left the choke on. I figured this was normal considering the fuel sat in the carb for 6 months. Anyways I ended up buying it in hope that a good carb clean would do it.... NOPE, not even several cleans. Next, I assumed getting a carb rebuild kit would do it.... also a "no" So next, I brought it to honda dealer and shelled out hundreds of dollars, but still no fix. (side note: the mechanics there were fantastic and they gave me some free hours of work but they were just checking essentially everything besides the carb, because we had decided the carb was not the issue for some reason.) I was getting frustrated, and went back to the carb.... i got it, it must be the jetting. I had always been curious about it especially considering that the bike had the typical air mods (no snorkel, full FMF system, no cage and screen, twin air, etc.) and all the jetting and AP stuff was Stock. So I cracked down and got the JD jetting kit and a smaller leak jet, and did all that business. Now the bike is running awesome and I love it. I would love to hear your guys' comments, etc, and I hope this may have been helpful. Im trying not to cross post, but am posting this because when I was doing all my research I did not find anything specifically down this alley (not to say there isn't, i just didn't find it) If anyone has questions i would love to try to answer them my best. Good riding to you all!
  24. Hey guys I've been using the forum for advice for a long time, usually just searching but I've created a weird enough problem for myself I had to make an account and post. I have a 1998 WR400, bought it used and abused. Last season I got her running well enough, drilled the exhaust, and opened up the air box but didn't change anything else. Getting it ready for this season I was getting into it and found that the slide in the stock carb was cracked, and the inside was all gummed up. After a bit of searching it sounded like the consensus was that you can just throw a WR450 carburetor in no problems. So I bought this https://www.ebay.com/itm/Yamaha-WR-400-F-WR400F-Carburetor-Carb-1998-2000-NEW/123166146174?hash=item1cad463e7e:g:hmcAAOSwZKBZJcS1:sc:USPSPriority!93446!US!-1 and popped her in. I blocked off the old hot start hole with some vacuum plugs but I cant get it to start for the life of me. I've tried kicking it a billion times with settings on the fuel screw from about 1/4 to 2, with no success. I did get it to start once at about 1.5 turns, but it died fairly quickly and I couldn't replicate it. I threw a new plug and air filter in as well, but no other changes. Anyone able to point out what stupid thing I'm doing?
  25. Hi All, I've been recently embarked on jetting my DRZ400E FCR39 Slant equipped bike for optimal performance. Especially considering that I had a DRZ400S with a the CV carb before that I swear would outrun my current bike, it was faster, responded better and had way better fuel economy. That despite the fact that the E is supposed to have the high compression gasket, better carb and cams. I have since made some changes and conducted some tests and frankly need some input from the community and those with more technical knowledge or maybe just practical experience. The bike is a 2009 DRZ400E Australia model. I have Tumbleton & Twist tachometer / electronic speedo installed. Starting point ( should be standard ) Main jet 165 Main air jet 200 Pilot 45 Pilot air jet 60 Starter jet 65 Fuel screw turns unkown AP arm linkage worn a little more than the standard 0.6 mm play. OBDXP needle on 4th clip Snorkel removed Restriction between carb/engine removed. Throttle stop cut down poorly to only allow about 80% slide opening. Staintune full exhaust system ( the header barely fits through the frame, shock canister section ) Changes 1. Checked float level at 9mm ( no adjustment ) 2. Cut throttle stop properly to allow slide to just clear the bore of the carb, adjust throttle cables. This should already add some performance! There was still about 1cm or so slide left in the carb at "WOT" 3. Did the Eddie Mod on the AP. However I'm not yet convinced this was a good thing. Even though I removed the stopper from the AP diaphragm I still get separation from the black linkage and the "timing fork" at WOT. Now I have to sacrifice the timing to be later than it should be in order to not bind at WOT. The moment you adjust the timing to be correct you get bigger separation at WOT, and if you don't wire the linkage to the timing fork then you now have a really really long squirt without the stopper in place. 4. Purchased/installed JDS0001 James Dean Jetting kit. As part of the kit you get 155, 160, 162, 168 main jets, new 45 pilot jet and blue/red needle and a useless fuel screw because it won't work on bikes with an electric starter. 5. The carb was fully cleaned, inspected. 6. Fuel screw 1 3/4 turns. ( doesn't seem to make any difference to idling ) I live at about 1000ft but my riding could go from sea level to about 4000ft. After reading all I could about tuning carbs and what is established knowledge, I set about making the changes to my carb and may have wandered too deep down the rabbit hole. There are a number of things not touched on / explained in forum posts which are 1. Throttle position vs RPM All the talk is about throttle position as if RPM doesn't matter. But that cannot be accurate. An engine at WOT at 5k rpm has different needs than at 10k. The amount of air needed is different and therefore the amount of fuel. The air fuel ratio needed for best performance could also be different at various point across the RPM range. One of the few tuning articles around that did talk about this was tuning the Weber carbs on 911's. http://www.performanceoriented.com/performance-tuning-2/ This article also touch on how air is going to behave very differently to liquid and therefore fuel metering is affected and don't behave in a linear fashion regarding air speed / fuel metered. 2. Lean/rich condition testing on Main jet In my quest for determining the main jet for my bike I have questions instead of answers. Doing some tests I installed 160, 162, 165 and 168 jets all with the blue JD needle at 4th clip to keep things constant. This is done going WOT in 3rd/4th from about 5500 to 10000+. 160 is rubbish - airbox cover/ no airbox cover. 162 feels good and smooth across rpm range, with no airbox cover the mid range is stronger and top is almost as strong but seem "rougher" 165 doesn't feels as good as 162 with cover, with no cover it feels stronger than any 162 config by far in the mid range and equal at or stronger at top. 168 felt worse than 165/162 in different ways. The outcome then was not clear to me. I expected to have find a jet that runs better than any other configuration WITH the cover on. Instead I found a jet that ran better than anything else with the cover OFF. The only conclusion I could come to is that the airbox was actually restricting the bike at mid/high rpm and I should go with a 165 and modify the airbox. Which is what I did..... This made me question if this type of testing is valid. Removing the aircover make the bike leaner?? How really? The engine will only use the amount of air it needs. The only way it will go leaner is if before it was actually being restricted. This became quite apparent after modifying my air box at the top in a "reversible" way by making 5 x 2cm holes. After doing this, taking off the cover made almost no difference at all anymore because the engine was not being restricted anymore - at least that is my take away. 3. Airbox modifications I also decided I needed to be a bit more data driven in my approach, but it is hard without having access to a test track with very long straights or a dyno. Tuning a liter bike would be impossible because of the speeds involved. Especially considering I am now breaking away a bit from the "3x3" mod and established jetting. That however is already the case seeing as the DRZ400E just need the "snorkel" removed and then the hole is actually bigger than "3x3" and every bike will be different, with different exhausts and engine state and cams for all the different variants out there. Testing my bike over a set distance and measuring the max speed in 4th gear from 5500rpm going WOT. This is from about 75km/h at 5500rpm over a short distance accelerating for about 4-5 seconds almost revving out. Standard airbox - 113km/h, 113km/h, 114km/h Standard no cover - 114km/h, 114km/h, 115km/h Drilled with cover - 115km/h, 115km/h, 117km/h Drilled no cover - 116km/h, 116km/h, 118km/h Even after drilling the air box, running without the cover still had a higher speed at the end of the run. The issue I have is how much the needle could be interfering because of starting at 5500rpm. Because the run is so short and not really a max speed, maybe other circuits are having an impact. I have not repeated this test with the 162 or 168 main or any other needle. I'm willing to bet however that a 162 is going to perform worse than the 165. Not sure about 168 but I'm not going to open the airbox any more and will just jet for what I have drilled. Also, the AP could be interfering over such short runs... 3. Airbox mod / Low RPM/Mid RPM power? What I have observed is that with the cover off and post drilled airbox the mid range RPM power have really suffered on initial roll-on from a closed, 1/4 throttle position. Now I have not yet started adjusting the rest and playing with the needles as I wanted to get the Main jet sorted first, however I am concerned that the needle will not solve the issue and was wondering if there was some method to the madness and that by restricting the airflow of the airbox at WOT/high RPM it actually improves mid range power. This would probably depend on the engine design, cams and exhaust system. I suppose I will find out soon enough if I start changing the needle and can't get the bike to behave properly in the mid-range. Luckily it will be easy for me to close the 5 holes one at a time until I get the desired airflow if needed. More to follow......very happy for some input and thoughts..... Another really worthwhile read, very detailed and clear with pictures not found elsewhere like emulsion tube cut away etc. http://www.49ccscoot.com/faq/carbtune.html