03 wr450 missing/skipping/sputtering.

hey all. so i have searched plenty and found numerous things to try and still no luck. in searching, i have found some post that were exactly what was happening to me but there were no replies back so they were of no help. hense my posting now. i have an 03 wr450 with an fmf q4 slip-on, grey, wire pulled, no-toil air filter and a dynojet jet kit with their 140 main jet and their needle with the clip third position down. dynojet recommends second position down but it was still popping a little on decel so i dropped the clip down one.

now for my issues. #1 it starts hard, if i choke it and hold the throttle at 1/8 turn or so, it fires up and idles second kick. otherwise im kicking for a while. #2 at any throttle position, 1/4, 1/2, 3/4, or full, as soon as it gets to the max rpm at any of those postions in any gear, it starts stuttering. what ever it is its kind of hard to tell. almost seems like a misfire. i have checked everything, exhaust leaks, tps, timing, pulled the carb apart, etc. all things i have found searching. i pulled the plug and the entire thing was black. last time i knew that ment it was rich but the symptoms make me feels its lean. what do you all think. could it be a fould plug or an electrical issue? also, with the jetting setup the was it is it ran fine for two short rides and numerous rips around the house so im having a hard time thinking its the jetting but i am no pro so it sure could be. any help would be great.



140 main????? that is too small. its probably what i would use at 20,000 feet.

check the jetting guide and follow the examples. or get the jd jetting kit.

i think you have a problem that's easily solvable.

well with a slip-on pipe and no air box mods, thats what dynojet recommends. seems weird to me that they dont just go with normal jetting numbers. and i know its not running lean because the plug is black as black can get. but i do agree, i feel you are right in saying that it is something simple.

Agree, It probably a carb issue.. But if it ran good before and you didn't change anything I'd look for blockage in pilot jet and the passage from the pilot. Check or just replace the o-ring on the fuel screw along with the oring on the slide plate (make sure its in the right way (good pic on JD jetting webpage).

As far as starting the pilot jet and fuel screw along with the pilot air jet control idle and therefore starting (most people don't mess with the pilot air jet, and most of us have found nirvana with 45-48 pilots and roundabout 1.5 turns on the fuel screw, give or take) (great sticky on choosing pilot and air screw adj).

The needle is the main control for 1/4-3/4 throttle, god knows what the dynojet needle is, again if it ran good before should be ok.

The main jet only controls the 3/4 to wide open but 140 sounds waaaaay lean reguardless of the needle so recheck that while your in there... Most folks seem to come in around a 165-170 main. I'd call dynojet if that is their recomendation and ask &%$#@!?

Providing the carb checks out, including the acc pump and your getting gas metered in correctly and all seals and o-rings are 100% then I'd get the multimeter out and start checking ignition parts, could be something as simple as a bad sparkplug or corrosion on a connector somewhere.. Start cheap and work your way up.

thanks for the help. i changed the plug tonight and took it for a burn. still no change. when i got back the fuel line from the tank had torn. at that point i figured i'd found my problem. replaced it and took it for a burn. still no change. it started pooring here so testing kind of came to a halt. tomorrow i'll take the carb back off(again) and check everything. i did forget to mention that i have a works connection fuel screw but im pretty positive that that doesnt mean anything. just makes it easier to adjust. i looked at some of the pictures on jd's website and some of them were helpful. just wish the had more information about the ignition stutter.

Fuel screw should be fine as long as its not damaged. Make sure it's o-ring then washer then spring on the screw and like I said make sure the o-ring is good, read in I'd just get a new one. My 03 had almost every rubber part in the carb dried out or cracked. The seal on the slide plate can cause a world of issues also. So if in doubt get new ones. Cheap parts that are easy to replace while your in there. If the pilot jet even looks a little funny I get a new one also as even the best cleaning can't get them 100% open again. Make sure you can easily shoot brake cleaner through all the passages in the carb body and blow out with compressed air. Took me three goes at mine till it was clean. Could take the damn thing apart in my sleep...

Check the float level too it could be running out of fuel at sustained rpm. A long shot but again free. The previous owner of mine had the float way off.

And funny you mentioned cracked fuel line as mine was rotten plus the petcock was so dirty not much could get through it. And the tank was nasty inside again worth a look free and somewhat easy to fix.

It's always great if a friend has a 450 something with a FCR carb so you can swap to see if that fixes it too.

After that it's time to get a good multi meter and start checking electronics. Luckily the owners manual has pretty good info on checking all of your goodies.

thanks rocky. i'll post back with findings.

While working in yard I came up with some more thoughts.

First you said no air box mods, I hope you pulled the snorkel out to help it breathe easier esp with a aftermarket exuast letting more air out there has to be enough going in. I opened up the right side of my box to help a bit as it seems very restrictive compaired it's mx cousins.

Second you say it starts to run rough after it comes up to rpm. I assume you mean when the engine isn't under a load and freely revving. Have you checked your acc pump duration and squirt? It could be running well with the squirt but when it stops the small main jet can't supply enough fuel to satisfy a thirsty 450 and it goes lean. If you rode where the pump was working all the time it would be less noticeable but if you were somewhere more open it would stand out. Kinda like a bike set up for supercross doesn't work outdoors.

This is always my issue with jetting kits, they are expensive, don't come with all the jets and parts to make a bike run 100% and usually the jetting is a bit suspect as its done on a dyno or exuast monitor and not in the real world.

There is some great info on here under jetting 101 or something like that over in jetting and intake area and I even typed something similar to a post awhile back here in wr world.


Here is the AP 101 I was thinking about, this is what I used to set mine up plus some now missing posts. Some of the info is a bit old (the BK mod is a thing of the past with the MX version of the FCR). But the timing and explanation is dead on.

Does a good job of explaining the miss, pop, bog, funk that modern 4t's face.. My diaphram was rotten also BTW, so I replaced it with the shorter pin version through Yamaha. It was perfect.. I'll leave you alone now!

haha no need to leave me alone. any and all information is much needed and appreciated. i read the AP 101 post and that was great information. im hoping the weather straightens out so i can try dropping the needle back down a clip and try that. i was reading the jd jetting FAQ's and there was one that was about the tps stutter and the said that you can either raise or lower the needle a clip and that is a temp fix. i dont think that is my problem but it was running fine before i raised it and for a very short time after. i may just order up and jd jetting kit anyway seeing as from what i am reading they ar far superior to the dynojet kit. not bashing dynojet just seems like you get alot more for not much more money!.

If you want to save money I can tell you how to order a "rocky jetting" kit, you can get everyting from Yamaha (I did get my acc pump spring from KTM as Yamaha doesn't offer one through GYTR but KTM does through their SXS parts for 5.00 bucks, and my brother is a KTM dealer so that helps) for a lot less than JD and you get hard coated OEM needles, which are way better than aftermarket brass ones (IMO) in the JD kit. People seem to like the JD kits but a lot of folks can be fooled into emperor's new clothes syndrome on things like that.. And as far as a jetting chart, Merge has a spreadsheet that does that for free on their website, computers are cooler than paper.

I even have all the part numbers saved on my computer because I'm a nerd..

what/who is merge? and what asortment of jets do you get for the cheaper price?

never mind. figured out what merge is. in their chart it shows different letter for the needles. what do those correspond to?

http://www.mergeracing.com/products/ They sell nice goodies for FCR carbs and have the HD springs (but KTM sells same spring for less so if you have a KTM dealer close by or you can mail order..) http://www.thecycleshed.com/index.htm is my pick but I'm partial. Its part# SXS08250030.

The pinned thread "WR performance index" has all the jet part numbers from yamaha listed, yep its a bit more work but you will save a lot of cash and get OEM kehin parts, (thats huge to me). I used http://www.ronayers.com/ to order everything. Yamaha sells jets cheaper than KTM dealers can order them for..

If you do a 165, 168 and 170 main. 45,48 pilot and a 40, 50, 60 leak jet along with a NCVS and NCVQ needles (03-04 yz) you will have all you need to jet away to perfection plus you can pick and choose any random seals, diaphrams and o-rings out that you need. I recommend the 5JG-14940-17-00 acc pump diaphram (I have zero bind with this one) stock is the "18" longer pin diaphram.. Plus the advantage that I got new screws for my carb that the PO had rounded out with a cheapo screwdriver.

My jetting so far.. it's still evolving and always will be I suppose (If I had fuel injection I'd be so bored)

168 main

48 pilot

NCVS needle 4th clip

50 leak= just less than 1 sec squirt

fuel screw 1.5 turns, changes daily it seems with 20 deg temps range..

Oh and don't bother looking for the OEM sizes aftermarket even in Keihin parts, they supply different needles for manufacturers than what they sell plus they are brass versus hard ando alumium.. Even JD said hard ando is better back in the day but he ca't get access to them either. Trust me back when factory race bikes had carbs they had OEM needles in them :cry:

You have to buy needles through Yamaha (cheapest), Honda etc.. The rest of the jets are the same as long as they are OE Keihin. Jetsrus.com, sudco are good sources..

sweet. thanks again rocky. that is all incredible info and will be all kept for future refrence. i went home at lunch today and dropped the needle one clip and problem was mostly cleaned up. and from what im reading the wr's have a slight stutter to them at partial throttle anyway that yamaha supposedly did to keep the engine cooler when cruising. i learned that i should always trust the plug. it was black as the night so that told me it was running rich. and by leaning out the middle a position on the needle it pretty much fixed it. is it perfect, no. but its back running the way it was so im going to run it for a bit and see what happens. ill probably end up doing what you recommended and order those jets and such when im feeling more ambisious and not want to ride as bad. next thing i need to teach myself is how to adjust valves. if you have any good info on that, send some of it to my in box if you dont mind. im going to do some searching and the manual seems to have pretty good info so im sure it wont be a problem.

thanks again. that was all great info and much appreciates. i learned alot from it.


Cool, so it was running a little rich, easy fix! You can try unplugging the TPS sensor also and see if this cleans up the rest, seems mixed reveiws on it but only takes a few seconds to unplug and test.

The air cut valve can affect this also, its suppose to help with decel popping but I disabled mine and really helped with erratic hanging, floating idle and again only takes a few min to change and can be put back if you don't like it.. Real dirt bikes should pop a bit I recon..

Far as valves go the manual is the bible.. Figure out what shims you need and a dealer should sell them by the piece.

Happy trails!

ok so im bringing this one back. after i dropped the needle it was fine. went on a club ride and was fine. riding around the house a little bit was fine. now last night i went and did a few laps in the woods and the same issue is back. im thinking im going to go back to stock jetting and see if that fixes it. maybe the dynojet kit just isnt working. i dont know. im lucky to have a gas station in town that sells non-ethenol gas and i have been running that so i know its not bad gas. i have gone over all of the other things that have been recommended and have found nothing. only thing i havent done is go through all the wiring with a meter and make sure all the voltages are correct. i just feel that this is something stupidly little.

I don't believe Dynojet's numbers actually mean anything compared to "stock" jets numbers so don't go by the "140 is WAY too small" thoughts....as long as you stay with Dynojet jets, you can use their info.

Hope that didn't throw you too far off.

Try stock jetting, adjust from there.


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