99 WR400/FCR carb jetting guru's come forth!

More or less a general jetting solution, I hope. I've exhuasted searching for jetting help. I'm a lifetime rider, other than the last few years. Had a 2000 WR400 I liked a lot, selling seven years ago when the first kid came along. Kids are riding now and it's time for me to jump back in to the fun at 40!

I recently bought a as much as can fit the word, a 12+y.o. mint-condition stock 99 WR400, after selling that much ridden and last owned bike, a 2000 WR400 tuned spot on six years ago. FCR39H carb on this one - turns out to be way different (and crappy) when compared to the carb on the 2000.

Knowing I played the jet game to get there on the 2000, I just bought the JDJetting kit to simplify things. I live at 8000 feet in Colorado. My altitude riding is typically 5500 to 10000, occassionlly higher. I used the red needle in the 5th clip position and 165 main jet as suggested for 6k-9k in in the JDY001 (98-06) for the bike. Only change from stock is a White Bros. E-Series and mid-pipe, also found in "mint" condition on ebay (same outstanding pipe I had on my 2000 several years ago). The bike ran pretty good stock with the throttlestop fixed, stock pipe uncorked.

Now, with this kit, I'm about to just go back to uncorked stock - but I would like the gains I've felt glimpses of while trying to jet and tune it. The bike runs pretty good with the choke fully on. Now, the bike with choke off has a low-end bog, back-fire and it seems to even out in the mid-range and then seems to miss in the top end, maybe lean again. Seems to be running very lean, but the kit seem to be generally set to lean it out from prior stock jets/needle. Seems to me I need to go up in both my main back to at least stock 168, maybe higher to a 170 and a higher pilot jet from the stock pilot jet of a 45, not down to a 42 as is put in the kit, which would lean me out further in the low end, right?

Is there anyone out there specifically familiar with the bike that started it all? I know it will run strong all the way from low to high-end, as my 2000 WR did, but so far the kit seems to be off. I need someone that knows more about jetting than me. Thanks!

Edited by ColoradoXR

Probably not what you want to hear but reading around on here these past few years shows those older FCR's to be a pretty shitty carb with a lot of the issues you are describing so most people simply swap them with newer ones.

Not too expensive, as you don't need a super new one (03+ maybe?) but I'm thinking you will need a new JD kit of you go that route. Prolly $150 for the carb, $75 for the JD kit.

Other than that follow the chart, even if it doesn't make sense, that's why you bought the kit. Otherwise there used to be a pinned sticky at the top where people listed their jetting and elevations. Read through that and see what you find.


Well, to begin with, it must be running seriously lean off the bottom to be needing the choke on. Is the pilot plugged? Float height set properly? Or possibly is the hot start not seating properly and is hanging open slightly? That's where I would start. And also, if I remember correctly, the accelerator pump on the older models carbs sometimes get finicky by getting plugged up with mud and such. Have you looked into seeing if the pump is working properly? Maniac

I agree with Maniac.There has to be something causing a lean condition. I have the same bike and ride at the same elevations and it runs near perfection.(9.5 of10) I didn't see that you metioned where your fuel screw is set.My bike has a 42 with the screw set a 2 turns out.The exhaust change should have almost no affect on the pilot.For sure need to make sure the accelerator pump is working. But it should idle nice without the choke on.

Edited by VinceDC

I'm not sure if your bike has the same carb as my 02 426 but if it does I don't think I would swap it out for a later one unless you found a good deal.

I bought my 02 426 (FMF header-stock muffler with insert) and it was running extremely poor. After a good carb cleaning I had to really fiddle with the pilot and needle jetting to get it right. It backfired like crazy, idled like crap, bogged every time I hit the throttle, coolant would bubble in the technical sections, it didn't like to start, and the header glowed like a cherry at night :thumbsup: It now runs better than I would have ever thought possible. ALL of those symptoms are completely gone. I would get the 42 pilot out of the mix and go straight to a 45 with an adjustable (tool less) fuel screw. I have the Tusk piece and it was really cheap and works great! Write everything you change down in a notebook so you remember what jets you changed and what adjustments you made--its so easy to forget lol.

Here's where I'm at now: 4500-9500' - 45 pilot - 3rd clip (stock needle) - O-ring mod - Tusk adjustable fuel screw - can't remember the main jet lol. That said it all went out the window when I installed a WB E-series muffler LOL. I'm sure I could have tuned it in, but I switched back to the stock one because I liked the quietness :thumbsup: Hope this helps,,


PS tight valves can give the similar symptoms. It would be a good idea to check those before you fiddle with the jets too much.

Any bike that has been stood for a few years WILL have a blocked pilot jet....

Petrol evaporates and leaves a goo behind.

You may have seen my new thread - my WR400 had a blocked pilot and it started and ran fine apart from at low revs/light throttle - ran like crap. Now not.

42 pilot, was 165 main but runs much better with a 170

JDJetting said the same thing about the likely clog on the pilot jet. I went to my local Yamaha dealer, which seems like the ultimate s%@thead shop. I get the attitude that a 40 year old, heavier than he used to be guys can't possibly ride a dirtbike worth a crap. I'm probably slower than most of em, but have been riding since they were a twinkle in daddy's eye. What tops it off is "oh, we have to order those, check back in a week" for a friggin' FCR 42 pilot jet and 100PAJ? What? Isn't this a common carb & the local Yamaha shop doesn't have it.

Anyone know a shop with less attituded and better stock than the one on West Colfax at Wadsworth in Denver? They suck.

I hardly ever bother with the dealers anymore. I get most everything from rocky mountain MC. It is not great for one small item because of shipping.

I have a story about the "old guy" thing.

One day I'm out riding in an area where the young kids like to jump. There all hitting this table top that is about 40 feet across the top and has a nice long down hill landing.They were just clearing the top.So I go over and roll it a few times to get a feel for it.Than i take a run and clear the whole thing and land at the bottom of the landing.Total distance about 60-65 feet.You should have seen the look on there faces when I took my helmet off and they realized how old I was . Also the fact that my bike had a head light on it made it even better.

Yes, I don't do the things a use to but I can still pull off some air if I want. Don't listen to those guys. It's all about the fun of riding and not matter you age or skill level there is nothing wrong with wanting a bike that works right.

Edited by VinceDC

OK, being busy as heck with work and kids, I've finally gotten this thing "improved". Current symptoms are that the bike runs pretty good and strong with the choke on, idles well and can adjust the idle speed with the idle adjustment. Almost no popping on decel. The bike runs OK, but with less power with the choke off. Will idle very low and for a very short time with the choke off, then dies. Runs, but not really rideable. Will stall at low speeds. Pops like hell on decel. The fuels screw does nothing. In can close it all the way up and it still runs with no discernible difference.

Unplugged White Bros exhaust and mid pipe and an opened up air box. No grey wire mod - can't find it & don't think it applies to a 99.

Here is what I am running, tuning is being done at 8000 feet.

165 main (down from the stock 168, per JD Jetting Kit)

Pilot jet 45

JD Blue needle, 5th from the top per JD Jetting 6000-9000

100 pilot air jet, emissions removed and plugged "de-octopussed"

Adjustable fuel screw, but as mentioned adjustment does nothing and I can close it all the way up with no effect.

In addition to what is currently on the bike, I have:

150 155 160 168 and 170 main jets

40 and 42 pilot jets

75 pilot air jet

I'm close, but where do I need to go and with which jet?

Fuel screw in means you need a smaller pilot jet, go down till it is correct at around 1.5 turns out give or take a turn.. Make sure you have a new OEM pilots, never try to clean and reuse them as a tiny coating inside the jet can change the size a lot. Also make sure the passages in the carb body are fully open. Check your seal that goes on the slide plate if its not 100% that will cause a lot of issues also. Read all the FAQ and pinned threads about this and you will find its pretty easy.

Try http://www.jetsrus.com/index.html for carb parts they are reasonable, offer cheap shipping on parts and are super fast..

You've definately got something going on! It should not need the choke on to run!! Back to the basics. Have you ever checked the valve clearances? And then after that, like Rocky said, the slide plate seal is critical!!! Did you by chance remove the slide for any reason? This is commonly re-installed backwards, and will cause terrible running issues too. Check all of these things, and get back to us with some more info.


I may not be able to solve your problem, but I may be able to steer it back on course.

I have a 2008 WR450F that ran great with really good throttle response. It's always had some minor popping on decel but it was getting annoying in Moab last week.

I was thinking I needed to open-up the fuel screw a little, but didn't have an easy-access one on the carb, so bought an R&D Flex-Jet. On my bike I have to take the back-end apart and remove the shock to get to the carb. I did so then I tipped the carb out the back (leaving everything hooked-up to it) and removed the bowl. There was a little red (very fine) sand in the bottom of the bowl from Moab that I cleaned out. Other than that, all looked good. I installed the R&D fuel screw and reassembled eveything. Fired right up and ran smooth. I let it warm-up and adjusted the R&D needle... no change. Blip the throttle and it dies. Close the needle all the way and it still runs, no RPM change. I start thinking that the profile on the R&D needle is different and I need a smaller Pilot Jet.... that I didn't have on hand. I take it all apart and remove the R&D needle and compare it to the OEM and it looks like the very tip of the R&D is a little smaller than the OEM, so I put the OEM back in and set it to 1/2-turn out, just as it was. Fire it up and it's exactly the same as with the R&D in it, has a serious bog, backfires worse that it did, and if you blip it too fast it wil quit. Took it for a ride and could feel it surging a little and if you let it back down to just above idle and open it up, it chokes and almost throws you over the bars. All I did was swap-out the fuel screws!

So now what? I plan to take the carb completely out and throughly clean it all, especially the Pilot Jet. I have a JD kit on the way but may put it all back together just as I had it and see if cleaning it will at least get me back to where I started. I know it doesn't answer your question, but after running this bike exactly like it's been for the last 4 years, changing the fuel screws around has really jacked the thing up.... similar to your situation. The only thing I can think of is that somewhere in the process I got some loose crap in the jets.

Edited by Mtn-Track

Do you have the o-ring as the last part above the spring and washer? Be sure its there, and be sure it hasnt gone bigger so it folds up there. Fcr carbs just dont work right without...

Funny as people always have a scorch the Earth policy when it come to a poor running bike... If it ran good before a simple change then retrace your steps and look for human error, it's always the root of all my issues carb wise...These carbs are a lot more touchy than the carbs of twenty years ago were with the acc pumps and fuel screws making correct set up a little tougher but all the trendy jetting kits in the world won't fix pluged jets, passages and bad seals, missing incorrect orings etc... take the time to clean and set it up correct ( it took me about three weeks of reading, ordering parts then cleaning, reassembling and adjusting to get to a solid place where I will start to do final adjustments ie; needle jet/clip position and main jet).

I had mine apart several times and spent a lot of time looking at pictures and reading on here and the JD jetting and Toyko mods websites to make sure I had it 100% correct (just the acc pump squirt took a couple days alone, I only get a hour or two in the evenings).

Its super easy to put the slide plate in upside down (cutaway at the bottom is correct). get the seal backwards... Not to mention getting the damn acc pump tube and pilot jet passages clean.

Mine was a 2003 and every rubber part had to be replaced as they were dried out or cracked. A few dollars worth of new parts and a lot of elbow grease will get these scooters running better than new.

Opened the airbox, changed the pipe, removed emissions, not really simple changes from a good running bike. I guess I could have left it running good as stock with a pinky sized vortip on the stock exhaust, but I'm into gaining performance - which my mods have done - just need to get it jetted right now. Other than the starter jet, all jets are new, needle is new, carb broken down and cleaned, slide confirmed correct. AP squirt is still a bit long, but not going to mess with the burp on the bottom until the bike runs good.

I guess I'm a bit confused by the suggestion (which I see is also in the sticky thread for adjusting the screw and pilot jet) of going down on the pilot if the fuel screw can be turned all the way in. If I need to put the choke on to get it to run good, doesn't that mean I'm introducing a heavier, richer fuel mixture? Wouldn't going smaller from a 45 to 42 actually lean it our even further?

Those changes have been done by almost everyone on here including myself as they are nessary to make the WR rideable (even yamaha knows this, hence the GYTR kit). Allowing more air makes more HP as long as there is fuel to go along with it, thats why most changes to open airbox and exhaust flow require a re-jet to larger size jets to supply more fuel to the increased air volume.

What I think and everyone else has eluded to is that your bike is not getting gas from the pilot jet and that is why the fuel screw and pilot jet changes are not having any effect on idle (your bike is needing the choke circuit to run because the pilot circuit can't take up fuel so the choke is acting as your pilot circuit). We hope you have checked the valves and any chance of air leaks or other outside the carb causes of lean idle

On my carb I had to soak the passage in real carb cleaner (the nasty stuff in a bucket that destroys stuff) but I put it into the curcuit with a disposable eye dropper and put a rag into the carb body to catch it. I had to do the same to the AP tube as both were completly blocked. Once they were opened I sprayed them out with aerosol carb cleaner and compressed air till I was satafied they were 100% open. I had to polish the jet needle (tube that needle jet sits in and main jet screws into, very confusing) because even after a week soak in carb dip it still had a residue on the inside (I used a soft cloth fed through and aerosol cab cleaner to ge the rest of the nasties out). It took about four goes for me to be somewhat satifed its good enough to work.

Think of your pilot jet and fuel screw like this and anyone can correct me if I'm wrong... The pilot jet controls the fuel mixture from idle through 1/8 throttle and by 1/4 throttle it has very little effect (the needle handles most jetting from above 1/8 through 3/4, the main really only effects 3/4 to wide open).

The fuel screw acts as a fine adjustment to the pilot jet (opening the fuel air screw richens the pilot slightly closing leans the pilot slightly so you can easily adjust the pilot without jet changes for small temp and alitude changes). If you can turn the screw past three turns to get correct pilot jet operation then you need to meter the fuel with a larger jet to get the adjustment ability of the screw back. On the flip side if you can turn the screw in all the way (cut off all fuel through the screw then you need a smaller jet to regain useage of the screw so that fine adjustments can be made to basicly allow you a infinetily adjustable pilot jet so that any off idle issue can be fixed in seconds (providing you have a aftermarket screw with a adj knob). The goal is to get perfect jetting with about .5 to 1 1/2 turns so that you can adjust for a wide range without changing jets.

The AP squirt makes it a bit difficult to determine correct jetting on the pilot as its there to add a shot of fuel when you go from idle to a lot of throttle quickly and the engine can't create enough vacum to pull the fuel up through the jets quick enough ie; bog. So the leak jet and pump timing must be 100% dialed for your type of riding as the squirt too late it's useless and you bog and if the pump squirt is too long then it can flood the engine in slow sections causing a false rich condition. From what I can gather about .8 second squirt is about right for woods riding and it needs to be instant (stiff spring, o-ring or wired linkage) and adjusted to just miss the slide, I was able to get all this with a 50 leak jet and a stiff spring. If I gently roll the throttle on the ap does not squirt as the leak jet leaks faster than the fuel volume is pushed out of the pump, this is where I could tell the bike had a slght lean spot along with the fuel screw opened to three turns I determined that I needed a size larger pilot from a 45 to a 48, I changed and the screw went to about 1 1/4 turns and the starting improved dramatically with no lean spot at 1/8 throttle.

If the throttle is wacked wide open like coming out of a corner onto a straightaway or a wack of throttle to clear a log or a rut the AP pump gives me that squirt of gas to allow the jets time to catch up and supply the fuel without dumping unneeded fuel into the engine but if I am using slight finesse to get through tight trails then it doesnt add much fuel so the engine isnt rich.

I know these things are friggin complicated compaired to the old two stroke jetting I was used to but when set up correct they will work better than most fuel injected bikes out there. I'd bet that 95% of all modern four strokes are not set up correctly so I was determined to be in the 5% group it just takes some time and tuning.

Just a suggestion- replace the pathetic Gen1 FCR for the Gen 2 carb. You will find these on- 2000 WR400, WR426,YZ426, RMZ450 05-07.

Or the Gen 3 from wr450's, YZ450's etc. You won't be disappointed :thumbsup:

The Gen 1 FCR is a POS and has all sorts of drama's like the slide breaking into bits and going through the engine CLICK HERE FOR MORE INFO. This is not to mention that it is on a slant and it makes it hard to set the float height + there is not much fuel in the bowl to work with.

If you want to fix it...

That carb does not have a hot-start so it cant be that :thumbsup: Tick!

The rest should be covered above.

I had to polish the jet needle (tube that needle jet sits in and main jet screws into, very confusing)

Not to be a dick but...

To make things less confusing for you- the thing you call a ''jet needle'' is the main jet needle. The thing that you are talking about is called the emulsion tube or main nozzle.

I thought I had better add my insight to this because not everyone is lucky enough to find a great deal on a Gen 2 carburettor like myself.

First of all- remove carb to check if the AP is working correctly. If it is you should see a nice stream of fuel when the throttle is applied (with fuel in the carb). If not- check that the AP nozzle is not blocked (common) and that the diaphragm is not perished (it will have cracks if bad). There are mods that you can do to this but that is up to you, use the search bar above. See part number 31 in exploded view below.

Check that the float needle valve and float needle are 100% clean! Now- check that your float needle valve 0-ring is not perished by blocking the fuel with the float needle- If it drips, leaks or still flows- replace the O-ring or float needle valve. See part number 24 in exploded view below.

Blow air jets clean and air passage with compressed air or carb cleaner. These supply air flow to the jets. See parts 19 and 22 in exploded view below.

Check that the fuel metering screw 0-ring is intact (if it isn't it will idle high or rev out completely) and the circuit is not blocked . For your info it goes fuel screw, spring, washer, O-ring. See part number 23 in exploded view below.

Now check that the pilot jet is not blocked or varnished and that the pilot circuit is not blocked- Replace if necessary. For your info- if the pilot jet is too small it will have trouble idling and stall easy. In addition it will run best with the choke on and the fuel metering screw past 2.5 turns out. On the other hand- if the pilot jet is too big it will flood with the choke on or stall and or idle high. In addition it will run best at 1~1.5 turns out and or do nothing when the fuel metering screw is turned all the way in (it should stall if correct). See part 21 below.

Check that the leak jet and circuit is not blocked. Some people like to go smaller on this jet by a few sizes to reduce bog off idle. See part number 20 below.

Check that the main jet, main nozzle/emulsion tube circuit is not blocked. For your info- if you are spluttering through high throttle range ie. 3/4 when the throttle is applied , do not have crisp response, backfire when throttle is let off, motor takes time to wind down from 3/4 throttle, you may be running rich. On the other hand- if you are overheating, revving out before peak performance, backfiring when throttle is applied, lacking performance, you may be running lean. See part numbers 17 and 18 below.

Check that the float height is set right- see manual for specifications.


Just for a reference- Standard jets should work fine (apart from the leak jet which can be too large) . If mods have been done try an increase of 2~3 sizes on pilot and main jet.

This info is relevant for all the FCR carbs.

I forgot to add- you can make mild ~ moderate changes to the main jet by raising or lowering the jet needle position.

To do this- if you raise the clip position up it will lean out the fuel mixture through 3/4 throttle. In reverse- if you lower the jet needle clip position it will make it ''richer''. See below...


Once the correct main jet setting has been found, adjust clip position until it smooths out to your liking.

Edited by Barra8

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