James/Clark up date on jetting.

I do thank you guys for the help. I did what Clark said, it helped a bit. BUT I am fouling plugs now. Just to let you know I got my elevation wrong I am at 3500...not at 2200. I decided to check it, I was way off. And it has been raining here so I am running without the entire air box lid. :D It was too dusty to take it off before. :) Should I keep the 170 main and go back to a 42 pilot? And drop the clip to a the #3 slot? (from the top) Or maybe go to a 168 main? Just keep experimenting, right? :D More info would be great.I have read most of the jetting suggestions and hardly anybody is running a completely stock bike...which makes it difficult to judge what to use. I am waiting to get the nice stuff pipe, header, etc...

Kermit: While I wouldn't presume to tell you exactly what jetting to use since my bike is a 426, I can give you the benefit of my experience in sorting out jetting. Like most 426's, mine also had the hesitation when the throttle was wicked open suddenly. It took some experimentation, but now I have the carburation spot on. If you are fouling plugs, either your jetting is rich or you have oil getting into the combustion chamber....or both. Let's assume that it's a matter or jetting. Oil in the combustion chamber is another animal and you would have a much larger problem to solve. The target in jetting is to match the stoichromatic ratio as perfectly as possible throughout the RPM range. The slow speed circuit is usually quite easy to jet since you only deviate one jet size typically and then adjust your fuel screw accordingly. It would be difficult, though not impossible, to get so rich via the pilot circuit to foul the plug. Keep in mind that changing the pilot jet also affects jetting throughout the RPM range since fuel ALWAYS flows through the pilot jet. Changing the air flow characteristics, such as removing the airbox lid, will necessitate jetting changes. An overly rich jetting will cause the engine to emit a particularly unpleasant exhaust odor. One that is jetted correctly will give off an aroma that I find kind of nice. At least it's not offensive. An overly rich main jet will cause the engine to blubber at high RPM's. To test for this, warm the engine thoroughly and then while getting up into the midrange in 3rd gear, roll the throttle quickly to wide open and see if it revs cleanly to the rev limiter while paying close attention to how the engine sounds. When mine was too rich it would suddenly start blubbering well before hitting the limiter. I went down one main jet size and tried it again. It revved higher before blubbering so I knew I was heading in the right direction. I went down one more size and the engine came alive like it was supercharged and it revved quickly and cleanly til it hit the limiter. Once I had the proper main jet, I simply adjusted the fuel screw properly and now the throttle response is perfect and I don't worry about fouling the plug. I pulled the plug a few days ago and it is nice and clean and is a light tan color. As you go through the jetting process, only make one change at a time and then test. If you change the main jet, pilot jet, and needle clip position all at once and then it runs worse, how will you know which change made it worse? While at a local track recently, another rider showed up with his 426 and he fouled a plug immediately because he started the engine and let it warm up at a fast idle with the choke on. After about 2 minutes, the engine backfired violently and stalled. He would kick and kick and about every 10th kick, it would backfire again without starting. The moral of this is that you take the choke off as soon as possible. What amazed me about this guy was that he said the bike did that often. DUH!! He couldn't seem to make the connection between leaving the choke on far too long and fouling his plug. He had to change back into his street clothes, load the bike back up, and leave since he didn't have a spare plug. I hope this was of assistance to you.

Boit is right on. Get the low rpm problems fixed by the pilot/screw. Here's how I found to do this easily. Warm your bike up. Not a 5 minute run, try 20 full minutes. Stop, turn your idle down ever so slightly until it starts running rough but not near stall. Adjust pilot very slowly until the roughness starts to get a little better. The trick is to let the bike run for 5 seconds in between turns (1/8 at atime) and listen for roughness to get better overall. When you feel it has got better a little, turn idle back to normal, test throttle at idle for hestitation, and go for short spin. Ride real slow and feel for surging, sounds like chirping, squeaking or metal to metal contact (other than normal engine noise, if you can figure that out)- if so you're probably a little lean. If you're more than 2.75 out on the fuel screw, up the pilot one number and reset screw to around 1.5 and it again. It's important that you warm the bike, turn the screw slightly, wait for a few seconds, listen, and go for a ride.

And Boit's advise on the MJ is right on.

Now after you master the PJ and MJ, ignore everyone but James and Clark :). Seriously the next step is to dial in your needle. This litle baby is the difference between a WR that runs good (or so you think) and one that RIPS!!!!

My guess is (if you have a '00, WR timed, stock exhaust insert out, airbox lid off, riding at medium elevation and it's getting cold---45 PJ, Fuel screw around 1.75 to start, then turn out (counterclock) slowly. 170MJ w/ stock needle clip #4. This is only a guess. Remember to do lot's of plug checks in between, and right down your setting, temp, alt, and riding observations. Also now that it's muddy, you're air filter willget dirty fast and your carb vent lines will clog easily which will make the bike spit, kick, backfire, hesitate, etc-consider rerouting or kicking the mud off at rest breaks :D


You said that you tried my advise and now the bike runs a little better but is fouling plugs. I gave you some advise on how to clean up the lean spot but left plenty of room for you to experiment. Please post what your jetting is right now with the fouling problem and we will go from there.



I do thank all of you guys for the info. I know there is other guys on this site that know alot too. I have just read more post by James and Clark, so I that is why I asked these two. Clark my MJ is 170, PJ 45, fuel screw out 2.75 ,air box lid off, clip at #4, DRS needle. Clark I am not blaming you for the fouling. I just wanted some advice on jetting the thing. Also it seems like since I took the airbox lid off it falls flat at top end. I am thinking of putting the stock set up back in. It ran great on that, except for a bobble on the bottom. I think I am tring to do too many things at once. I am coming off of two strokes they seemed easier to set up, at least for me. Oh yeah the bike only has 1400 miles on it.


When jetting you count the clip position from the top of the needle. 1st clip is the one closest to the top and 7th is the last one closest to the bottom.

I would recommend the following jetting based on what you have done so far. DRS #4 from top, 45 PJ at with the fuel screw at 2.25 turns or a 48 PJ with the fuel screw at 3/4 to 1.25 turns out (PREFERRED). The main jet selection is not causing your fouling problem. So jet the main based on top end performance, its easy to experiment with.

Good Luck



You are at a higher altitude and are using a richer #4 clip position than stock #3. The needle clip position affects fuel mostly at 1/4 - 1/2 throttle and you have said that it sputters at 1/4 throttle.

My suggestion again is to try clip #3.

(The standard YZ400 needle setting is DVR#4, your DRS#4 setting is close to 2 clips richer)

[This message has been edited by James Dean (edited 10-23-2000).]

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