Altitude Jetting

I'm riding a '00 WR400F YZ, timed, UNI w/cover off and stock exhaust w/o baffle. I ride in NM @ 5,000-6,000 ft. I'm currently running PJ=42, MJ=170, DTM needle #2 from top Air 1-3/4 turns. This was recommended by the Yamaha dealer. Bike seems to run rich at mid throttle. I'm thinkin' about changin to the DVR needle #4 and PJ=45. Bike runs okay, but a little sluggish in the low end and transition to wide-open isn't crisp. Any suggestions from the experts?

DVR#4 is very close to your original "stock" needle DRS#2. DVP#4 would split the difference and be a little leaner off idle and a little richer in the midrange than the DTM#2 you are at right now. If you are rich in the midrange and not down low, then the DVR#4 should be shifted one clip leaner to DVR#3 or #2. #170 main sounds like it could be increased, see Clark's tech section. articles.


Try this with your YZ timing between 5000 and 6000 feet and I'll bet you'll like it:

Pilot: 48

Needle: DVP #4

Main: 178

I run the 45 pilot with YZ timing in Colorado all the way up to 13000 feet so I think the 48 would be better at your elevation. That's what I run in Moab which is only a little lower than you.

You can even go a little higher on the main if you dare. This setup will even allow you to go a little higher in elevation if you want. Let me know how you like it.


Thanks guys. Looks to be running a bit rich in the mid-range with my current setup, so leaning out the mid-range should help. I'll try the changes. Gotta order the new needle jet, though. I'll post a reply when I get the parts and get a chance to try them out.


After riding the PJ=45, MJ=172, DRS#2 PS=1 1/8, I rejetted to PJ=42, MJ=170 and DRS#2 and PS=1 3/4. Bike runs better, but pops through exhaust on deceleration from intermediate speed w/PJ=42. PJ=45 no popping, but plug fouls going downhill w/o throttle on. MJ=172 and bike stumbled at WOT. Read Clarks technical writings but still wondering what to try next? DRS#3? DVP on order!

-gary in albuquerque. . . .

I was wrong on the needle jet. I have a DRS #4 (on suggest of local Yamaha dealer). Runs rich at intermediate speeds. I'm rejetting to PJ=45, MJ=172, DRS#2 and PS=1 1/2 turns. I have to order all the jets Bryan suggested. Albuquerque Yamaha dealers don't carry much stock. I'll repost my findings once I get the new jets.


I've been experimenting with jetting, and doing more reading from Clarks technical section and I'm a bit mystified by some symptoms that my bike is exhibiting.

I'm running PJ=45, DRS#2 and MJ=170 PS=1.75 turns, airfilter is clean, no baffle w/stock exhaust. I'm riding at 5,600 ft in NM.

My bike loses power on acceleration and on the high end. It makes this sputtering noise. It pops on deceleration through the exhaust. It also runs hot now. I think it's too lean on the DRS and MJ. Any suggestions. I think my diagnostic skills are impaired because of too much exhaust fumes!!

I have a DVP on order as well as a DSP exhaust - thank goodness!


Does your bike have YZ cam timing?? Clark has noted overheating with lean jetting and WR cam timing in the tech section article.

Looks like DRS#3 is the only setting in between too rich and too lean. Your PJ=45 and PS=1 1/8 setting had no popping, try it again. Make sure there are no vacuum leaks.


James - Yep, got the YZ timing; that's what started all this carb stuff. I'm going back to PJ=42, DRS#4 and MJ=172 with PS=1 1/8. The only symptom I had was some sluggishness on acceleration and WOT would cut-out with a little popping on decel.

My confustion is on the symptoms: is cutting out/bogging a symptom of too lean or too rich. (Thank gawd I'm not a doctor!)



DRS #2 or 3, 48PJ, 1.5 turns on the fuel scrw and a 178 or 175 MJ.

Good Luck


[This message has been edited by Clark Mason (edited 08-11-2000).]

Clark -

Don't own a PJ=48 (yet!) so I'm jetted as follows to compensate(5,600 ft):

PJ=45, DRS#4, mj=180 and PS=1.75

It seems to run pretty well. A little rich on the high end, and a little leaner on the low end. I think that will go away when my DVP needle comes. I may take it down to MJ=175 if the plug fouls. . . .

I tend to ride my WR in the upper RPM's most of the time, so I really notice when the MJ is too lean. You think a DRS#3 setting will give me a more linear and predictable power curve? It does tend to take off at 1/4-1/2 throttle and then again at WOT.


Gary (who's tired of taking off the float bowl on his WR) :)

Maybe just hold off until the DVP and other exhaust come. The middle letter "V" is 2 clips leaner in the midrange compared to the "R" (DRS). In the midrange 1/4-3/4 throttle DRS#2 equals DVP#4. Because DVP will be richer down at 0-1/4 throttle DVP#4 will be a good starting point.


Thanks for the information. . .now I understand the difference between the two needles. PJ=42, MJ=172 DRS#2 was pretty lean w/lots of popping on deceleration and stumbling on acceleration. I like how it felt in the mid-range (DRS#4 isn't as strong), but the overall performance is more consistent now. If the DVP needle is richer in the low end, that would help. I'll try DRS#3 and see if that helps. I think Clark is right: PJ=48 DRS#3 and MJ=175/178 w/stock pipe no baffle would be best.

I have a DRT and and EKR needle needle. Would either of those help the midrange?

DSP exhaust system will be here next week. Oh boy, get to do this all over again. . .I need to find a dynomometer here in Albuquerque!

Clark and James, would you to consider posting some graphs if fuel/air ratios over the RPM range using some of your jetting suggestions using a standard bike setup? Or better yet, a jetting spreadsheet! You put in your altitude, setup and it gives you suggestions on how to jet your bike!!

Thanks guys for all your help!! I'm close, so close! :)


ps - just rejetted my son's YZ125 w/FMS SST pipe and it runs GREAT up here!!

Got it!

Clark, your setup works. I like it with DRS#3, PJ=48 and MJ=180/175. I'll ride this until the DVP comes and the DSP pipe.

Thanks!!! :)

[This message has been edited by Gary Kessler (edited 08-13-2000).]

Gary K.

Glad to see you are making progress.

With the new DSP exhaust keep everthing the same and run a DVP in the #4 from the top and you should be in the ballpark.

Once James and I finish collaborating on what is turning into a joint needle development project we will post the results for all to share in.

Good Luck


Clark, James and Brian -

Got the DSP pipe and rejetted to the following: PJ=45, MJ=178, PS=1.75 and DRS#3. Bike runs well at 5000 ft. Only comment is the stumble at WOT, which Clark mentioned is the rev limiter, and I think so too. It does want to stumble at load when I go quickly from 1/2 to WOT. If I back off, then do it more slowly, it's fine until I hit WOT. Going to put in a slightly longer throttle stop and see if that helps. Now just waiting on the DVP needle. I have an EKR needle. Would that make any difference?

-Gary (patiently waiting on my DVP needle)


'00 WR; YZ modified, DSP exhaust system, UNI air, Dunlop D756, ProTapers, Devol Rad Guards, P-38, Mason/Dean Jetting specs. . .and lovin' it!

[This message has been edited by Gary Kessler (edited 08-18-2000).]

Guys - thanks I finally got it perfect!

As suggested by everyone, with the

DVP needle I'm using the following:

PJ=48, DVP#4, MJ=178, PS=1.75; YZ timing

aftermarket DSP Factory series exhaust @5300ft

It runs strong!!

For those that don't have the DVP needle (they seem hard to get). Using the stock '00 needle (DRS), I found the following worked well w/o aftermarket exhaust

PJ=45, DRS#3, MJ=172/175, PS=1.5 w/YZ timing

@5300ft. Results may vary w/your local terrain! :)

Thanks again!! Ride with the rubber down!


'00 WR; YZ modified, DSP exhaust system, UNI air, Dunlop D756, ProTapers, Devol Rad Guards, P-38, Mason/Dean Jetting specs. . .and lovin' it!

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