High-altitude Jetting results - Colorado

I finally have some time to share my jetting results from past rides. I appologize for being "all over the map" on my first few settings, as I really didnt have any sort of strategy.


Patrick Burns Keihin Carb Tuning Manual

First of all, I digested the Patrick Burns Keihin Carb manual and came out with 5 distinct tests to perform with each jetting setup:

1. With a warm engine, rev the bike in neutral...and let the bike return to idle. This measures the # of Pilot Screw turns required to adjust the Pilot Jet (PJ)/Pilot Air Jet (PAJ) mixture.

- If engine speed stays high, then slowly returns to normal, too lean.

- If engine speed drops below idle, then slowly returns to normal, too rich.

2. Gradually accelerate from 0 - 1/8 throttle. This measures the setting of the PAJ/Jet Needle straight diameter. See #3 also.

3. When riding the bike at higher REVS, check the response at:

a. 1/8 throttle under minimal load. This tests the jet needle straight diameter.

b. 0 throttle. This tests the PAJ setting.

4. Test the acceleration at both lower and higher REVS at:

a. 1/4 throttle

b. 1/2 throttle

c. 3/4 throttle.

This tests the combination of the jet needle clip position and main jet settings.

5. Test the acceleration of wide open throttle (WOT) at:

a. WOT max TORQUE. This tests the main jet (MJ) setting.

b. WOT max REVS. This tests the MJ and Main Air Jet (MAJ) settings.

- If WOT max torque is good, and it seems lean on WOT max REVS, go down on the MAJ.

- If WOT max torque is good, and it seems rich on WOT max REVS, go up on the MAJ.

Also, according to Patrick Burns, one should work on setting your jets in the following order:

1. Main Jet (MJ) & Main Air Jet (MAJ)

2. Jet Needle (straight diameter, taper - D or E I'm assuming, clip position)

3. Pilot Jet (PJ) and Pilot Air Jet (PAJ)

4. Pilot Fuel Screw (PScrew).


Given all of that mumbo-jumbo, my bike has WR timing, uncorked stock WR exhaust, BK mod, airbox cover off, White Bros. air filter, grey wire cut, 91 octane pump gas.


A. Summer - 60-90F/7K-9.5K ft (Rampart) - 42PJ/75PAJ/1.5 turns/DRR#3/155 main/200MAJ

- Average accel thru throttle range, lean on PJ/PAJ/Jet needle diameter, lean on main jet. An overall OK setting...

B. 7/4 - 50-70F/10K-13K+ ft - 42PJ/75PAJ/1 turn/DRR#2/155 main/200MAJ

- Very mediocre setting at best...but atleast it got me up to the tops of the 13K ft. peaks in Ouray, CO! Amazing place to ride by the way. Need a lot of work here...

C. 11/3 - 40-50F/7.5K ft - 45PJ/85PAJ/0.5 turns/EJQ#3/160 main/200MAJ

- Sucky idle & 0-1/8 throttle (very rich), explosive accel 1/4-FT compared to D__ jet needle.

D. 11/3 - 40-50F/7.5K ft - 45PJ/85PAJ/0.5 turns/EJQ#2/160 main/200MAJ

- Sucky idle & 0-1/8 throttle (very rich), average accel 1/4-FT (too lean)...clip #3 better choice.

E. 11/11 - 40-60F/7.5K-9.5K ft (Rampart) - 42PJ/75PAJ/1.5 turns/EJQ#3/160 main/200MAJ

(First started documenting results from my tests listed above).

1. 1 turn idle doesnt come down (lean), 1.5 turns good. FYI...First kick start (no hot start) for a warm engine.

2. Accel 0 -1/4 throttle good.

3. Slight hesitation at 1/8 throttle and higher revs. Maybe try E_P needle diameter.

4. 1/4, 1/2, 3/4 throttle acceleration good. Maybe slightly lean on main and/or jet needle clip position.

5. WOT max torque good (maybe lean). WOT max revs slightly lean.

F. 11/17 (this Sat.) - 40-60F/7.5K-9.5K ft (Rampart) - 42PJ/75PAJ/1.5 turns/EJQ#3/162 main/200MAJ

Results TBD.

Subsequent tests:

- Take best of 160/162 MJ and look at smaller MAJ.

- Try EKP jet needle with best MJ/MAJ

- Try 40PJ with best MJ/MAJ/jet needle.

BTW...If anyone's interested in joining, I'm meeting my bud Jim at 6AM at the gas station at 470/Sante Fe and heading up to Rampart. Will probably ride until 2PM or so...

More results to come!

[ November 16, 2001: Message edited by: LarryCO ]


That was a very good attempt at summarizing Patrick Burns' jetting article. It is not a simple task. There are so many things interacting at 0-1/4 throttle that it is difficult to summarize and there are never enough words.

There are a few choices that are critical to the order of selecting jets that Patrick made little mention of. The float level, needle taper, and needle straight diameter. If any of these are changed, you are best to go back and start over. The taper throws the previous main jet choice out the window from "D" to "E". The YZ went from DVR/#175 to EKR/#162 from '99 to '00. The straight diameter interacts with both the pilot circuit and the clip position. An EMP might run #5 clip and an EMM with a leaner #2 clip position AND leaner pilot circuit settings.

A year ago I ran tests with D, E, F tapers and L,M,N,P,R straight diameters. The conclusion was E_M needles(EMN,ELN,EKN,EJN) were best near sea level and 60F, E_P were best for warmer temps and higher altitude(EMP,ELP,EKP,EJP).

You have already seen the benefit of switching to taper E and are headed for the P straight diameter. Hope it works out.

How did the test ride go?


[ November 19, 2001: Message edited by: James Dean ]

[ November 19, 2001: Message edited by: James Dean ]


Have you guys seen the carberation pictures in the 426 shop manual? They show graphically how all the jets interact vs throttle position. If you have seen them, do you think they are right?


Yes they are very informative. The percentage change is interesting. The needle straight diameter does not extend as high in the throttle range and taper effect can actually cross from lean to rich which the graph does not show.



I am sad to inform you that I was unable to complete my tests this past weekend as listed previously. A little too much drinky drink at the Av's game this past Friday night contributed to a serious headache on Sat! :)

I was able to do a quick test ride around the neighborhood using 42PJ/75PAJ/EJQ#3/162main/200MAJ...but probably didnt ride long enough to get an accurate measurement of the change in main jet from my previous setup.

Given that there's snow on the ground now (as many of you probably saw falling during the pathetic Broncos/Redskins game Sunday afternoon), it'll probably be a little while before I'm back out testing. I do feel pretty confident though that the "close to best" setup for cold, winter days in the Denver area is a 160 - 162 main with an EJQ#3 or EJP#3 needle and a 42PJ/75PAJ.

Thanks for confirmation on the tests, etc. I think the bigest problem I'll have going forward is determining what subtle changes in jetting cause (e.g. if I have a slightly richer condition than what I had, etc.)...I think there's a point where you need some sort of measuring equipment (dyno/gas analyzer) to really notice the differences.

Oh, one last thing...now that I've done a bit of testing, I'm very seriously considering the YZ timing mod (and probably throwing all test results to date out the window!). Would you expect that mod to make the bike want a slightly richer jetting...or leaner? And if so, in what throttle ranges (0 - 1/8, 1/8 - 3/4, and/or 3/4 - FT)? I'm assuming that the biggest impact would be 1/8 - 3/4 since that's where the biggest performance gain is...so changes in the jet needle clip position and/or main jet...but that might be pretty short-sighted of me.


[ November 19, 2001: Message edited by: LarryCO ]

[ November 19, 2001: Message edited by: LarryCO ]

I wish you had a 99 400. That would have saved me a lot of work :D

I can't believe you would have the time and patience to tear into the carb that much during a ride. Everytime I get the idea I will do the carb testing, I get on the bike and then get very lazy. I'm like, screw it, it's running, it's a nice day, I want to ride instead of test. I admire your tenacity.

Maybe I'll just have to buy a 2001 426 and jet to your tests. I'd rather spend the money than take the time to jet my own bike correctly :)

Only a couple of more chances to ride Rampart before it's too late. Soon the snows will come.


I sure have you fooled! You've misjudged me! If given the opportunity to ride 65 miles at Rampart(and feel like I was hit by a truck the next day!) or ride 10 miles and make 5 different jetting adjustments, the choice is clear!

I do, however, monkey around in the garage every now and then with my jetting...and those results are from basically the last 6 months or so (once I started paying attention!). I live in Genesee (Golden) at 7500 ft...so that makes testing jetting changes nice from an altitude standpoint. But I appreciate the admiration, albiet undeserved!

Yes, Rampart is all but a dream for the rest of the winter I'm afraid. However, I do see a few Fruita trips in my near future. If you're interested in joining in, I'm hoping to get out there in mid January or so. Great riding out there if you've never been...the "Rim trail" (I think that's what it's called...along the CO river) is pretty cool...and of course, the start of the Kokapelli Trail isnt too bad either. I'll let you know when we finally come up with a date. I believe my amigo Jim will be heading out there on 12/1-2...so let me know if there's interest there as well.


Glad to see you are considering the smaller MAJ, I am too. My notes from jetting q's have me with the PAS at 75 or 85 and then honing in the MJ/MAJ ratio. It was pointed out in that thread that these ratios are really important to get right. I got MAJ 160-170-180 from Sudco, waiting to try them.

How do you like the Michelin M12's?

I can't experiment at Rampart either, too much adrenaline. I'll usually go to Berthoud to do it.


Could you do me a favor? When you get a hold of the correct part numbers for those MAJ's, please post them on this thread for me...so I can get them as well. I'm assuming a 190 and 180 would be appropriate, and maybe a 185...but without playing around with them, I have no idea of how "sensitive" the change would be.

Havent ridden the M12's enough to make a call either way. I dont think it matters what tires you use sometimes out at Rampart, because you're going to slide off the trail anyway (with all that loose, sandy, rocky crap). I've put 65 Rampart miles on them so far and a few road miles and they're still looking pretty freekin' new (which they should). I'll keep ya posted as I get some more miles in.

Heh, that offer of Fruita in Jan. is obviously extended to yourself as well. Let me know if you'd be interested in a long weekend out there, as my bud Jim and myself surely will make it out there sometime.



To get these parts, I just called sudco and asked for them by name: i.e., Kehin FCR-39 MAJ160...

I agree that you and I will probably need slightly bigger on the MJ/MAJ ratio to compensate for altitude. For example, given 160MJ, likely in the 165-180MAJ range. I will know soon.

Here are some MAJ part numbers:

Sudco part number:

KE 019.202 #160MAJ

KE 019.210 #180MAJ

I also got a MJ:

KE 021.235 #165MJ

Sudco 1-800-998-3529.

The paper was ripped for the #170.

What is the riding like in Frutia?

[ November 21, 2001: Message edited by: Stefe9999 ]

Thanks for the part numbers....

Fruita is not unlike Moab in a lot of ways...just subtract a little slickrock and add more sand. It's a great alternative for riding (both mountain bike and motocross) and is 90+ minutes closer to home. I really enjoy the "Rabbit Valley" area (exit #1 just before getting to the state line on I70)...lots of trails out there for all riding abilities. Not to mention, some pretty cool scenery ta boot...especially on the trails overlooking the CO river. It's also the start of the Kokapelli Trail to Moab (~110 miles).

I'll let you know about our pending trip when it's planned...


4/12-4/13 Moab, UT (4500ft...60-80F) & Golden, CO (7500ft...70F):

38PJ/65PAJ/1.5 turns/EKP#3/150MJ/160MAJ

('01, WR timing, uncorked US exhaust, BK mod)

-Bike started cold first kick every time with choke. Always started first kick warm (no choke or hotstart). Idled well and always returned to idle immediately.

- VERY CRISP AND SNAPPY from 0 - 1/8 throttle. Smooth acceleration upon gradual twist of throttle.

- Good accel from 1/4, 1/2, and 3/4 throttle.

- Excellent high-end performance at max torque and max revs.

- Very little backfiring upon decel. at high revs and 1/4 and 0 throttle.

By far, the BEST JETTING YET! Very satisfied with 150MJ/160MAJ on the high-end...and extremely satisfied with the snappiness of the bike from 0 - 1/8 throttle! Wish I would have tried clip position #4 at the lower elevations of Moab to try to improve an already great midrange. Although, at the higher elevations of Denver and Rampart (7500 - 10K), clip #3 is the way to go. Would HIGHLY RECOMMEND this setup to anyone at the higher elevations!

Next test this weekend at Rampart: 35PJ/55PAJ/EKN#3/150MJ/160MAJ

After the results of this next weekend are in, I'm definitely sticking with one or the other. Much better performance than my previous 42PJ/75PAJ/EKQ#3/160MJ/200MAJ setup. Another Taffy jetting believer!


I assume your bike has the air cut valve intact. There is a lot of rumor that the crispness of jetting changes, esp on the pilot, are muddied by it. Have you experimented with it or did you have no fuzziness in the Patrick Burns low end tests with it intact?


I dropped mine to a 162 main and 42 pilot. Will try it out at Rampart this weekend. I also set up the Taffy mod but I haven't even started it yet. (had to get some yardwork done that I have been putting off for a while).

Larry, what day do you plan on going to Rampart? I can meet you up there on Fri or Sat.

work that pilot screw to see which setting works best.

try dropping the needle BEFORE you go to even smaller pilot settings.

does it have the Taff Mod APJ?


Taffy, I have not done your "Taffy mod" to the APJ. What I really want to accomplish with the jetting was to get rid of the off idle bog.

i'm sat on the other side of the riverbank and am happy with the bike. how you get there is up to you. thing is i can't help you if it goes wrong.

don't forget that i disconnected the APJ after just three tests and only put it back on when all other jetting changes were done.

you can't recogniise what's going on with it connected. it gets between you and the bike.

i raced mine but never thought of reconnecting it because the bike got better every ride without it.

i hope you'll consider getting on without it.


Hey Larry, I tried the EKQ #3 165 Main last week at Rampart on my '99 WR400. It ran terrible was blubering at 3/4 to full throttle and sounded like the plug was going to foul. It still wanted to stall if you whacked the throttle right off idle.

Will try clip #2 this week to lean it out a little more. How did you get rid of the off idle bog?

This is how I did the Taffy Mod, is this correct?

I used a wheel coller for an RC car and slipped it on the rod of the accelerator pump. I then tightened it down so that it would only allow a small squirt. I didn't have anything to accurately measure the squirt length so I counted "one one thousand" :) I only got to "one one" of that count when the squirt died. It was approx 3-4 seconds before the mod.

I am currently jetted as follows (have not tested yet though):

Main: 162

Pilot: 42

Needle: EKQ #3

I have not de-octopussed nor have I changed the air jets (which I have been told not to change until I have removed the octopuss). I'll post results after the weekend.

sounds fine dougie.

the simple mod results are the best doncha think :)

surely you had a a pair $2 feeler guages in your tool kit? micrometer? vernier guage?

.024" or .6mm will do fine.

i don't know "their" reason for saying you shouldn't change the PAJ until you've deoctopussed? thats a new one!


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