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LarryCO posted this good summary of the Patric Burns article a while ago. If your willing, I'd like to ask you guys to elaborate on a few things on it and then maybe we can put this up in the tech section on jetting. I'm hoping this will help a lot of beginners understand how to start the jetting process and save you guys lots of questions. If it looks like too much work, no sweat, thanks anyway.

Here are my requests/questions.

1. I've re-ordered the testing based on how you should do the testing listed at the end of the post. Agree?

2. I don't see anywhere it mentions on what symptom to look for to adjust the PScrew

3. At the end of item 1, it says if it seems rich or if it seems lean. I believe lean would seem starved for air, and rich would be rough running. Agree?

4. On item 2, it says that these tests test the jet needle clip and main jet settings. What symptom per each throttle position is a MJ problem and what symptom is a needle clip problem.

5. On item 3&4, what symptom is the PAJ and symptom indicates the needle straight.

6. Test 4 sounds confusing, it sounds like hi-rev the motor, back off to 1/8 and gas it to check response. Correct?

Larry's post, with my re-ordering:

1. 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.

2. 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.

3. Gradually accelerate from 0 - 1/8 throttle. This measures the setting of the PAJ/Jet Needle straight diameter. See #4 also.

4. 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.

5. 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.

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).

[ November 28, 2001: Message edited by: Stefe9999 ]

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Great idea! Although, I'd be the first to admit that JD/Taffy/etc. should provide their experiences as appropriate beforehand...and we could certainly add content to help diagnose lean or rich conditions for each test (i.e. what to look for).

Here's a few comments on your questions below...sorry I wasnt being as clear as possible the first time...but my fingers were tiring out from all of the typing! :)

1. Reordering of the tests: I ordered them that way because you generally start with your bike at idle...accelerate through the throttle ranges...and end up at WOT. I wasnt trying to put them in the order of jetting changes (as listed below in your post). Doesnt really matter the order you do the tests in...just that all the tests are done I suppose.

2. PJ/PScrew symptoms: The symptoms are listed in test #5 in your post. Test #5 was meant to test the PJ/Pscrew setting. You're looking for the return of the engine speed above/below the original idle speed. If it returns quickly to the original idle speed, you're good. If you have to go under 1 turn or over 2 turns on the PScrew to correct, make the appropriate PJ change (larger or smaller).

3. WOT tests: I agree...I would think that a rich WOT response would be "burbling" and running rough. Never really tried it with an enormous MJ or a severly-undersized MAJ to really know firsthand...but I suppose I'll be doing that soon enough after getting my recent purchase of smaller MAJ's from Sudco...thanks for the pn's! My take is to use the biggest MJ you can to cause a rich condition (easier to notice I suppose than a lean condition) and then back off a little bit at a time on the MJ until you're happy.

4. MJ/Needle Clip problems: (As I understand it) since the MJ overlaps so much down in the 1/4T - 3/4T area, it's hard to separate the two. If you've already jetted at WOT (Test #1 in your post) and are happy with the MJ, you should only have to play with the needle clip position. My experience has been that it's pretty obvious what clip position you should run at given a MJ...you'll know if you're off because the bike runs like crap!

6. Test 4 explanation: Test 4a means you're running at higher rpm's (moving) and are simply maintaining that speed in a higher gear by only using 1/8 throttle. This supposedly tests the jet needle straight diameter setting. Test 4b means you're doing the same thing (moving at higher rpms) and let off the throttle completely. I suppose you're listening for backfiring...which if you hear, you're lean (increase PAJ). I'll admit, I'm not as clear on this area yet, but notice the overlap on this test vs. test 5 (idling engine test).

5. Needle straight/PAJ settings: See #6 above...running at higher rpms at 1/8 throttle (maintain speed on level ground for eg.) tests jet needle straight...running at higher rpms and closing throttle tests the PAJ.

NOTE: All of these tests assume that you're using a D taper or E taper jet and are sticking with one or the other. I didnt include any mention of finding out which taper best suits your bike/conditions. That part could be added by JD/Taffy...as they've played around with other tapers, etc. I'm sticking with the E and going from there...

Anyone else have any feedback?

[ November 29, 2001: Message edited by: LarryCO ]

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well done stefe, good idea

would you like to have an intro that is relevant to us and dirt bikes etc rather than using long words etc.

your write up could explain how it feels on "our" bikes as opposed to just taking PB's write up over to us. after all it would be easier to supply a link to PB than to do it all yourself. this i would therefore say is your chance to personalise it.

section on easy hints on removing carb

JD undoes the throttle cables,, Taffy moved both engine plates etc

what kind of day and things to do and tools

Taffy; i would chew over the area that i'm going to try to improve and go check PB then i would put a 3 and 4 allen key, ratchet... etc in my rucksack etc...

what kind of areas to ride to check each thing. how long should you ride for on each change.

general tips for exhausts

general tips for airbox lid off

general tips for YZ timing the jetting;

general tips for altitude/europe, east west coast.

i think larry you meant to say PC and not PAJ.

also when lean at low revs turning out the PAS makes it WORSE.

perhaps you should say to people look don't do everything just rejet your exhaust so you've learnt something first.

i hope this has helped the start. if you say what you want i'll have a go. it's worth pointing out that we can't even get a consensus on the MAJ! our PJ's are everywhere. all the old school are happy with 48/100 and nothing is going to change them.

perhaps you need to pick some priceless quotes out and do something like this

Taffy; i like very low jets because blah, blah...

JD; the 48/100 is a great set up .......

how does it feel when it's rich or lean. don't forget you've got ALL your senses.

there's even smell!!

sight (plug, pipe, header etc)

feel (through the bars, bum, feet)

hear (engine, exhaust, airbox)

taste. er..! we'll forget about taste shall we!

even gearing effects jetting and feel. it's no good saying my bike flies to the limiter on 13/53 gearing is it!!!

likewise on 15/48 like me today, you won't get the back popping quite as rauocously (sp?)on a closed throttle. this could come under things to remember.

oh! and you've got conditions etc. i've just caused a ruckus over on the 250 site because they said that the air is denser in the winter so you should jet up. i explained that the weatherman tells you that that last LOW will be followed by another LOW on Saturday etc. Low pressure means jet down and not up.

as i mentioned there i've sat on that one for the 18 months i've been here.

the other one that i've sat on is the one about pushing the rear wheel forward. if you push the rear wheel forward you take weight off the front and put it on the rear so the front goes light and we all fall off instead of turning slowly!

just an aside chaps ok. start a new thread for that if your gonna chip me. keep OUR focus here shall we?

anyway well done.


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Like I've said before, there are never enough words to cover it all. Patrick Burns work is a very good article. ( www.lifenet.com/brm/carbkei.htm ) It is difficult to make a blanket statement of what order to tune and the symptoms of each. Pick a needle taper and straight diameter and hope that it's right because if it's not EVERYTHING will have to change. Then start over.

Something like:

-Main jet (rough estimate)

-Tapers (D,E,F) and clip (rough est.)

-Main jet (again for taper changes) and main air jet

-straight diameter and clip

-pilot jet and plot air

-refine clip position

-refine plot air and pilot screw

-recheck the main jet

To make a complete assessment it takes iterations rechecking the overlapping circuits. As Taffy said, all the senses provide some measure of how the bike responds. Varied conditions are needed to establish a measure of what will work. Long uphills and downhills of different steepness. We have 6000ft ridgetops and can easily do 10,000ft of net elevation rise and fall in a days riding here. :) Things like that aren't easily tested and measured on dyno's.

I'm working on a tuning guide myself and it will include an simple way to compare jetting. Take a current setting and key it into an Excel spreadsheet program with pilot, needle, clip, main, temp, and altitude and it will plot the needle profile. Key in 2 alternate settings and compare percentage jetting change versus throttle position at the same or different altitude or temps. Someone in snowy Colorado can compare to sea level in the California desert. A rider with a stock OBDRS needle can see how far off it really is from the YZ OBEJP. The main, needle diameter, needle clip, needle taper, and pilot jet effects will be similar to those graphs shown in the back of your manual. More than that, it will show interactions of ALL of these together. I feel reluctant to ask a price for this, but it has been a great amount of work putting it together. Would it be worth $20 to see any of the over 100 needle profiles available? 100 needle profiles times 7 clip positions makes 700 available settings to compare by the stroke of a key. Multiply that by 5 main jets and 5 pilot jets gives thousands of settings at any altitude and temp. This is an Excel spreadsheet and I think it will give the public a view of something only the factory engineers have had up until now. I will let you know when it nears completion.


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check out the current addition of Motorcross action mag. On page 170 you'll find a step by step procedure on adjusting your fuel screw. I'd try that first, the worry about the jetting.

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You MENSA guys should work up a jet kit and sell it.

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