Jetting Recommendations?

[Aus_Rider_22]

I was just after some opinions and recommendations for the jetting of my 2000 YZ125.

I currently have stock Jets: Pilot => 35
Main=> 350
The clip and jet I do not know, but assume they are stock sizes and postions. My air screw is tuned by the mechanic.

What I would like it some input into what changes, if any, you would recommend. It's spring here and about to come into summer. The temps I wil be riding in will be about 25-35 degrees.
Iam at see level basically.

My bike is stock except for Carbon Fibre reeds and a slightly shortened silencer.
I alternate between stock and aftermarket filters. Would this have much affect on jetting?

I was thinking maybe going richer from stock to accomodate for the reeds.

The bike is currently in getting fresh top-end and I was thinking now would be the best time to try different jets. I am happy with the bike's performance, but Iam looking for better response. The only real thing that I would complain about is that from slow speed the bike doesn't pick up or accelerate as what it probably could. It doesn't bog or die out, it just is kind of mediocre.

Maybe I could get it crisper.

What would you do and what size jets would you recommend trying?? Btw, when I do a plug test, it usually comes out slightly oily.

Thanks for your time.

Regards
Geoff

[Faded]

What I would like it some input into what changes, if any, you would recommend. It's spring here and about to come into summer. The temps I wil be riding in will be about 25-35 degrees.
I am at see level basically.

With the weather warming up you might benefit from leaning it out.

I alternate between stock and aftermarket filters. Would this have much affect on jetting?

Not usually, but if it does the affect is usually negligible.

The bike is currently in getting fresh top-end and I was thinking now would be the best time to try different jets. I am happy with the bike's performance, but I am looking for better response. The only real thing that I would complain about is that from slow speed the bike doesn't pick up or accelerate as what it probably could. It doesn't bog or die out, it just is kind of mediocre. Maybe I could get it crisper.

Now would be the perfect time to try and clean up the jetting. Some people make the mistake of trying to rejet their bike when it’s NOT 100% mechanically sound. You also need to eliminate as many variables as possible. This means figuring out which oil type, fuel type and premix ratio you’re going to run and stick with it. Make sure you’re using a freshly cleaned air filter and that you’re reeds are in new or like new condition. Another thing I would add is that you repack your silencer and check your float level before you even think about swapping brass.


Click Here for directions on setting your float level.


By your description is sounds as if the slow(er) speed circuits could use some dialing in. Try my recommendation on determining pilot size, setting the airscrew and setting/adjusting idle speed. It’s best to start at the bottom and work your way to the top. All jetting circuits overlap and will affect each other as you begin rejetting, so you need a good starting point. Time to start at the bottom…

You need to make sure you have the correct pilot jet size and properly adjust your air screw before you try and set the idle. First you need to run the bike and bring it up to opperating temperature. Now find the airscrew. If you're looking at the carb from the left side of the bike it will be on the far right side just under the bell of the carb. Look for a small brass flat-head screw. Try adjusting it until you hear the idle come up. (If your bike doesn't idle try turning the idle speed up a little and resume this test.) You want to set your air screw at the point in which it idles the highest. If the idle get out of hand and takes off while adjusting the air screw then use your idle adjustment to keep it in check.

After this test ride the bike by getting the clutch out and putting along and then quickly roll the throttle on/wick it open. If it bogs and then takes off you're lean, turn the air screw IN (clockwise) an 1/8 of a turn and recheck. If its slow/sluggish to respond and then takes off you're rich, turn the air screw OUT (counter clockwise) an 1/8 of a turn and recheck. Keep doing this until you have achieved optimal throttle response.

After you've tuned for the best throttle response kill the bike and turn the air screw in until lightly seated making sure you count the number of turns it takes to get there. If you end up being more than 2.5 turns out on the air screw you'll want to try the next smaller pilot jet, less than 1 turn out and you'll need the next richer pilot.

If your pilot needs changed you'll have to repeat the process (start back at the top of this post) of setting the air screw. After all this if the idle is too high/low then you need to reset the idle screw.

What would you do and what size jets would you recommend trying?? Btw, when I do a plug test, it usually comes out slightly oily.

All bikes are not created equal so to say that xx jets will work in yy bike is usually far from true. Your best bet is to determine what is best for your bike in your environment under your variables.

Remember that a plug chop is only good for a few things like determining correct plug heat range, ignition timing and main jet strength. A plug chop needs to be performed on a brand new, properly gapped plug. When reading the plug it must be done on a new plug otherwise you're going off of multiple conditions with varying loads, conditions, and RPMs. How do you know what circuit/load/condition you're reading from? There are many variables. The weather, fuel, etc. are constantly changing and if you're trying to read a plug with weeks worth of changing information then you're shooting in the dark. You could have two out of four circuits messed up and still come out with a tan plug. The idea is to eliminate the variables and start fresh. ***Usually the fatter wire plugs (NGK BrxxES series) read faster and are cheaper which makes them ideal for a plug chop.***

As far as jetting is concerned, the plug is only useful in diagnosing the main jet strength. You need to look for the mixture ring, which will form at the very base of the insulator (way down behind the threaded portion of the plug). This will be hard to see unless you have a good magnifying glass and some good light or you cut the threaded portion away. You can't read main jet strength off the nose of the plug. If you install a new plug, warm it up, take it down the street and do a throttle chop the nose will still be white and like new, but the mixture ring will form at the base of the insulator.



But, we’re getting a little ahead of ourselves. Start at the bottom and work to the top, right? After the pilot/air screw/idle is sorted you’ll want to move on to the needle and needle clip position. The best way to do this is the ride-and-feel method.

You need to start by marking your throttle housing with something you can write on. I like to use a strip of white medical tape and stick it on the throttle housing next to the grip. Now, go dig through your Mom's sewing supplies and find one of those needles with the colored head on it. Stick the needle into the big doughnut part of the grip next to where your thumb and forefinger contact the grip. Take a marker and draw a line on the tape next to where the needle sticks out of the grip, this mark will be "0" or closed throttle. Next, rotate the throttle all the way to WFO and make a mark on the tape where the needle sits. Obviously this mark will be WFO, WOT, full throttle, etc. Measure the distance between 0 and WFO and make a mark directly in the center for ½ throttle. Do the same for 1/8, ¼ and ¾ throttle. Now you’re set to do some roll-on throttle test to evaluate your needle/clip position.

Start at the bottom and work your way up by doing roll-on tests going from 1/8 to ¼, ¼ to ½, etc. Use the colored head on the needle and the marks you made on the tape for reference. Remember that a bog or a hesitation will indicate a lean condition while sluggish, slow to respond symptoms indicate that you’re rich. Sometimes it’s hard to distinguish between the two, so make a change going either richer or leaner and see if it helps or hurts the problem. Basic trial and error will be your best course of action until you better get the hang of things. Below is a chart to help guide you on what changes to make as you work your way through the circuits.


(Note that this chart is Keihin specific and the Mikuni equivalent will vary slightly)

After you’ve finished with that then it’s time to check the main jet. You can continue to use the ride-and-feel method or you can opt to do it with a plug chop. The ride-and-feel method consists of using that same trial and error methodology. Try swapping out main jets until you find the one that pulls the hardest and longest on top. After you’ve found that magic jet, use the next size larger main jet. This will give you a little bit of insurance and help you avoid any major meltdowns.

If you’re going to do a plug chop remember to install a new, properly gapped plug and make sure your machine is up to operating temps before making any attempt. After you’ve brought the bike up to operating temp jump on and find an open stretch that will lead you back to your tools. Run it through the gears until you stick it in 4th (or 5th). Start at the very bottom of the gear and then wick it WFO. Hold it until the top of the gear and then some (just a little) and then hit the kill switch. When you hit the kill switch remember to close the throttle at the same time and coast back to your tools. Remove the plug and inspect.

Here is a helpful chart that will guide you on what to look for after you’ve done your plug chop.




I know I’m probably forgetting a few things here and there, but this should be enough to get you started. One thing will never change though, and that is…

AVOID DETONATION AT ALL COSTS!!!

There you go, have fun.

[Aus_Rider_22]

Thanks for that Faded! You do know your stuff!!!

I think I may go in and order some jets. 1 size leaner and 1 size richer and experiment like you said.

The air screw is usually tuned nicely, but I will experiment with Pilot first. I will try lean and see if it gets better. Then I will put in richer jet.

Thanks for your help again!!

[char393]

Um, I nominate this one for the KB

[Aus_Rider_22]

Yep, plenty of info here for everyone!!

[Faded]

Originally posted by aus_rider_22
The air screw is usually tuned nicely, but I will experiment with Pilot first. I will try lean and see if it gets better. Then I will put in richer jet.

Geoff, let the air screw determine the pilot size by following the first part of the procedure that I wrote. You might find that you don't need to change it at all. Also, you'll probably have to change the air screw setting often, especially when you start moving the needle and changing the jets. I usually check/set mine just about everytime I ride as the riding enviornment/conditions are never the same.

[twoofeach]

Originally posted by aus_rider_22
Thanks for that Faded! You do know your stuff!!!

Faded, you know me. So, you'll know how rare it is for me to be throwing the compliments around. Hopefully, you will appreciate it because it may be awhile, if ever, that you will receive another one.

That post was top notch! My friend you have elevated yourself to a new level. ATM occassionally has informative posts. It's not always about Alessi's dad or the Texas/Utah weekend riding report.

Kidding, I love the pictures of the "big air" and the "clean air'.