Jetting...I can't believe the difference


KnoxKDX

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Jan 29, 2003
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After riding around since February with my needle jet in the top position as a free way to lean out my '98, I finally decided to take the advice our veteran posters and re-jet my baby. I ordered a 155 main and 45 pilot from Rage Performance and installed them this evening in 20 minutes. I can't believe the difference. This bike absolutely screams on every level. What's more, I really didn't think be able to tell the difference. I am now a believer in this and HIGHLY recommend it to anyone riding a KDX. Thanks to all the posters on this topic. BTW...it's only $20 and 20 minutes 20/20.
 

KDXFreestyle

Damn Yankees
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Nov 19, 2002
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my bike came jetted rich stock. i drilled out 3 1 1/4 hole sin my airbox cover, and removed the snorkel. it was still alittle rich. I added the FMF Powercore 2 silencer, now my jetting is spot on. tan plug
 

canyncarvr

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Oct 14, 1999
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(I have no idea why I bother...)

re: '..jetting is spot on. tan plug'

To be frank and to the point....those two items have nothing to do with each other. The fact that your plug is 'tan' might be nice when taken as a fashion statement (Oooohh...I like the tan with the kawi green combo!). As far as it showing anything about jetting......it doesn't.

knoxkdx...but wait.......there's more!! ;)
 

KDXFreestyle

Damn Yankees
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Nov 19, 2002
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on motocross.com , it says if your running a tan plug, your jetting is correct. Black spoogy plug means to rich, and like white, or carbon means to lean. Just thought Id let ya know.
 

jboomer

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Jan 5, 2002
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As far as it showing anything about jetting......it doesn't.
That's a bold statement Canyncarvr!

That being a blanket statement....you're wrong. If he's performing the correct technique for a plug chop, then I'd say he's pretty close.
 

Braahp

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Jan 20, 2001
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He is right..................plug color is useless anymore with todays fuels. Thats just the only thing anybody ever hears as to how to check. I've seen it preached on here thousands of times. Sure it will get you in the ballpark but thats about it. Most people don't even know where to look at the plug.
 

jboomer

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Jan 5, 2002
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The ONLY other technique I've seen posted is the "Feel" method. Are you saying that is the most reliable technique? Hardly. You couldn't possibly be referring to jetting for spooge and smoke. The only other alternative is to jet using the plug chop. If one were to run a search on the subject he would immediately have access to a wealth of knowledge on the technique and where to look if searching for optimum results---I think it would get you way closer than a ballpark setting. If you've got a tried and true, BETTER technique, then by all means, spare us the suspense!
 

Braahp

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Jan 20, 2001
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Yep....the ol' "feel" method for throttle response and transitions. Thats what your jetting for anyways isn't it? You can tell if its rich or lean just by how it runs. For the main jet plug chops just aren't that reliable unless you can run at full throttle/ full load for 4-5 minutes to get a good coloring. Almost impossible to do that on a dirtbike. Plug chops are the most basic for most to do and thats all that has been shoved at you. Hey I still look at plug coloring too but don't rely on it totally. I use a PET digital tach on mine. At WOT RPMs will drop if you are lean.....drastically if you are real lean.....they won't reach peak RPM if you are rich......they will stabilize if you've got it right on. There again.....you can feel if its running rich or lean. Common sense tells you if its smokin' or spoogin' you know its over rich. Do you really think the top tuners rely on plug chops?.....Not.
 

Rich Rohrich

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Originally posted by Braahp
For the main jet plug chops just aren't that reliable unless you can run at full throttle/ full load for 4-5 minutes to get a good coloring.


4 to 5 mintues at WOT? :scream:

You can get a totally reliable and dead on accurate plug chop on a fresh plug from a nine second dragstrip pass. The same can be done in a similar time frame on a dirtbike running up a short hill or moderate incline that puts consistent WOT load on the engine. Longer time frames at the same load will tend to make reading easier. Unleaded fuels will still produce a good plug chop if you are looking in the right place on the plug. Looking in the right place is the KEY, and probably the area where there is the most confusion.

I don't know anyone other than the guys at Bonneville who can run WOT for 4-5 minutes. I'm not sure if there is a KDX class out there though. :laugh:
 

Jaybird

Apprentice Goon
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http://www.strappe.com/plugs.html

The guy who wrote this ^ may have known a thing or two about it.
Perhaps one day we will venture into stator adjustments. (shrug)

Naaaa...arguing about plug colors is much more fun. It has to be fun, it gets done weekly it seems.
 

canyncarvr

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Oct 14, 1999
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That's a bold statement Canyncarvr!

That being a blanket statement....you're wrong.

Thanks for straightening me out! I appreciate your showing me the error of your ways.

..wait.....is that right? ;)

Only weekly? You need to check in more often!
 

Jaybird

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On the defense of those who look at the pretty tan plug color...I should have said: "Gets done weakly" :)
 

skipro3

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Dec 14, 2002
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Plug color is good for one thing: heat range. The tan color will tell you that your plug is the proper heat range for your conditions. Otherwise look at the ring at the base of the insulater. Cut off the threads of the plug to see it well. The thickness of this ring will tell you the correctness of your jetting. A 1mm ring is considered safe. Less and you might find problems with altitude changes, air density, ambient temperatures. Warm up your bike with a used plug, change to a fresh plug and run the bike under load for 20 seconds more or less. A hill provides good load but if a hill isn't available, then drag the rear brake. There's more on CD's site on how to perform the chop portion.
If I am incorrect on anything, let me know. This is how I understand to do it anyway.
 

Braahp

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Jan 20, 2001
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Rich of all people preaches the differneces in fuels available. I just don't think you can get a good coloring of the plug in that short amount of time. The digital tach is my preferred method.
 

Rich Rohrich

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Originally posted by Braahp
Rich of all people preaches the differneces in fuels available. I just don't think you can get a good coloring of the plug in that short amount of time.

Differences in distillations curves among other things certainly exist in fuels, but all fuels from pump to Pro Stock fuels will produce a readable fuel ring in a short period of time if you know where to look, and what you are looking for. Different fuels will produce a different looking ring but it will be there regardless.

Whether or not you choose to believe me is certainly your perogative, but it doesn't change what's happening in the engine. Having cut open hundreds of plugs I've had the opportunity to see some interesting things. You should sacrifice a few plugs and chop them open. It's pretty interesting and has helped me understand a lot more about what's happening inside the engine.

A reliable consistent tach is an excellent way to determine main jet settings. Lots of road & kart racers use that method as an adjunct to reading the plug and piston crown.

Like I've said lots of times, any method that works consistently is a good method in my book. :thumb:
 

Rich Rohrich

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Originally posted by shr
What "reliable consistent tach " do you recommend for the KDX rider? Rich

Ron this is the one I like :
http://www.sendec.com/meters/html/fs500.html

The Sendec FS500 is more expensive than some of the cheapy pulse tachs, but you can count on it to read correctly time and again.
 

shr

Uhhh...
Apr 8, 2002
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Rich, thank you for the info on the sendec tach, I am very interested in learning more about them.

I have been using a digatron tach for my engine RPM testing needs, but have not been that pleased with it. Some times it's erratic and numbers jump around a lot.

It uses a similar antenna type pick up................ Do you find this problem with the sendec tach? Is it a steady and smooth read out?

Your impute will be greatly appreciated and I will have to contact sendec and listen to what they have to say, but I did that with digatron and I still have some problems with there answers to fix the read out problems.

Thank you
 

Jaybird

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For what digitron charges for their products, they should have a home service man visit.
 

Rich Rohrich

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Originally posted by shr
It uses a similar antenna type pick up................ Do you find this problem with the sendec tach? Is it a steady and smooth read out?

I haven't tried a digitron so I can't really compare them. The Sendec is far from perfect but seems to have a good sampling rate and filtering setup keeping it reasonably stable for a simple unit. For what it's worth I found that using a better quality ignition wire then the japanese OEM cost reduced junk helped stability. Dielectric grease and shrink tubing helped as well.

A few years back I switched to a real data acquisition system with an inductive clip feeding to a small dashboard type setup for on bike data readout as well as data logging of a number of different sensors like TPS, water temp, wideband exhaust O2, among other things. Much easier when you can come back and look at data on the computer. No surprise there I guess :) For the type of work you are doing this type of system would be a good investment in the long run.

http://www.aim-sportline.com/moto/logger.htm

Even being able to do simple rpm (or throttle position) analysis for a given track like the links below can be incredibly helpful. When you overlay that data with exhaust gas O2 and intake air temp data it opens up a whole new world of possibilities for understanding tuning and rider preferences. Some days I think the analysis is more fun than the riding :)

http://www.dirtrider.net/teamdrn/showphoto.php?photo=4438&size=big&papass=&sort=1&thecat=

http://www.dirtrider.net/teamdrn/showphoto.php?photo=4437&size=big&papass=&sort=1&thecat=

http://www.dirtrider.net/teamdrn/showphoto.php?photo=2571&size=big&papass=&sort=1&thecat=500
 

Rich Rohrich

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If anyone is interested in seeing how the data acqusition analysis software works it can downloaded for free, and there are data files avaliable on various net sites. I'll be happy to post links and share some data files if there is any interest.
 


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