Fiddling with the carb and RB mod

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Fiddling with the carb and RB mod

Post by DeSoto Flyer »

I've read most of the posts about jetting, RB carb mods and stuff. Seems like a lot of folks are fiddling with the air screw, changing jets, and needles quite a bit. I have ridden my 2006 KDX for well over a year through the 105 degree summer, the 15 degree winter and humidities from zero to 100%. I have never adjusted the air screw, changed jets (until I got my Woods pipe), adjusted idle or anything and the KDX runs perfect. I've run the stock carb settings and all seems well. Starts first kick hot or cold, crisp throttle response. Changed the plug once, and that was because I was bored. All my riding is within 100 miles of Memphis, TN (altitude 342 feet).

Why do people have to mess with air screw/jetting/etc? Am I missing something? Does the RB mod make the bike more susceptible to temp/humidity/pressure altitude changes? I am considering the RB carb mod, but not if I have to constantly fiddle around with the carb.
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Post by Mr. Wibbens »

If it runs perfect why bother?'cept yer bike running perfect prolly runs like crap to most of us :wink:

We jet and fiddle with our a/s to get the most out of our bikes for given situations.

I have not touched my a/s in +100 miles , but if I need to I can, even while riding.
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Post by Jeb »

The KDX is a "robust" machine meaning it'll run pretty good most things left alone. Most riders in this world don't fool with jetting, wouldn't know how and don't know what they're missing. But jetting, needle, and air screw changes can move performance to the optimal end of things . . .

for example Rick's KDX 250 ran well enough but then he dropped the main jet a size. You could see the difference and he could sure feel the difference.


A few BTWs (in case you don't already know):

The pilot jet's primary contribution is from idle to I think about 1/2 throttle or so. The bigger the jet size, the more fuel is metered during operation. The air screw is a sort of a fine adjustment between pilot jet sizes.

The main jet's contribution is for 1/2 to full throttle openings. The bigger the jet size, the more fuel is metered during operation.

The needle (it's actually called the jet needle) contributes just off idle to just before full wide open. With the throttle closed it restricts fuel flow through the needle jet (notice: jet needle, needle jet), and as the throttle is opened the taper & diameter affects how much fuel is metered through the needle jet AND the main jet (the main impacts flow from the bowl to the needle jet / jet needle arrangement).

All of the jets contribute to fuel metering during pretty much all throttle positions, but they are primary contributors at the given throttle openings.

I'm no vet at this stuff so if others see something about what I've stated that's needs correcting by all means chip in! I think I'm close, though.

Do some searches, here and elsewhere, and learn what you can.
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Post by firffighter »

The KDX runs "good" in stock carb settings. I ran my '92 KDX200 for nearly a year with stock carb settings, then after reading about the RB mods, did the carb mod.

All I can say is WOW! What a differcence. Rip, Zip, Clean, Crisp, etc., all words to describe the RB'd carb. I never messed with jetting after it was RB'd, just adjusted the mixture screw to find the second sweet spot.

I now have a '97 KDX220, because I wanted more torque for the terrain I like to ride, and in stock carb settings, the bike is "very nice". But, I am getting the RB mods this week, because I know it will transform the bike's performance from "very nice", to "amazing".

Ron at RB is great about setting your jetting up to your specific situation. Send him a line and he can help you out.
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Post by Matt-itude »

I have asked this question if it makes the bikes more susceptible) 2 times on different forums and never got a solid yes or no. Anybody have a before and after rb altitude sensitivity experience?
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Post by billys04kdx200 »

DeSoto, come over to Columbia, tn and ride mine. You will see and feel it is worth it. I rode all weekend at highland park resort in Ga. and never once messed with the air screw. And from my understanding unless you are making major elevation and temp changes then you won't have to mess with it to often, i don't.
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Post by Indawoods »

I just tweak the air screw... not much, maybe an eighth of a turn either way during the day. Temps change and for your bike to run optimum this is required.... RB'd or not.
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Post by KarlP »

I don't think the RB mod makes the bike more susceptible to running poorly if anything is off a wee bit. That has not been my experience, anyway.

I prefer to tune kind of middle of the road. I'm not looking for my tuning to be on the nats edge of optimal performance because when it falls off that, you miss it.

I messed with sports cars for years and tuned, tuned, tuned. I don't want to do that anymore. I want my bike to run "right", but not necesarily all it can be.
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Post by DeSoto Flyer »

Billys - thanks for the invite! Might have to take you up on that sometime.
Karl - I'm with you 100%, don't have time, and don't want to be constantly fiddling with the bike to get it to run "optimum". I DO want it to run good, reliable, and consistent. Think I'll save up some money for the RB carb mod later this year.
Thanks for the replies:)
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Post by billys04kdx200 »

DeSoto, no problem. We are always having cookouts up here. I have roughly a mile long track we can play on, and plenty of rooms if ya wanna stay over.
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Post by DeSoto Flyer »

Thanks, Billys! If you get down around the Memphis area, I've got 11+ miles of trails behind my neighborhood. Nothing too challenging, but a lot of fun! Must have quiet bike, or neighbors will get ruin it for me.
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Post by canyncarvr »

Curious, what is the reasoning path that takes one from here:
RB carb mods and stuff. Seems like a lot of folks are fiddling with the air screw, changing jets, and needles quite a bit...
I've run the stock carb settings and all seems well. Starts first kick hot or cold, crisp throttle response.

Why do people have to mess with air screw/jetting/etc? Am I missing something?
To here:
Think I'll save up some money for the RB carb mod later this year.

??

Seriously..it's an honest question.


Consider a couple of things. 342' el. with what amount of change on a ride?

A normal riding day for me this time of year will start in the mid 30ºs with temp changes upward of 50º by the time the day is over. Considering such a temp swing AND elevation changes of over 3000' common..would you really consider a carburetor to be capable of adjusting itself somehow to compensate for those conditions.

Summer rides have temp swings wider than that...and el. changes WAY over that.


If you consider a simple air screw adjustment to be 'constantly fiddling' with the carb, save your money. You will hate a modified carb.

Honestly.

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Post by DeSoto Flyer »

CC,
To answer your question, I have become semi-educated on the subject, thanks to this site. There have been some great responses, and I went back and read thru a lot of the other postings. Not flip-flopping like certain politicians, I've just become a little more knowledgable. RB's air screw seems like it would make adjustments fairly easy. Where I ride, there is nothing like 1000' elevation changes. And to be honest, I don't usually ride for more than a couple hours at a time.
It appears to me that I was, indeed, "missing something" and that was my ignorance on the subject.
I can deal with air screw adjustments, but jets/needles/etc is something I don't want to mess with more than once or twice a year. Could be I'm a little lazy, or don't realize just how good my bike can run with some "fiddlin".
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Post by canyncarvr »

Re: 'I've just become a little more knowledgable.'

You don't ever lose with that.

Re: 'I can deal with air screw adjustments, but jets/needles/etc is something I don't want to mess with more than once or twice a year.'


That's plenty good enough. :wink:

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Post by DeSoto Flyer »

Contrary to urban legend, us Mississippi folks are capable of reading, learning, processing information, and coming to new conclusions:) Thanks to all the posters on this site and whoever makes this site possible! Truly a wealth of information!
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Post by Mr. Wibbens »

Once you get your jeting sorted, you really never have to mess with it again

well unless you gonna run it in the dunes
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Post by kicknrocks »

I just tweaked the CEK needle in my RBd carb for the first time (firt time adjusting a needle also). The bike was screaming already, but I figured if it ain't broke, I can probably make it broke. Seriously though, it ran really rich my first time out, so I figured I'd try moving the needle clip from its mid position.

First I moved the clip up one, effectively moving the needle down. I thought that this would lean the mixture, but it seemed to have the opposite effect. So I moved it back down to the number four position (second from bottom, am I right?) and the plug showed nice light brown throughout.

Point being, I had never removed or adjusted the needle before. The last adjustment took me less than five minutes, and I threw the tools in my fender bag with some nice clean shop cloths. If it sucks, I wouldn't hesitate to change it on the trail. A couple of spare clips will soon be in my bag also (I'd hate to get cocky and lose the only one of those in the woods).

As for now, I'm going to go sift through the carb posts looking for more useful information, now that I have actually seen the needle jet and jet needle, and have a rudimentary understanding of what they do. It's amazing how this stuff starts to make sense.
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Post by canyncarvr »

Re: 'First I moved the clip up one, effectively moving the needle down. I thought that this would lean the mixture..'

It did.


Lowering the needle (raising the clip..the clip slots are numbered 1-x from the blunt end) does lean the air/fuel mixture, raising the needle does richen the air/fuel mixture.


Image

Where the rub comes in is determining the direction the bike needs to go. A 'too rich' condition has often been described as a 'booo-WHAH...' exhaust note when wicking the throttle. By 'often' I mean a lot of riders have used the term. That sound is considered relative to 'too rich'.

Except...that's the same sound you get when you're too lean, too.

While there is a possiblity clip-2 would run better for you than clip-4, it's not likely...'cuz that's not what other riders have found to be true.

By the time you read through 11'd-dozen threads on jetting, you will likely run across this: Plug color means pretty much nothing in regard to an air/fuel mixture tattle. Neither does spoo.

This:

Image

showed this:

Image


The two pics relate to something completely different..the point is, don't jet by spoo..don't jet by plug color.

You will likely hear lots of 'experts' that have been 'jetting bikes for years' and know exactly how to jet 'perfectly'...and use plug color as a jetting guide. Had one'a those on this forum recently. He had no clue what he was doing...but no one could TELL him anything, either...because he was an expert (def: ex= has been, spurt= drip under pressure).

Something noted by ski in another thread..a good place to keep spare clips is ON a spare needle. Make sure any clips you buy as spares actually fit. One-each SAE (not metric) clips don't.


Re: 'It's amazing how this stuff starts to make sense.'

That's a fun part of the whole thing, ain't it!



btw: I'm no jetting expert, either (Ha..that's been made 'clear' to me quite recently. :roll: ). I've spent hours-days-weeks sorting out other people's problems. Me? I jet my bike to run to suit me..and ride.


I gather you haven't ridden with the CEK-4 yet?

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Post by 2001kdx »

At the most, you would only need a slightly richer Main jet/Pilot jet combonation as your "Winter Jetting". On a bike such as the KDX, however, it'd be pretty easy to find a happy balance between performance and flexibility I.E not being too rich when you see an elevation change, not too lean when riding in the lower temps of morning. Simply fiddling with the air screw would be the most you'd have to do with such a setup.
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Post by DeSoto Flyer »

You folks are a wealth of info! It is humbling to realize just how much I did not know. Really like the needle/clip graphic, CC. Pictures are so much better than words.
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