Explain RB carb mod for me?

[dave04kdx]

I'm interested in getting the RB carb mod. I understand he can do the OEM carb or buy one from him, right? What exactly does he do physically to improve carb? What can I expect perfomance wise? How much does it cost? Can I jump further? Does it come with a black rear fender?

thanks

David

[KDXer]

Yep, yep and yep, just make sure you get the go fast stickers too.

If you go search the links page it has his site listed with all the info... Oh I'm starting to sound like the search parrots over dere. Look ere http://www.rb-designs.com/

[Indawoods]

Boring carb to 36 mm (gives you a better top end)

Installing divider plate (gives that low end response, like a smaller carb)

Modify slide (do not forget to send your slide with your carb) Cut Leaner

Install new needle (gives the carb that personality)

Change jets (if necessary)

Modify jet block (helps with the over all performance)

Modify low speed circuitry This part of the modification along with the other mods really brings the air mixture screw into play. With only very small adjustments, you can keep your bike running at its peak as you change altitude or with weather changes during your ride.

Modify your carb for $165 AirStryker is $325
http://www.rb-designs.com for more information.

[KDXer]

Well said...

[dave04kdx]

muchas Gracias mis amigos

[canyncarvr]

Check his website:

http://www.rb-designs.com

Call Ron if you want specifics..

Answers to your questions:

He can 'do yours' or you can 'buy his'. The advantage of buying one is that it would be a 4-vent AirStriker Keihin. This model carb looks just like the PWK you have...except for the extra hoses (you can see a pic of one on my bike in the gallery). On the intake throat of the AS carb there are little 'wings' that focus or improve the signal to the jet block.

And...you could always then sell your PWK or keep it as a spare.

What does he do: The carb is bored to 36mm. A divider plate is placed on the reed side of the carb. It snugs up against the slide on the inside of the carb. Think of your carb now as a 2BBL (two barrel) carb. Velocity of the incoming charge is important. Velocity is improved when you have the same amount of air going through a physically smaller space. So...'barrel 1' is when the slide is below the divider and 'barrel 2' opens up when the slide is pulled past the divider. There are air rarification (smoothness) things that take place, too, due to the lessening of turbulence of the incoming charge dumping past the slide. (None of that is from Ron...it's my own understanding of some basic carb stuff...call it BS if you like...fine with me) He will cut your slide to a 7(mm cutaway) from the 5 it is now. You can purchase a slide from him if you want to keep yours as a spare or somesuch.

In addition to boring the carb and setting the divider, the pilot circuit is changed...casting cleanup/flow improvements. His mod does NOT 'come with' his finger adjustable air screw. If you want one'a them, make sure he knows it...and add $20 to the deal.

If you buy and AirStriker, it will come with the jet block height modified to match your PWK. That way needle/jet settings correlate better one carb to the other. You won't have to 'convert' your (oem) AirStriker setting to try to match some other PWK reference.

Performance-wise: Better off-idle response...smoother throttle response up through the mid-throttle range. If you have a 220, expect a good bit of hi-rev improvement...cuz the oem 220 carb is only 33mm.

$165.

Jump: Yes.

Fender: Yes. (that's a lie...but what the heck!)

Send it in clean. No hoses. If you get the AirStriker you will have to either use your PWK cap (angled entry tube on the cap) or buy another with the radiused cable tube.

Tell him where you ride (environs..temps, elevation, type of riding and such) and he will send it back to you real close jet-wise. If you ride at higher el. and ride a 220, consider sending your head in at the same time. Specify what kind of fuel you will be using and he will optimize your head for that fuel.

How's that?

It's a reasonable investment in your bike. If you ride WOT all the time on your 200, then it won't make a huge difference (bottom end response and mid-throttle smoothness being important points). If you ride a 220 in the woods, you're going to be impressed!!

***edit*** You guys are too fast...or I type too slow....I was the only one here when I started!

[IdahoCharley]

Anyone know the details on the pilot circuit clean-up?

Got to admit the boy's air screw on the pilot circuit (35mm PWK on 200) seems pretty worthless. I even removed the pilot circuit casting from the carborator main body to see if it was plugged. The ports were open and the o-rings looked good. I can tell changes in PJ but AS changes have the least affect of any carborator that I have worked on in some time.

[canyncarvr]

The oem PWK airscrew is a lot less effective than after it has been modified.

The oem PWK airscrew is a lot less effective if the pilot jet choice is out of whack, too.

So...don't know what 'least' in your experience means...but it's not a whole lot on the oem carb.

I have no details other than what I said. You're looking for a 'how to' for a DIYer?

Sorry. Can't say.

[Indawoods]

IC... The combination of all the modifications that Ron applies makes it much more sensitive to changes. At least that's how I understand it.

[Indawoods]

I ride a variety of different terrain from slow nasty rooted and rutted trails to wide open areas where you can just go.... With this carb mod I should be able to be happy in any situation since it doesn't tame the carb on the high end. It is mainly for the low and medium circuits.

I can't wait to hit the river bottoms with the SS (second sweet spot) and just moto on through without shifting so much... It's gotta save on the arm pump even more on top of the stabilizer.

The knurled AS is going to be soooo nice because I won't have to carry a screwdriver with me that I always end up losing somewhere... probably in someones tire.

I just can't wait!

[canyncarvr]

I agree with Inda.

How much of it is the low speed circuit..how much the slide modification, how much the jet screen modification (if done) etc. etc....I don't know how each of those work separately.

I know some of them work oddly together. For example...finding the AS on a modified AirStriker to be sensitive to the tune of 1/16s of a turn in the 2.5 out range doesn't fit any oem PWK 'profile'. But that is what you will find on one of Ron's modified carbs. There are some (been posted on this site) riders that will tell you such a setting is 'impossible' ..that 'something else must be wrong'.

Well, no.

Not trying to be elusive or mystical about it all. The 'package' works. It works to the extent that trying to sort it all out so I can do the same thing myself is a waste of time.

Yeah. Give me a rattail and a #5TV. I'll have it whipped into a #7 in a jiffy!

Ha!

Kind of off topic a bit...but I'd always assumed that a bored carb would be concentrically bored. Never thought a whole lot about it, but that would have been my guess. It's not bored that way.

Glad I didn't go after my PWK with a dremel!

...wait....I don't even have a dremel......

[IdahoCharley]

CC just a FWIW background - I've worked on quite a number of the PWK 38-39 mm carborators for the last 7-8 years. Mostly on KTMs and CRs. I've modified about a half dozen slides on the PWKs and about double that if you want to count the round slide muniki carbs. I've been running a SLP-power paks in 39 mm Keihn carbs since '99. currently running a 40mm PWK (bored 39 PWK) with SLP Power Pak on a '00 380 KTM. Familiar with Sudco needles, JD jetting guide and needles and general carborator tuning. I have a good handle on the basics; but realize I will never know it all.

I never heard of the second sweet spot until I read it on this forum; did not know of RB's extended AS needles until this forum. (bought two of the needles - they are great!!)

Bottom line - I'm familiar with the added signal a divider plate will give the low speed circuit and familiar with PJ, AS, needle, MJ changes and their effects. I've just really been impressed with the 38-39 PWK carbs and find the PJ/AS 35 PWK on the 200 not quite upto what I've come to expect from Keihn. I'm happy with the 200's response with a 50PJ AS1.25 CEK3.5 152MJ and slide modified to a little over a 7 cut. (Original cut TV to a 6.5 seemed too rich - had to run a 35 PJ. Cut the TV a little more to better balance the transition between PJ and needle.)

I would like the AS to be more effective though - we ride from 5,000 to over 10,000 feet elevation, in a single ride, several times a year. It nice to be able to set-up and ride these elevation changes without having to stop and rejet the bikes on the trail. We have usually been successful in setting up jetting which will allow us to ride these elevation changes with just opening the AS to lean the PJ circuit as we gain altitude. Of course the bikes are running rich at the high altitudes but we are working them hard so its the low speed circuit which causes problems as we descend off one mountain before we get to the next climb. If the bike is going to give you carb problems it is fouling a plug due to an overly rich PJ/low speed setting. If we can lean out the PJ circuit with an effective AS then life tis good.


DIY's ?- If there was a modification to the carborator pilot circuit that wasn't too extensive and made sense to me I would likely try it.

The primary thing I like about these forums is the sharing of knowledge of what works. I've always enjoyed working on bikes and sleds. Anything with motors really. I also find riding/talking with people who are always working to make their bike or sled a little better (i.e. faster, better handling, etc) more to my liking than those people who are riding for the seanry or because they have a bike in the garage and want to blow the dust off of it because the wife wants them to sell it.

Sorry I was so long winded and got off-topic, but I think (hope) some of you will identify.

[m0rie]

Bottom line - I'm familiar with the added signal a divider plate will give the low speed circuit and familiar with PJ, AS, needle, MJ changes and their effects. I've just really been impressed with the 38-39 PWK carbs and find the PJ/AS 35 PWK on the 200 not quite upto what I've come to expect from Keihn. I'm happy with the 200's response with a 50PJ AS1.25 CEK3.5 152MJ and slide modified to a little over a 7 cut. (Original cut TV to a 6.5 seemed too rich - had to run a 35 PJ. Cut the TV a little more to better balance the transition between PJ and needle.)

I curious if the 50 PJ that you've got listed is a typo? The stock 48 PJ that comes in most KDX is bubbling rich at sea level, much less 5,000 to 10,000 feet. My AS was basically useless until I cut the PJ down to a 40 or 42. Now it will semi compensate for the changes thru the day. My throttle response was terrible with a 48 PJ (having a rev profile pipe didn't help things either) but cleaned up a lot when I lowered the PJ and got pretty darn good when I switched to a CGK-3 needle.

*edit* My carb has not been worked over by RB (coming soon hopefully)

-Maurice

[RBD]

IdahoCharley,

If the majority of your riding is over 5000 feet, you would really benefit from milling your head. This will help compensate for the lack of air at 5000 to 10000 feet. It will also make the fine tuning of carb with the AS much more effective

Ron

[IdahoCharley]

RBD - You performed that work on the KDX's 200 head last month. Just been riding the bike a little to break it in and fine tune it before the trails clear around here. The head modification BTW is great and the quality of your work is EXCELLENT!!!

mOrie - No typo on the PJ - it is a 50 with AS 1.25. (I'll may be able to back it off to a 48PJ when the weather warms and we start riding and going up in elevation.) Please note the slide is modified significantly.

The CEK/DEK needle seemed to the recommended needles to use with a 6.5 to 7.0 slide. Cut the original slide to an approximate 6.5 and tried the CEK and had to back the PJ down to a 35. It would still load up some while warming up but clear out quick and once warm and underway it worked fine. I like to run a little larger pilot jet so I modified the slide a second time (actually went further than I should have) and ended up having to run a 50 PJ due to the larger slide cut. Warms-up now and runs very clean and crisp. No need to "clean-out" the engine after a 3-4 minute idle warm-up. If I were to do it over again with a 6.5 slide I should have gotten a DEK needle to install. (I had a CEK along with a couple of other series 13XX needles to try already in the toolbox.)

EDIT - I do realize that I seem to be jetted a little richer than many posts that I have read. I'm NOT jetted rich as it is currently jetted although.
I'm setting up the bike for my 14 yr old who is moving off a TTR 125L and I want good clean power with no big hits. (Actually setting the bike up just like I would for myself but with lighter suspension settings.)

Engine Related Changes - Air box mod, Twin Filter, RB Head, stated Carb mods, PC2 pipe, stock silencer, Boyseen two-stage reeds, stock timing.

[canyncarvr]

IC:
I had a pretty good idea of your 'background' just from the tenor of your posts. Nothing I said was intended to denigrate or question anybody other than myself...certainly not you.

That said, I guess I am a little on the defensive when it comes to RB-Designs..or anyone else like him. I am aware of the huge amount of time, work and effort it took him to get his modification to where it is today. Besides that, he's one heck'uva nice guy and will go to practically any extent to help anyone...whether a paying customer or not. Put all that together and I do baulk at the idea of 'What was done?' so someone can do it themselves. If I knew to the micron what was done, I wouldn't say. ..But I don't know.


I've not run a #7TV in an OEM carb..don't know how it would jet out. Generally speaking, if you have to go too far in one direction in circuit 'B' to make up for having gone too far in the other direction in circuit 'A'...something isn't right.

Not harping or dinging on you at all....but making a jetting choice based on how the bike runs (clean) after a 3-4 minute warm up idle is going to lead you to a wrong jetting choice...(IMO and all that...read the sig! ). While I read of a couple of instances where/how the bike runs better with its current setup....those instances don't apply to how that circuit is setup. When the end result is a much richer jet selection than anyone else uses...AND the bike is run @ +5000'el., you have to expect a bit of wondering about it all.

So...if you would?...how do you 'test' the lower couple circuits?

Certainly, it's all about having your bikes run the way you want them to..regardless of what anyone else's point of view is. ...and if that's the way things sit now, great!

Cheers!

[skipro3]

The CEK/DEK needle seemed to the recommended needles to use with a 6.5 to 7.0 slide. Cut the original slide to an approximate 6.5 and tried the CEK and had to back the PJ down to a 35. It would still load up some while warming up but clear out quick and once warm and underway it worked fine. I like to run a little larger pilot jet so I modified the slide a second time (actually went further than I should have) and ended up having to run a 50 PJ due to the larger slide cut. Warms-up now and runs very clean and crisp. No need to "clean-out" the engine after a 3-4 minute idle warm-up. If I were to do it over again with a 6.5 slide I should have gotten a DEK needle to install. (I had a CEK along with a couple of other series 13XX needles to try already in the toolbox.)

Hi IC;
While I have a 220 and yours is a 200, I've found the same conditions with my jetting. (Not the main but probably because I'm running a race gas that has a very effective evaporation curve for my bike. ) With the CEK and the slide cut at 7.0, I'm running a 38 pilot on my RB modified AirStryker carb. I've not experianced the warm-up issue you mentioned though, but you are right; it's hard to jet for 5K+ feet when snowline is at 4K. Ha!! Anyways, I am keeping my jetting at this point because I get some great fuel economy with that 38 PJ. How has the 50 PJ affected that part of your fuel economy? Also, my bike starts extremely easy when cold, but once warmed up, it takes a few stabbs at the kicker with the throttle held slightly open to light up. It never loads up or fouls a plug though.
Your idea to set up for your 14 year old son sounds like the right direction to go; a DEK needle, and find the second sweet spot with that AS. I've heard it makes the motor friendly and torquey at the same time. The CEK with the first AS sweet spot is a hit big time right off idle, but now that I have an auto clutch, it's easier to deal with. (Besides, I LOVE the seat of my pants feel from my setup).

[IdahoCharley]

CC - No offense was taken. Been on the forums enough to know better and I truely try to be a straight shooter. I also respect straight shooters, guess that makes me an analytical driver type of person.

I also truely enjoy tinking with the toys till I get it "right" for me.

You asked how I checked the lower circuits. Good question. I've never put this into words before so it will likely be long winded - WARNING!!!! - might want to grab a beer or not read it until you need to fall asleep.

Since the bikes we use are for trail riding purposes I'm looking for as much low - mid end torque as the motor will give me. Also want a good punch in the mid-throttle area. Upper end of the engine's rpm range is not as important but I due want the bike to rev out and be responsive to throttle input. So really looking for a fairly uniform balance across the low-mid-upper power ranges. Relatively crisp throttle response is necessary for the water bars, etc but it does not have to be supercross crisp. (I hope that makes sense to people)

Testing various carborator settings is by the seat of the pants combined with the ear and the hand. Whoa you say - let me explain.

First like most everyone I try to get the MJ very close. WOT plug chop and BJH's gallery shows a great WOT plug chop Main Jet IMO. Then to answer your question concerning the lower-mid throttle settings.

1. During the warm up you get an indication whether your fat or excessively lean by the exhaust sound and by how fast the pipe gains temperature. If your are fat on the PJ circuit the bike will be alot slower to warm the pipe. If your lean, the sound will be tinny and the pipe gains heat quickly. Of course if you sit on the bike and rev it while its warming up - this point is mute. (Throttle response off-idle never has told me too much if the engine doesn't have a load on it: Once you have a PJ that is close. If you warm-up your bike and then during take-off or shortly after you need to "clean your engine out" before it will run clean you may have an imbalance between the PJ, needle straight dia, AS, or TV cut away. This really isn't important if the carboration works for you once the engine is up to temperature - UNLESS you cold foul a plug occasionally when you take-off.)

2. After the cylinder and pipe are warm to the touch when you take off then engine should pull from a dead stop with minimum throttle. Idle away in gear is normally a good sign. If there is a bog or need to give it more than a few hundred revs to smoothly take off then the lower circuit may benefit from changes. Taking off from a dead stop in 2 or 3 should be easily accomplished with minimial clutch slippage.

3. (a) Mid-throttle - best way I've found to test this is by riding up various short hills for 100 -200 feet and seeing how the bike pulls the gearing and the grade. (This is very subjective but if you've ridden various sized bikes you have an idea.) Say I hit the hill in 3 gear mid-throttle and the bike pulls it well for the bike's size - Life is good.

(b) Next I like the bike to be able to lug down to a little over a fast idle while still pulling the incline; even if I need to slip the clutch to keep the bike at a low rpm because I going too slow. Throttle position between 3/8 to 3/4 range. If the bike starts to lug down and then the engine's power drops off very fast I'm lean normally in the needle. If it starts to gurgle or hiccup I'm not balanced in the low-mid transition throttle and likely rich in one of them OR I have the TV wide open. Also the engine exhaust sound gives an indication of the bike's jetting.

Since the circuits are interrelated it takes a few trys to get the carb close. I do NOT do plug chop checks for part throttle settings. But I do look to the plug as an general indicator of overal jetting.

IMO a properly carborated engine when put under load will always have fairly good combustion temperatures and the plug insulator gives an indication of the combustion temperature. Also the engine's pipe temperature will remain - lets say 'hot to the touch' through out the various throttle openings. This is what helps give the crisp throttle response at the various throttle openings. It says to me anyway - the pipe is working at charging the cylinder with fuel.

IMHO - getting the squish close and then jetting the engine combination to get the most out of parts you have is the biggest bang for the buck. Doesn't matter if you have a stock bike or a fully modified one like yours. Getting the combustion right pays big dividends for whatever engine we are tinkering with.

OK - I'm done!! LOL

[IdahoCharley]

Skipro - I read a bunch of posts on jetting and expected to jet the 200 quite a bit leaner than I ended up with.

I ended up jetting the bike close to what people at 1000' level were doing but even with a bigger pilot and we are at 4750' elevation. Of course the pilot may be because my hand cut slide may be closer to a 7.25 or so TV cut. The fuel curve indicates that I'm about 8-10 percent over what Fishead and his friends ended up with when they tested jetting. The bike runs well and I could go safely 1/2 clip leaner on the needle with no problem other than giving up a little torque for the current weather conditions, but a full clip position leads to a very detectable drop off in power.

We really don't have enough miles on the bike to know what it will get for milage but from past experience if you jet for torque output your milage will be better than bikes which have stock jetting adjusted for altitude. Also the spooge factor should not be significant in that you always have fairly good pipe temperature and relatively little condensation of the fuel which does pass through the exhaust system.

I still may try a DEK. Although the DEK may even cause me to have to increase the PJ size upward for the low-end pull.

Starting is very, very, easy. 1 or 2 kicks and its running cold: 1 half hearted jab when warm.

The warm-up issue is really not a big deal. (Well for most people.) When I have a bike that loads up when I drive off from allowing a idle warm-up; I figure that excess fuel is condensating in the crankcase during the warm-up which is to be expected - until you bring the engine up to temperature. It more the idea of a very quick clean out of the engine verses one where it takes 50-100 yards before the engine is cleaned out. Does that make sense?

Concerning the sweet spot. I may play with trying to find this sss which you guys have talked about.

The first ss with the CEK on the boy's 200 is not a big hit as it is currently set up. Prior to redoing the slide for the second time it was a harder hit - since the engine was fuel rich. That was certainly an indication in my mind that I could improve the throttle response. Don't want my boy to come up to that 8-10 inch log and attempt to loft the front end and instead nail it dead center putting him over the bars!!

One of the important things for the jetting to allow is to be able to predictably loft the front wheel in first and second gear. The KDX seems to be doing that fine and although I haven't hit the official trails with it yet I believe third gear lofting will be possible in the meat of the power band.

The way I see it - If I set the jetting up for me and it works for a riding weight of ~210 lbs think how much fun my 130 Lb son will have!!!!

I still have a number of other needles I may test. This set-up seems to work extremely well on this bike. Is there a better set-up - probably. That IS the challenge isn't it? Eliminate the other possibilities until your satisfied nothing is better for the riding style you have.

Tinker, tinker, tinkering away!!!!.

[m0rie]

I ended up jetting the bike close to what people at 1000' level were doing but even with a bigger pilot and we are at 4750' elevation. Of course the pilot may be because my hand cut slide may be closer to a 7.25 or so TV cut. The fuel curve indicates that I'm about 8-10 percent over what Fishead and his friends ended up with when they tested jetting. The bike runs well and I could go safely 1/2 clip leaner on the needle with no problem other than giving up a little torque for the current weather conditions, but a full clip position leads to a very detectable drop off in power.

We really don't have enough miles on the bike to know what it will get for milage but from past experience if you jet for torque output your milage will be better than bikes which have stock jetting adjusted for altitude. Also the spooge factor should not be significant in that you always have fairly good pipe temperature and relatively little condensation of the fuel which does pass through the exhaust system.

I still may try a DEK. Although the DEK may even cause me to have to increase the PJ size upward for the low-end pull.

The first ss with the CEK on the boy's 200 is not a big hit as it is currently set up. Prior to redoing the slide for the second time it was a harder hit - since the engine was fuel rich. That was certainly an indication in my mind that I could improve the throttle response. Don't want my boy to come up to that 8-10 inch log and attempt to loft the front end and instead nail it dead center putting him over the bars!!

One of the important things for the jetting to allow is to be able to predictably loft the front wheel in first and second gear. The KDX seems to be doing that fine and although I haven't hit the official trails with it yet I believe third gear lofting will be possible in the meat of the power band.

IC, Call me a stick in mud but its still not clicking for me. How are you verifying that your slide is cut to a 7? Inquiring minds want to know. Your descriptions of it having a harder hit before you recut the slide a second time make it sound like you cut the slide too far (well past 7) and are having to compensate with a overly large PJ. There is a lot of people who run a 7 slide with the RB carb mod and don't run a PJ near that large.

-Maurice