RB Carb Mods are a Mystery

[TurboStew]

Hey all, I have spent hours in the bowels of the Internet researching the details of the RB mods to a PWK carb in general and on a KDX200 H-series in particular. Other than the divider plate, and thumb screw, the other details are very vague, like what is done to the idle circuit and jet block? I understand that there are many, many happy people out there. What I am not sure of is would these people would have been 95% just as happy if they had a jetting guru go through the various needle combos and spend the hours to get the jetting really dialed? I have CNC equipment and can make a nice divider, Is this the 5% left to do?

I would like to hear from anyone who has tried the CEL and DEL needles, been all around the world with pilots, mains and clips positions, and then finally did the RB mod.

[SS109]

RB mods are independent of jetting. I don't think that having a jetting "guru" do your jetting would negate the benefits of the RB mods. BTW, what is "guru" supposed to mean anyways? Like us regular people can't jet our bikes properly or what? As for what he does internally to the carb, well, that is what is known as trade secrets. You can always have him do one for you and then reverse engineer it to find out if you really want to know!

[JJRace]

zip ty racing,Tokoyo mods, RB all do carb mods.

I had him do my cr125 head squish band and it came out excellent! I made sure I sent the squish solders to him.

But he is local to me within 30 minutes and super happy with the work. He does list what he mods out in the carb generally since he wants to keep is trade secrets:

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)

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.

[TurboStew]

SS109, my goal is not to copy what he does, it's to make sure I don't spend a bunch of money for a "I think it runs a little better" experience. I always want proof, not speculation (engineer's curse).

I want to see a post like:

"I spent 2 weeks jetting every day, I bought like 5 jets, 20 needles, 4 slow jets, and 4 slides from Sudco, I tried every combo of needle and slide riding my 5 mile test track with both super tight sections and fast ripping sections, I ended up settling on a DEL needle on clip number 2, a 150 main jet, a 42 slow, it was awesome lugged like a John Deere, ripped like a MXer.":thumbsup:

with a conclusion like:

"then I decided to do the RM mods......OMG it was unreal, the off idle torque response was double!!!!":thumbsup:?

OR

"then I decided to do the RM mods......I think the RB mods may have helped the low end a little.":confused:?

Anyone have a non-fiction story like this?

[shrubitup]

Look at my signature. Everybody hates the TMX carb on the Honda CR250. I was told that you simply could not get rid of learn surge easily at idle. I followed Honda's jetting chart and got rid of it. Lugs well but not great. Top end still rips. Odd thing is that you let it cool for 5 minutes and hit it again and all of the sudden it's rich for a bit and then gets normal - so claims that it is finicky to find right setting always are applicable.

In the end I will very likely just get the Keihin PWK but I had to at least discover options on Mikuni first.

Will say that adding a JD jet kit to my already jetted YZ250F actually made it worse.

[OLHILLBILLY]

I had him re-cut the squish band along with the carb mods, and I added a VForce 3 reed block to the mix, so I don't know exactly how much the carb mod did by itself, but the whole package change was amazing. The bike has a hit now that rivals a MX bike, and flat rips all through the rev range. When I sent the carb to him it had 40/150 jetting, he left the 40 and bumped it up to a 152 main. I had a CEL needle in it and he changed it to a CEK. Very little in the way of jetting and needle changes, so I can only presume the biggest majority of the improvement came from the rest of his work on the carb. Was $$ very well spent in my opinion.

[asg5265]

or you can just get a kdx200 carb. Im not a big believer in carb mods because all they do is make the holes and the carb more hydrodynamic. its called a dremel, get started.

[SS109]

asg5265, you totally lost me there!

OP: Hey, you are the one who asked "...like what is done to the idle circuit and jet block?" and I replied to it. Sorry if that wasn't what you were looking for but that is one of the questions you asked! Well, if you want to read a lot of reports about what people have experienced with RB's carb mods and the KDX, including jetting before and after RB mods, then go read up on it at KDXrider.net. You will not find a larger group of KDX owners anywhere that have used RB's carb mods. There are plenty of reports over there on it and I haven't ever heard one person who wasn't very happy with the results.

[shrubitup]

a RB'd out KDX200/220 feels like a weak YZ250 (unlike its stock condition). Sweet mods for that motor.

[JJRace]

do a search on here:

http://www.justkdx.dirtrider.net/

search their forum. you will probably find the answer you want there.

[AtomicGeo]

I've had my KDX220 (2003 H) RB'd (carb and head) and I've gone to a CEK needle with new reeds. I ride at 7-10K ft in elevation. It was jetted properly before, but with the RB mod, it really made it snappier with response that I did not have before. IMO. it was worth it. I also have a mostly stock (except for exp chamber and pipe) 2003 KX250; no comparison, the KX250 is stronger and faster though out the range. The KDX is a fantastic trail bike though.