125-200cc RB Designs Carb Mods on CR/WR125 bikes

I'm interested in hearing all rider feedback on any RB Designs carb mods on 125 cc bikes. I'm primarily interested in 38mm PWK and before/after results.

More info on RB Designs carb mods

WOW!!! This is must-have item for all bikes

A few weeks ago, I sent my 38mm PWK air striker to RB Designs. We exchanged several emails and I went with his recommendations. Ron installed his divider plate on the outlet side (i.e., reed valve side), did some low speed circuit mods, and added his RB mixture screw.

First Impressions:
His turnaround time was very quick. I got my carb back in a few days and my first impressions were good. The carb was machined for the divider plate and overall workmanship was top-notch! The divider plate reaches all the way to the slide and matches the slide profile very closely. So the carb bore greatly reduces it effective diameter at smaller throttle openings. The RB mixture screw is also very nice! There is a second spring to help maintain your adjustment. The knob is knurled for easy turning and has an indicator line to help note your setting.

Ride Impressions:
I rode my bike just before shipping out the carb, so I had a good baseline to go from. I also left my winter jetting setup exactly the same before his modifications. My first observations was that my bike started up easier. A couple quick jabs on the kicker and the littler 125 fired to life.

I did my usual warm-up and rode easy for a few minutes down the trail. I immediately realized that this was not the same bike! This modification has made a major transformation. Previously, my throttle roll-on was tame and lacked low end response. I used to think this was just a "125 trait" and lived with it. Well, let me tell you... 125's can have some serious bark and low end response... you just need Ron to work his magic. My bike feels like someone just installed a 144cc big bore kit. The changes from idle through mid range were that dramatic.

My CR125 now runs flawlessly throughout the entire range. You get a very small bore at low end (increasing torque), then as you roll-on the throttle, the bore opens proportional to the slide action and results in the same screaming top end of the 38mm bore. I purposely short shifted and tried to bog my bike. It always pulled strongly when rolling on the throttle. This thing performed awesome!

I would highly recommend to other members.

John,

I did it to the PWK 38mm AS that I bought (from Motosportz). Seeing the amount of difference that it made, I'm sure that you would also get excellent results with the Mikuni TMX.

No boring was done. Since my carb was already 38mm and my bike is a 125, his recommendation was the divider plate and circuit mods that I described above. The divider was done only on the one side. Ron has been doing carb mods for many years and many types of bikes. I went with his expertise.

Other bikes and engine sizes may require a totally different approach. For example, KTM's with the smaller 36mm carbs need the bore increased up to 39mm and divider plate installed. The GasGas bikes get even more tweaks and slide modifications. It would best to talk with Ron about the characteristics that you are after. He responds to his emails and provides great service.

RB divider plate is nothing like the Power Now products. I had a Power Now on my last bike and there was little improvement. The Power Now divider is shaped differently and only covers a portion of the carb bore. The RB divider is machined and welded to fit perfectly in the carb bore. It covers the entire length up to the slide. While installing the divider plate, Ron can also make the other low circuit mods to work together. The overall package works excellent and it boosts the signal strength of the low speed circuit making it work more effectively.

His site does have examples of carb mods for different bikes. You can only get his mixture screw if you have him do the mods. It requires some machine work to work properly.
http://www.rb-designs.com/carb.html

You may want to go to his home page to see all of his services.

Photos of my carb modifications

Here is a couple pics of the divider plate.







Here is a view with the slide installed. Note the divider plate covers the full length of the bore up to the slide.



Here is a comparison of the RB mixture screw vs. the stock screw. Note the second spring to help maintain your adjustment setting. Nice!



Here is a close-up of the RB mixture screw installed. Adjustments are very easy now with gloves on.



Note: the OD is remachined to provide the proper profile to mate with the manifold boot.



Thanks Ron! You did an awesome job.