Jetting the RB carb mod

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#1
Ok, this is my buddies bike but I mostly end up jetting his stuff.

I am not looking for specific jetting recomendations, but just in general jetting with the RB carb mod. I think that is what it is called with the plate cut in on the reed side of the carb. Also this bike has had the crapy mikuni carb replaced with the PWK.

The way it is jetted now

PJ- 55
slide-?
Needle- R1469M which is 1/2 clip leaner than DGK
clip-3
main- 155

the way it is set up now, it has a very slight rich coming off of idle, the air screw has little if no effect. The idle screw is turned almost all the way in. To ride it, it feels almost spot on below 1/2 throttle, but is a reluctant starter.

I think that the 55 pj is overwhelming the idle circut and requireing the idle screw to be turned in all the way for the air, renderig the air screw useless.

I am thinking of changing the needle taper to a "C" less taper, but go richer on the straight section to a "J" and drop the pilot to a 48 or less in hopes of droping the slide back to a normal position to help starting.

Does the RB cause strange jetting woes that I will not be able to jet around or is it just a matter of sorting out?
 
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#2
This thread got move a couple of times, so dont know if anyone saw it here, so Ill bump it up. ;)

I know it isnt a KDX, but many of you have the RB mod and some prolly have some input.

Thanx

Chris
 

Rich Rohrich

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#3
Originally posted by 2strok4fun
I know it isnt a KDX, but many of you have the RB mod and some prolly have some input.
That's why I moved it here. :thumb:
 
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#5
Here is an update for anyone who is interested.

The other day was the firs day I had a chance to ride with my buddy with this bike. He didnt drop the pilot like I had sugested before our ride, but I did have him back out the idle screw to a more "normal" position.

This allowed easy starting, cold or warm, but a short ride revealed that it was way too rich just off idle and spooged terribly. So I know we are on the right track. Dropped the pilot to a 50 from the 55 and it helped some but still too rich. I suspect it will get to a 45 or below. Then we will try to swap some needles.

So It appears that it should jet easily, just needs to be sorted out. :)
 

canyncarvr

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#6
There are some still stuck with their original point of view of the rb-modification on the pwk...that its jetting picayune-ish-ness rendered it unsatisfactory.

That's unfortunate. They're missing out on a huge benefit.

For starters, don't assume ron (of rb-designs) has ever seen the carb you're dealing with. Verify that. Just because it has a downstream divider does not mean it's ever seen ron's shop.

You mention a slide of '??' Ron's slides are marked specifically to indicate if he's cut it or not. If the slide has NOT been cut, then that's a for-sure indication that it's not an rb-modified carb.

Also, the mention of the AS not having any effect is odd. Granted, a 55 pilot is pretty big, but generally speaking, ron modifies the pilot circuit in such a way that makes it much more effective than oem.

What size carb?

Another clue.....if it hasn't been bored, it also is not an rb-modded carb. I don't think he bores EVERYTHING, but that is usually done to (at least) make up for the space the divider takes up in the airstream.

Where'd the R1469M come from? Ron has generally (emphasis on general) used 'C' and 'D' tapers in his carbs.

I hesitate to say, 'Call ron.' He would know the carb by the name of the owner (if it hasn't changed)..but please don't waste his time if it's a carb he's had nothing to do with. Even if it is one of his pieces of handiwork that doesn't sign you up for an hour of his on the phone.....and it's not even your bike.

If you do call, maybe have the slide in hand so he can tell you the identifying marks he stamps on his work.

Keep in mind that sometimes he modifies the jetblock (changes the heighth of it) of some PWKs such that what you think might be suitable needle-wise won't be (suitable, that is).

Like.....why such a long L1? You list an 'M', which is not a direct cross to a keihin number, but is longer than an 'K' and shorter than an 'N'..so it is shorter than an keihin 'G' L1. Why isn't that richer (you say 1/2 clip leaner)?

The proof of the pudding comes from JD's spreadsheet which shows that an R1469M/3 compared to a DGK/3 (both with a 55/155/#7TV) is richer from 3/16 all the way to 3/4 throttle.

So...there a few things amiss in the original post, methinks.

Generally: For starters, find out if it's an rb-modified carb.

A further btw.... mr. ron has modified carbs for far more than green bikes!

Let us know.
 
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fuzzy

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#7
Good read...I'm getting ready to send him the PWK that's on my WR. It's jetted OK, but it's a stock KDX200 carb and definitely needs a leaner slide by the way it's running. I know he can cut them for less than a new slide costs, but I'm just going to have him do the whole shibang. It's made a great torque improvement on the (already torquey) beast already. I can't wait to get some razor sharp response out of it! Prolly mount it to a DFII while I'm at it...

What does this mean? I'll prolly have some needle questions to bounce off ya in the future.... :) BTW, where can I get that spreadsheet?
 

canyncarvr

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#10
What's the frown for? You didn't think it was FREE did'ja??

It's worth it! :) :)
 
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#11
OK-

Thanks canyoncarver for the response, Ill try to provide more info:

This is not my bike, so it is not in my garage. The bike was purchased from a DRN'r that I trust and I am going on his word that it is indeed a RB mod. I put the ?? after the slide # because I dont recollect, it has been cut.

Yes I have the JD spread sheet, it has helped me dial in my ktm, That is where I get the DGK comparison from. R1469M-3 = DGK-3.5 exactly. BTW I ended up using a shim on my 250e to get a 1/2 clip.

Also this year RMX had a Mikuni as Original, the carb was swapped for the PWK possibly when the carb mod was done, not sure.

I belive it was just WAY over jetted on the pilot circut and was compensated with a the idle screw turned all the way in rendering the idle circut / air screw useless due to the low velocity / delta P difference.

I think it will jet out nicely and be a easy starter.

I just try to help my bud. If it were me, Id have it closer sooner. ;)
 

canyncarvr

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#12
The carbs come back from ron jetted pretty close. Well, based on what info ron had in the first place (where it's ridden, riding style, expectations and such).

Certainly that doesn't mean it wasn't changed from that.

So...this is a 35 bored to a 36? Or it's a 38 bored to a 39?

A 55 pilot with a 155 main seems way bassackwards to me, but I have no experience or knowledge of what your buddy's bike requires.

Yeah...I rejetted my riding buddy's punkin. Shouldn't have!! It was fun waxing his arse with my little green machine! He's back to leaving me behind...agin..... :(

Hope it's the sooner more than the later!

Good luck!
 
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#13
The next time he has his bike in my garage I'll measure the carb, but I am fairly certain it is a 38.

the 55/155 combo seems foreign to me as well, I dont know how much it may have changed from its original RB jetting spec if at all, I will try to find out.
 

canyncarvr

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#14
Keep that in mind if it comes to listening to any kdx'ers jetting ideas. I don't know of anyone other than dan (acutemp) that's run an rb-modified 38mm (bored to 39 I think) on a kdx.

Is it a 4-vent carb?
 

fuzzy

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#15
FWIW, the 'close' jetting on my WR250 w/ the 35mm PWK is 40/185-3rd on a 60deg day with 70% humidity...Neat, eh? :)

I say 'close' cause it runs decent(Main set w/ chop, pilot set w/ butt), but definitely needs a different slide and needle. It will now loft over anything off-idle, WAY off the pipe, and it's sinfully rich on the slide. :)

The WR came stock w/ a 38mm Mikuni VM just like the RMX. Mine had a 38mm TMX swapped out from a YZ before I got it. The jetting on it was generally 35/360.