RB Carb Mod Survey

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Feb 1, 2001
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#1
I have a modified 98 kdx 220. Pipe, reeds, Wiseco, air box and the normal mods. I neeeeeed more top end. I don't want to loose bottom either. From what I read this RB modification to add the divider and bore out to 36mm is the answer. Is there anyone with a bad experience with this modification? Everyone seems to be real happy with it and I am about ready to make the jump.

I rode a KTM exc200 a couple of weeks ago and I liked the way the bike revs high. I can cover alot of ground per gear shift. I still like the kdx better all around but I would like to get that type of rev into the kdx. I looked at the KTM and it uses a Keihin Carb but it is a 38 MM WOW!!! on a 200. Is there anyone that has put a 38 mm on the kdx 220 instead of boring it out to a 36mm? Or maybe larger? Am I insane? Probablly!!

You don't know until you ask the experts. :p
 

Sage

dirtbike riding roadracer
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Mar 28, 2001
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#2
I've got a 38mm plated carb on my 220, and have noooo problems dusting a exc200, i'm even 70 pounds heavier than a friend that rides tha exc. both are ported but mine has reeds & a rev pipe.

1 in 4 people are mentally ill, ask 3 friends, if there ok then your the one!
It's good to know I've got company.;)
 
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#3
Hey Sage
Where did you get the 38mm? Is it a new carb or a bore job? Did you have to change any of the air boots on either end of the carb? This sounds great! There is a hill climb in WV that I am getting ready for. It is a third gear wide open with a bike that revs just to make it. We had stock kdx's that could not get even half way up this. My modified kdx got almost to the top and did not have enough power to stay in the band. Oh yea, how is your bottem end with the 38mm? Did you loose much?
 

Mac

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#4
Hey Greenman, which pipe do you have?? I would think that going bigger than a 36mm carb would hurt low end.
 
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#5
Talk to dan (acutemp on DRN)

He's said he's getting a 39mm for his KDX.


It is from RB, so it's 'plated' (if I understand Sage's nomenclature..same as 'divided').

Generally, the bigger the carb, the worse the bottom-end, the better the top-end. The divider mitigates this somewhat. You keep the velocity up for the bottom-end, and still have enough volume to make the top-end scream.

Makes your carb kinda like a 2BBL carb.

Think of how lousy your car would run if it had one butterfly in the carb (your DO have a car with a carb, don't you?) that had about a 3" diameter.

I had 1300CFM on my 340 for a few years. It worked fine...but only 'cuz every throttle shaft was progressive.

Anyway...talk to dan about his 39mm. If I recall correctly, he just got it this week????

Oh, and I've ridden with a KTM exc200. The KDX can eat it alive! (Yes, I do have the RB carb mod).
 

acutemp

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#6
Greenman, I do have a RB modified 39mm PWK that I am going to install on my 200. I also want to try a buddies 36mm bored to 37mm carb that has been modded.
The 36,38,39 carbs are much larger than the stock kdx carb so there are mods that need to be done to the carb boots, it is not a simple bolt on mod.

I am lucky enough to have access to some pretty nice testing equipment (2-dyno's) so as soon as we get some baseline numbers with my modded stock carb , on goes the 39mm. I hope to gain some of the overrev that the ktm 200 has. Mine has a much nicer power curve up to that point but the the orange bike holds its peak power longer.
Dan
 
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#7
Thanks for the input guys. Dan, well you are the king. Please send an email when you get through this. I am ready to purchase a carb mod and you are doing the kind of testing that all of want to do. Some of us don't have the time. Some don't have the knowledge or the tools. Like me Some don't have any of the above. I jut wan my bike go fas.:confused: A thread on this would be helpful for the forum as well. I want to have the bottom and top end and as much as I can get. It sounds like every other person on two wheels. "GIVE ME MORE"

A thought that was spurred last week by canyncarvr was "why does noone make a 2bbl carb for a dirtbike?". Seems too obvious to not have been thought of. Maybe someone on the forum knows why this has not been done yet.

MAC, didn't mean to ignore you. I have the FMF rev pipe for the KDX 220. It made the bike a totally different bike. I lost so very little of the bottom end that I can't be sure that I lost any at all. I can still grind this thing up a hill without stalling it.