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Has anyone here fitted a larger carb to their bike?! I've just won a 39mm PWM, I know I have a little machining to do to make it fit, but has anyone else tried it?! Results atall?!

this again? Aren't you supposed to be some kind of engineer?

Bad idea. An IC engine can only pull so much air. Putting on a larger carb than needed is never a good idea. You decrease velocity, and ruin the throttle response. It takes a good bit of work to get 36mm to work well, going bigger.... Why?!

cuz bigger is betterrrr

Tried it, I won't say it isn't possible, but I have several hours in it. The dx isn't ported to handle the volume. If the motor was ported for xtreme top end(such as a 125) then it would be very possible. Also there is the pipe factor. A 2 stroke engine must also have the proper designed pipe for the carb. I again wont say it isnt possible, but alot of work and not so sure the gains would be desirable to have.

my ktm 380 has a 38..........a 39 on a 200??? .......wont the carb be about as big as the intake opening?

juliend is right..... its kinda like running a 850cfm carb on a 200hp v8 engine.

kx125 has a 38mm
rm125 has a 38mm
yz125 has a 38mm
exc200 has a 38mm
yz250 has a 39mm

juliend wrote:this again? Aren't you supposed to be some kind of engineer?

Bad idea. An IC engine can only pull so much air. Putting on a larger carb than needed is never a good idea. You decrease velocity, and ruin the throttle response. It takes a good bit of work to get 36mm to work well, going bigger.... Why?!

Why don't I just take a leaf out of RB's book and make a divider plate on the inlet side?

And all those bikes you mention are like the KDX in what way? Why is that relevant? Do what you're gonna do, man. I don't know why you bother asking.

MORE POWER!!!!!

moto_psycho wrote:kx125 has a 38mm
rm125 has a 38mm
yz125 has a 38mm
exc200 has a 38mm
yz250 has a 39mm

Almost all, if not all the two stroke KTMs have 36mm carbs. I'm also pretty sure that the YZ250 has a 38mm carb like most other jap MX bikes have.

I'm sure that you could get a KDX 200 to start and run with a 39mm carb. I'm not so sure that it would run well. The bike isn't ported for a lot of RPMs like an MX bike is so it doesn't need, and probably can't use, a carb that large. I would expect it to be very soft on bottom and not much better than a 36mm carb on top. Due to the weaker signal it is going to need large jets.

if you already have the carb, and seem to know it will work, go for it. you always ask questions when you already have your mind made up?

i know you was asking if anyone has ever tried this, not what everyone thinks...........but.......you have to understand the way a motor works. it can only pump in the air per rpm and ccs. the kdx really tuns lower rpms than most 200cc.........trail bike vrs motorcross, unless you ride thru the woods pined in 3rd gear all day, then a 39 carb will kill the kdx 200.

Search for 38mm carb on here and you'll likely dig up an old discussion concerning this. RB put extensive work into it, and gave up which is pretty much the end of the story. Any minor gains were offset by the fact that the larger carb body itself was just a PITA to deal with. Doesn't fit intake boot, doesn't fit airbox, top hits the tank, etc. This is why he bores the 35's out, and does his other magic. The other bikes came w/ 38's, but seem to all have gone back to 36's aside from MX/SX bikes. I just got a 36mm PWK for my 38mm TM equipped WR, but the bike did come with a keihin one year so I aquired that airbox/throttle pieces/etc, and the 36-up are same body. I once tried a 35mm KDX carb and had thte opposite problems of the KDX trying to go bigger.

I will be adjusting the porting on the bike when I next rip into the top end, mainly just cleaning up the transition area between the ports, but also using guides based on the KDX and other bikes I will modify the port height slightly.

Also, I have checked the measurements (info here http://www.sudco.com/Carburetor01.html) the carb WILL fit in the gap, I am not 100% certain whether it will squeeze into the rubber so I may have to shave a mil or so off the OD of the carb to get into the inlet (there is a 40mm model of the PWM so I will def have at least 1mm grace). The carb is 12mm shorter than the PWK (better response though I know you will argue added bore will counteract that but I am very much considering fabbing up a divider plate on the inlet side to aid low end flow), I may have to turn down 1mm or so on the airbox side also to fit into the rubber, or perhaps any more if I can then machine an adapter to make the carb effectively 12mm longer (not affecting response as carb jet opening is still closer to port).

Trust me people, I know all the arguments for and against, I don't care about that, if you really want to argue the toss, come to England, we will go to the pub, and argue engineering to our hearts content, I do it most weekends anyway! I want experience, no-one can tell me exactly how this bike will react, common sense says better top end but not 100% efficient due to other factors such as porting (which I have already stated I will be fettling soon) Ignition, reeds & exhaust (I have a new CDI http://www.dragonfly75.com/moto/KDXCDI.html and will eventually get a pipe made to compensate from these guys http://gibsonexhausts.co.uk/page_7.php?pgenme=about).

Queue the next comment "why did you buy a KDX if you want something with more power then? the KDX is a woods bike not an MXer" my answer, I like modifying ****, its fun, its a hobby.

But yes, you are right, I made my mind up already, but that wasn't why I was asking, I was just asking for anyone else's experiences doing similar, it interests me. (in which I will now be searching for RB doing the 38mm) Plus if the carb is crap I wont lose anything, it was badly listed on ebay and I got it for next to nothing!

Edit: Come on people spank away!

kx125 has a 38mm http://www.kawasaki.co.uk/product.asp?Id=34006758AA5
rm125 has a 38mm http://www.suzuki-gb.co.uk/bike/rm125k9/tech/
yz125 has a 38mm http://www.yamaha-motor.com/sport/produ ... specs.aspx
exc200 has a 38mm http://www.bikez.com/motorcycles/ktm_200_exc_2006.php (though they have returned to 36mm which you will find on official KTM site)
yz250 has a 39mm (they have gone down to a 38mm now http://www.yamaha-motor.co.uk/uk/produc ... =techspecs earlier models got the PWM 39 which is a higher model carb)

If you want some one that has experience with the larger carb in the KDX community then contact Ron Black.

yes the kdx is a woods bike, not a mxer.........what i want to know is why are you tring to make it into a mxer bike?? thats whats gonna happen if you port and run that big of a carb on it. you will have almost no bottom end, and really not so good on top either. i dont have no engineer degree, but i know my motors. yes, i do all my work myself, including porting. there is a fine line when it comes to porting. if you run that big of a carb, you will loose, not gain............i know. if you combine your port work, pipe, ignition and 39 carb you have in mind.......you will have no bottom end, and you will have to turn the **** out of the bike to make any power, and that = early engine failer.

Then we look into all the race technologies that have been used to improve low down torque whilst retaining top end reviness. Like water injection perhaps...
http://www.suzuki-rg500.com/water.htm

Or simply installing a divider plate to maintain flow and remove the dead zone at lower throttle openings (dead zone = gas no flow zone)

Good luck!

Feel free to read that as: "Have fun wasting your time!"

BTW, those specs you posted from bikez.com are inaccurate. All three of my buddies ktm200's have PWK 36's on them. Years range from 99 to 07. Specs will sometimes sometimes say 38, I don't know why. Laziness?

Sounds like you really need a 200EXC. Definitely search on the post I mentioned from RB Designs concerning the 38mm PWK on the KDX...I know he put in a fair amount of effort....Is a carburation expert, and machinist....and bagged it.

The KDX will likely run fine with a 38mm PWK. Little sacrifice off the bottom, little gain on the top. The logistics of the project are it's nemesis though. Plan on needing different slides/needles/etc to get it jetted right if you ever get it to physically fit. Speaking of fit. The KDX's cage is smaller than these other bikes originally intended for the larger body carb.

I personally love the smaller carb on my 250. However, again, in this case, smaller is actually the SAME size carb body.

Is the PWM an electric solenoid powerjet carb??????

http://keihincarbs.com/carbs/pwm/gate.html

http://keihincarbs.com/carbs/pwk/gate.html

PWM is smaller all round than the PWK.


PWK

PWM