35mm CARBS vs PWK28 comparisons vs PWK38 mm

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35mm CARBS vs PWK28 comparisons vs PWK38 mm

Post by KR-1R » Sat Sep 26, 2009 10:20 am

If youre only going to modify ONE single thing on a KR-1 or KR-1S,
upgrading from the 28mm carburetors to the 35mm is probably the mod most lusted after.
The PWK35 are standard equipment on the race orientated KR-1R (D1,D2) and the Sport Production KR-1SP (C3A)

while the original parts are no longer available many motocross bikes feature the PWK35...
KX125H (1990), KDX200/220 (1990-1996) , KTM 125/140.

The successor to the PWK35 is the PWK35 QAS (Quad Vent, Air Striker)
which features 4x vent points (in case of mud blockage)
and the Air Striker venturis in the mouth improving small opening throttle response.
?The Air Striker version are standard fitment on KDX200 from 1996?
around US$350 gets you a pair of Q.A.S.

the throat area of the PWK28 is 616 square mm (100%)
the throat area of the PWK35 is 962 square mm (156%) if added intake and exhaust noise (volume) is a problem for you (MOT), stick with 28's
the throat area of the PWK38 is 1195 square mm (193%)

The KR's left hand carb is unique in that the idle adjuster is on the right hand side...so its not squashed against the frame spar
unless you have 'R' or 'SP' carbs there is a blank port to tap a rightside idle adjuster into an aftermarket or secondhand 35, if so desired
the left most carb below is from an R/SP and is for left side with the adjuster on the rightside (confused yet?)
the KR's PWK28 (left carb) features a vacuum take for the fuel tap diaphragm to start the flow of fuel - the 35s only have fuel in

If the Enterprise was a PWK28 then the PWK35 would be The Doomsday Machine

Its not until you hold one next to another...
Compared to the 35mm the standard 28mm looks like a toy... someone say scooter?

PWK35 QAS is Quad Vented, Air Striker which features
4x vent points (in case of mud blockage in dirtbike applications)

and the Air Striker venturis in the mouth improving small opening throttle response

The non OEM needles are Brass rather than hard anodised aluminium
The QAS slide is a No.4 (richer) versus No.6 (leaner) on KR-1R, KDX, KX etc. Slides are most expensive component to replace

the needle for the 'R' or SP is the R1372J (N427-48CEN)
the R/SP came with 40pilot, 145main, CENneedle, 6.0 slide (mild lean motor)
the Q.A.S. ships with much richer settings (48pilot, 160main, DEKneedle, 4.5slide cutaway) intended for higher horsepower/ported motors
the easiest choice to make the 35 idle is to downsize the pilot jet affecting the zero to 1/8th opening range then...
needle choice: is controversial and can depend on other motor/pipe modifications
YES a R1372J (N427-48CEN) in a 35 will idle as good as a bike with 28s!

needle selection comparison against 'R' needle (vs needles from donor bikes)
DEL, DEK, DEJ, DEN are richer coming off idle and can be leaner than stock in the crucial 1/4 to 1/2 opening region.
A 48pilot raises the curve at zero opening compared to same needle with #40.
#48 pilot might be 25% richer than a #40 pilot jet

related replacements to accomodate the 35mm upgrade...
Fuel tap - larger bore two outlet (no vacuum takeoff)
Throttle cable - modify spittler or aftermarket (remember oil injection pump)
Reed side intake rubbers make or modify
Airbox side inlet rubbers (enlarge airbox holes) seek donor FZR1000 or bikes with >40mm FCR or CV carbs

answers on alternative solutions to the fitment problems can be found on this forum seek MJ43, FALCONMAN, WAKASAKI, THE-ELF, SATARN, KR-1R


PWK35 Needle Selection
Choose your column (Diameter) to richen or lean (Zero, 1/8th to 1/4 throttle opening)
Choose your row to select needle Taper (A) and also Length (L1) to alter 1/4 to W.O.T. running
28 <<< 35 <<< 38mm
Can you go as large as 38mm ?... YES but not on standard KR-1R rubbers
PWK 38mm vs PWK 35mm

So a pair of 38mm will happily occupy the space that were meant for 35mm
They are the same width and heights below bore centreline as 35mm - just slightly wider were the fuel pipe sticks out
A 38mm is almost certainly overkill on a mild road 125cc (x2) - you would probably want the help from a pair of these ...
(homemade of coarse because these are ridiculously expensive US$89 each)
...to regain some low end/mid response
The next time youre having an argument with an RGV owner theyll shut up when you ask them how big theirs is.

Spotting differences of PWK on Ebay etc....

There is a size step between the PWK33/35 and the 36/38/39/41mm models
The 36mm is just a narrowed bore 38 offset bored, NOT an overbored out 35
there are many slight differences in the larger PWK's
the later 90's models had electric powerjet

there are at least two regular Ebay sellers (usually listing only 1 carb at a time) that seem to have very good Keihin clones/fakes for sale
they seem almost new in appearance but notable irregularities are
the large Idle screw,
the allen head fuel bowl screws,
the new clear vent hoses,
the very chrome like slider,
the poorly cast fuel bowl drain port
the colour of the brass plug below the choke pull
if you look at their selling history they have sold more carbs than any other part that a bike breaker would have or have access to.

PWK28 vs PWK35 vs PWK38

the is a 9mm difference between the 35 and the 38 above the centre line
the engine side is 42mm dia on the 35, 46mm dia on 38

the 35 also retains a tapping point for right hand side idle, the 38 has none...

note the inlet for the fuel pipe angled more on the 38's
choke hole and air screw notable differences between 35 and 38...

casting for ?slide vacuum point? (?or sensor switch?) on 35, 38 is bare...

PWK28 versus PWK38... (the RED writing is because carb from racing RS125)

PWK28 versus PWK38...

PWK28 versus PWK38...
the throat area of the PWK38 is 1134 sq mm (+18% on the 35, +84% on 28mm)


other possible donor bikes...
the 1990 onwards KDX200 used 35mm
the 1990,91 KX125 (H1/H2) used 35mm
the 1992 KX125 onwards (J,K...) used 38mm?
the early 90's KX250 (H1, H2) used 38mm
?the 1992 KX250 onwards (J, K...) used 39mm ?
the KX500 used 39mm

this data isnt strictly correct...
Polaris 700XC 98-99 PWK41? 39? and for pair!
Polaris 600XC 98-99 PWK39 and for pair!
Kawasaki KX500 88-04 PWK39
h***a CR125R 99-00 PWK38
Kawasaki KX250 88-97 PWK38
Kawasaki KX250 98-07 PWK38
Yamaha YZ250 1996 PWK38
Yamaha YZ250 99-08 PWK38
Kawasaki KX125 92-95 PWK36
Kawasaki KX125 88-91 PWK35
Kawasaki KX125 96-97 PWK35
Kawasaki KDX200 88-06 PWK35
Kawasaki KDX220 97-05 PWK33
h***a CR85R/RB 05-08 PWK28
Kawasaki KX85A PWK28
Kawasaki KX100D PWK28
Yamaha YZ85 93-08 PWK28

after the early 90's the standard PWK casting was modified to accept 4 float bowl vents, and also became Air strikers (throat fins)

four (quad) vent from 95 KTM250 MXC...
95 250/300 EXC...

KTM spec 35 from 125/200 SX...

what you DONT want is a Keihin PJ from most 90's crossers because they are too wide...

the supposedly higher spec PWM is possibly also too wide for the frame spars because the fuel line goes in very wide of the throat centre (a problem for the left carb fitment like th PJ model and wide at the TPS assembly) - although, the PWM is said to be superior to the PWK in outright top end, because the carb on whole, is about 20mm shorter intake to outlet. There is also idle adjustment differences and the short years the PWM appeared on MX bikes (late 90 YZ250) suggests they were possibly unpopular to tune?

PM me for the $$cheapest$$ way to go bigger than 35

PJ, PWM and the later model PWK with TPS and powerjets won't fit a KR !!
Last edited by KR-1R on Wed Aug 29, 2012 11:31 pm, edited 6 times in total.

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PWK35 needles question

Post by scooble » Mon Aug 29, 2011 9:35 am

The needles that came with my carbs were marked as CG L (middle clip)
I also have two other new sets marked CEM and N99 E

I've no idea which is richer or leaner, which ones should I fit to a tuned engine?

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Post by KR-1R » Tue Aug 30, 2011 11:37 am

I dont think I have any N99 but arent they for 28's ? (57mm long needle)

the needles for PWK35 (33, 35, 36, 38, 39mm) carbs are 66mm long

CEM has basic diameter of 2.715mm
CGL has diameter 2.705mm (smaller) so is slightly richer (0-1/4 throttle)
in small opening (near closed) the diameter is limiting the fuel flow - however pilot jet/circuit is operating too

CEM is nearly identical to R1372J (stock KR-1R=CEN) but is approx 5% more fuel but only <1/8 throttle

bigger pilots will raise/drag the fuel delivery curve up in 0 - 1/8th
clip position will raise/lower the curve 1/8 - 1/2 throttle
main jet controls fuel (richens or leans) mixture 1/2 to WOT

diameter meters fuel at small openings
a larger opening is then controlled by taper
largest openings (needle mostly withdrawn) the orifice of main jet limits the fuel (pilot, diameter, taper no longer proportioning fuel)


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