The Keihin PWK28 Carb. Thread

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RC45
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#31 Post by RC45 » Fri Jan 20, 2012 3:31 pm

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Last edited by RC45 on Tue Apr 17, 2012 9:14 am, edited 1 time in total.

acarver
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#32 Post by acarver » Fri Jan 20, 2012 5:15 pm

Ah, got it.

For those who still want to use the stock petcock with auto on/off and reserve functions, it's a great time to rebuild it.
Mine had never been rebuilt and was beginning to allow flow when off and
was allowing the reserve fuel to flow during the on function effectively stranding you.
The SRX250 petcock is identical except its outlet is 1/4 instead of 3/8.
You can disassemble it and put the parts in your RZ/RD petcock for an effectively new unit.
Part # 51Y-24500-01-00 .....got mine at ronayers.com for $64.

http://www.ronayers.com/fiche/TypeID/26 ... /FUEL_TANK

One more additional thing regarding throttle force, the lighter fastracing slide springs I mentioned previously
were only for the larger body PWK's, so I cut mine 2 coils for better feel.
Stock PWK28 force x 4 along with pulling the oil pump arm is too stiff.
Now it is only a little stiffer than normal bike feel and still pushes the slides shut well.
Someone on premix without the pump cable might get away with only one coil cut.
Andrew

RC45
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#33 Post by RC45 » Sat Jan 21, 2012 5:55 am

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acarver
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#34 Post by acarver » Sat Jan 21, 2012 1:33 pm

It is more true for a race bike to jet for the specific conditions than a street bike due to
the piston-exhaust temperatures, loads involved and simply convenience.
For the street or people who want to jet less at the track, it makes sense
to jet perfectly for your most dense situation and accept some richness
as the temps/elevation rises. For example, the information that I gave
(for a streetbike) that at 60F/SL the bike seems to like 40/NAPE #2(or your NAPF @ #2-#3 assumed)/135-138 (assumed)
is the most dense situation I'm normally in so that setting is on the edge of being lean. That is intentional so that as the temps rise to 80F or
I climb in the mountains, the bike is moving in the right direction mixture wise.
That method is for convenience on a street bike where rejetting isn't always going on. That is also a good method for off-road when you're
going through temp/elevation changes and it's not practical to rejet up on some mountain or trail. So I jet on the edge of lean for the most dense situation and
let the conditions "come" to the bike. This way the two stroke along with using the PWK seems fairly flexible and accomodationg to changing conditions.

If you're competing, then yes, I'd rejet at
80F from a 55F perfect setting at the track to regain response and HP even though the bike would be more conservative if left alone albeit rich.

This is one of the reasons I switched to the PWK, in my MX experience it is a less sensitive carb to density changes and
will run well even when not jetted perfectly. Considering it is the #1 MX racing carburetor and also
the carburetor on the Kawasaki KR1 race bike, among others, I'd say it's a great choice for the track. I'm not knocking the Mikuni's,
I just have more jetting experience with the PWK, have learned it's behavior and like it.
Also, the primary assumed audience for this thread was street RZ500's,
understand your specific situation of a track bike would have had some
additional information we're talking about now in addition to installing
allen head screws, additional heat shielding/ducting etc.

Your description of setting a good known baseline and then adjusting for density is the way I do it. Jetting the PWK is very straightforward with
primarily an A/S, pilot/needle/main variable, assuming you like your
installed air jet and slide cut. Imho those two are fine for our application.
Below is a PWK38 chart for a YZ250 that I use for both MX and off road.
The PWK28 behaves and reacts the same way, you just have to offset
the sizes to the smaller PWK baseline
, otherwise it works.
Sudco can also assist you if you have specific questions for your situation.

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Last edited by acarver on Sat Apr 21, 2012 3:02 pm, edited 1 time in total.
Andrew

Dan_Austin
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cable compatible with Mikuni TM28?

#35 Post by Dan_Austin » Fri Apr 13, 2012 6:42 pm

I have a set of horrifically gummed up TM28s that have a cable
already setup for use on the RZ. I've started the clean-up and
checked to see what a full re-jet would cost, and thought I
might be better off with an all new setup.

Can I reuse the cable I had the Mikunis connected to, or is the
carb end completely different?

acarver
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#36 Post by acarver » Fri Apr 13, 2012 6:44 pm

The best person to ask that question to is Bill Wilson at Wilson Performance.
He custom made my cables for the PWK's, so he could tell you how they compare to his TM's.
Andrew

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#37 Post by acarver » Fri Apr 20, 2012 2:07 am

Just got done doing some final testing and here's the info I came up with.

Keep in mind this is for a stock engine with mild chambers, so prob. close to what works with stock pipes too.

Conditions: Sea level and 60F (most dense situation I'm normally in with the bike)
Street riding with a mix of city/open road

Tested:

Keihin PWK28 with stock float heights and Uni oiled foam custom cut pod filters/91 octane Chevron fuel
The fabric behind the carbs is ceramic header material cut to shield the carbs. from the cylinder/case heat.

Pilots:
#40/42/45

Needles:
#1/2/3 NAPE (triple taper)
#2/3 JJH (single taper)
#2/3 N68A (dual taper)

Mains:
#135/138/140/142/145

Conclusions:
For the best overall response/power, the bike liked 45/#2 N68A/135.
The plugs were a medium tan color and the throttle was responsive in all positions.
It seems lean from a numbers basis, but when going up on the mains/needles numbers, it was clear that the bike was rich with
four-stroking and stuttering at times unless the revs were kept up. For a track bike I'd prob go 45/#3 N68A or #2 JJH/138 lowers/140 uppers
for a little more wide open cooling. The bike would pull that without hesitation as long as the revs were kept above 6K, but for street riding while getting to that point,
you had to wait for the powervalves to open and for it to come on the pipe otherwise you'd get a little minor rich stutter when above 1/2 throttle and below 6K.
With the #2 N68A/135's, it pulls from any rpm and any throttle position.
Seat of the pants it feels like the bike picked up maybe 5hp from the stock 26's with stock airbox.
For those curious about the triple taper NAPE, it's a richer needle from MX and needed to be #1 to run well. It wasn't bad but
you could tell it was rich in the cruise portion where a MX bike needs to be able to snap the throttle open and we don't have to.
The KX 100 NAPF (leaner) would be better and might be an option for those buying that carb.
The JJH was almost as good as the N68A, but the N68A was just a little more crisp and universal in position/response.
Hope that helps people.

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Last edited by acarver on Fri Apr 20, 2012 1:02 pm, edited 1 time in total.
Andrew

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wolfgangh
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#38 Post by wolfgangh » Fri Apr 20, 2012 4:52 am

Thanks for the interesting info. On my medium tuned ANTENA 3 bike with pressurized airbox I run PJ #48 / MJ #130 (uppers) and MJ #140 (lowers). Dont ask me why the lower cylinders wanted richer jets. Tests were done on a dynojet with Lamda sond.

Is this a pressure regulator for a fuel pump in the fuel line or simply an on/off switch?

acarver
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#39 Post by acarver » Fri Apr 20, 2012 12:45 pm

You're welcome Wolfgangh.

Re: the jetting, I wouldn't be surprised if uppers/lowers varied like you said on different engines for optimum egt's.
My comment to run one step larger on the uppers was purely from the idea that the uppers tend to run hotter water temps
so a cooler egt will help to prevent detonation and minimum cylinder clearances on a track bike being run hard.

The fuel line issue is also a good question and requires it's own info.
Basically I tried to run the stock petcock initially......the problem was that
the PWK's have a higher fuel inlet which makes the gravity feed angles borderline.
So I tried to add a 1 psi pump but the floats would overflow intermittantly.
Rather than go back and redesign all the float pressures, I decided to remove
the stock petcock and run a 5/16 tee off the 3/8 reserve side of the tank.
So it goes 3/8 into (2) 5/16 lines to (2) 5/16 petcocks to (4) 1/4 carb lines.
The fuel flow is much better than the stock system, you just lose the reserve function
but gain back real off valves and can use the whole tank because it's off the reserve side.
Andrew

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#40 Post by JungleJustice » Fri Apr 20, 2012 7:21 pm

Sorry if this was already addressed;

What are you using for intake runners and boots on this conversion?

What cable are you using? With my TM35mm Mikuni flat-slides, the bike came with some weird 1-into-4 cable setup (cylindrical aluminium "splitter" that takes the main throttle cable out to four individual cables...) Retained the stock choke setup though!

Anybody want to buy some 35mm Mikunis for a different application perhaps? The bike SCREAMS up top, but below 3000-3500 rpm there's just nothing...
Anything with an engine in it...

acarver
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#41 Post by acarver » Fri Apr 20, 2012 8:20 pm

Stock boots/reeds and Wilson Performance custom cables (similar to what you have if not the same)
Honestly, although the bike pulls cleanly off the line, these low compression 125 cylinders
will never be real strong in that idle to 3K area no matter what you do.
Andrew

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#42 Post by JungleJustice » Sat Apr 21, 2012 10:55 am

You can see the aluminum throttle splitter and cables here...

Where is that from? I need to replace the main cable now too (just broke!) :mad:

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Anything with an engine in it...

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#43 Post by aaronmvrider » Sat Apr 21, 2012 12:09 pm

wilson performance... he is a member here..

you should be able to have a new inner cable made rather than buying a new one ???
my rides
rz 500 1984
2 x rz 350 1984
mv agusta f4 1000 2004

acarver
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#44 Post by acarver » Thu Apr 26, 2012 12:27 am

Ok, some new things learned and hopefully this helps those doing this conversion.

#1 The carbs seem to run better at a 21 mm float height vs. the stock 19 mm
This came about because I was intermittantly getting a mid-range burble
and noticed when I got back from test rides, the overflow had some dirt on the tip with intermittant dribble.
Reset the float heights to 21 mm and all is good, no burble, no dribble.

#2 The UNI foam filters are sensitive to length in relation to the signal they provide to the carbs.
Also, they're really not made to be shortened because the end has thicker foam to locate the spring.
Because of this, I switched to K&N filters in the 64x49mm size with the rubber caps. (RU-2760)
These are longer and flow better than the 2" ones you see sometimes and have ~1/2 inch of clearance from the panels.
Because they flow better, you have to compensate by richening the mixture approx. 1 step on the pilots/needles and 2 steps on the mains.

#3 The N68A needle is leaner than I thought, and that's a good thing.
I also now understand why it was the PWK28 needle for the KR1.
It was a little cooler today and before switching the filters noticed a very slight lean stumble at times with the N68A on #2 although it still ran well.
#3 cleared it up while staying great in all other areas.
This is why I think the N68A is the best needle. No matter where you put it, it seems to properly correct the whole throttle and RPM range mixture.
Part of the way it does this is it affects the mixture all the way through
WOT because it is thicker at the tip and stays down in the needle jet at WOT.
What you get are very smooth transitions during throttle and RPM changes.
As a result, your 3/4-WOT is no longer just the main, but the main+the needle.
Additionally, if you look at the chart I posted above of the PWK jetting areas of jets,
I would extend the "clip position" graph out to the right like the taper because
the taper on the N68A creates a wide enough needle at the end that the clip changes all areas.

Keep in mind all this data is at ~60-70F at sea level. Adjust via the chart above accordingly.
So 45/#3 N68A/145 with the A/S 1 turn out and my jetting is done.
(would prob. go 150-155's on a track bike at sea level w/the N68A's)

#4 Bonus - Because the PWK's have smaller bodies than the stock carbs and you lose the airbox/elephant ears and the filters don't block the carbs,
you can synch and rejet the needles without removing the side panels as long as you cut your fuel lines such that you can access the slide covers.

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Last edited by acarver on Sun Dec 06, 2015 1:03 pm, edited 6 times in total.
Andrew

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#45 Post by Vin » Thu Apr 26, 2012 2:37 am

Great work Andy and thanks for sharing your findings :cool:

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