PWK33 Slide Check

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Smoker
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PWK33 Slide Check

#1 Post by Smoker » Sat Jan 12, 2019 2:52 pm

ATM, I have 2 sets of carbs to use. PWK 28s and PWK 33s. Will probably use the 33s, not certain, yet. But, I will use the 33s for one of my bikes.

A friend [RS Raz], modifying a YZ, posted some interesting info. Will check my carbs for this. I doubt it'll reduce performance if the slide is above the bore at full throttle... but, it could disrupt airflow. This is his mod to get rid of the excess throttle travel:

"Just something to watch out for if you're using or plan to use a Keihin PWK33 (or possibly any carb which shares the same body as a larger bored variant).

The PWK 33mm that I'm using is just a 35mm PWK with a smaller bore drilled offset:

Imagenot my image


Which is fine coz it means all the parts from the 35mm fit it and vice versa but I noticed that when I open the throttle right up the slide still has 35mm of travel! So 2mm of travel is just wasted. The slide could be the wrong one but slides are expensive and this one works fine. I thought maybe there's a spacer or a different cap or something that my carb is missing but I can't find any info on that so I measured the recess in the cap:

Image

(32 OD, 13 ID, 2 deep)

The slide rests on the outer lip of that recess so I had a spacer made up which sits in the recess and protrudes by 2mm so that it limits the travel of the slide:

Image

Image

Image


And now at full throttle, the slide is flush with the top of the bore:"

Image

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Smoker
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Re: PWK33 Slide Check

#2 Post by Smoker » Sat Jan 12, 2019 2:56 pm

Busa1300 brought up a point about the slide mod.

At WOT, will performance be affected by the needle being 2mm lower?

Interesting question.

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Whymee
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Re: PWK33 Slide Check

#3 Post by Whymee » Sat Jan 12, 2019 3:23 pm

AS long as both slides are in the same position at WOT (or any other position) there should not be an issue. With the slides in the same position relative to throttle position, both needles should be the same height in the needle jets.
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Smoker
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Re: PWK33 Slide Check

#4 Post by Smoker » Sat Jan 12, 2019 3:59 pm

Whymee wrote:
Sat Jan 12, 2019 3:23 pm
AS long as both slides are in the same position at WOT (or any other position) there should not be an issue. With the slides in the same position relative to throttle position, both needles should be the same height in the needle jets.
I'm assuming that both slides will be adjusted/sync'd to an identical height.

After the mod, at WOT, the slide and needle will be 2mm lower. The slide will clear the bore, but how will carburetion be affected by the change in needle height at WOT?

Trying to figure out if this is a good mod, or not.

Needle position could also be altered. Just don't know enough about carbs to figure this out on my own. [smilie=dunno.gif]

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Re: PWK33 Slide Check

#5 Post by brrrappp » Sat Jan 12, 2019 5:04 pm

Smoker wrote:
Sat Jan 12, 2019 3:59 pm
Whymee wrote:
Sat Jan 12, 2019 3:23 pm
AS long as both slides are in the same position at WOT (or any other position) there should not be an issue. With the slides in the same position relative to throttle position, both needles should be the same height in the needle jets.
I'm assuming that both slides will be adjusted/sync'd to an identical height.

After the mod, at WOT, the slide and needle will be 2mm lower. The slide will clear the bore, but how will carburetion be affected by the change in needle height at WOT?

Trying to figure out if this is a good mod, or not.

Needle position could also be altered. Just don't know enough about carbs to figure this out on my own. [smilie=dunno.gif]
Looks to me like your assumption of needle position at WOT is in fact a concern especially if the needle taper is still progressing at the lower section.
Another concern I would have is does the throttle cap thread down completely? If that spacer is causing the cap to ride slightly higher, using fewer threads I would get nervous.
I had a YZ490 that the screw-on top popped off and sent me off a jump and into the woods at Full throttle. On my 490 it was a defect. It wasn't for lack of tightness- the threads just were not deep enough and vibration apparently took it's toll.
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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Smoker
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Re: PWK33 Slide Check

#6 Post by Smoker » Sat Jan 12, 2019 6:03 pm

brrrappp wrote:
Sat Jan 12, 2019 5:04 pm
Looks to me like your assumption of needle position at WOT is in fact a concern especially if the needle taper is still progressing at the lower section.
No assumption, just a question.

I've read that clip position influences 1/8 to 3/4 throttle opening, but my knowledge of carb tuning is limited. Someone more experienced than myself asked the question and I'm following up because I have a set of these exact carbs.

Are you saying that it will not be detrimental to performance, as long as the cap is still fully threaded on and tight?

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Re: PWK33 Slide Check

#7 Post by MK » Sat Jan 12, 2019 6:36 pm

The RGV carbs have the same issue and reducing max. lift adds top end performance.
Bye
Martin

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Re: PWK33 Slide Check

#8 Post by brrrappp » Sat Jan 12, 2019 7:28 pm

Smoker said "Are you saying that it will not be detrimental to performance, as long as the cap is still fully threaded on and tight?"

Two different issues. The cap-to-carb body thread issue only comes to mind for me because I had a YZ490 where the threads for the cap didn't have enough bite and the cap came off allowing the throttle to be stuck wide open.

As far as the needle position- yes adjusting needle position up/down generally only affects mid-range. That's because the further up the slide needle travels the less ability the needle has to restrict fuel flow coming up through the jet combined with the taper on the needle. Your spacer doesn't change the low to mid-throttle position so that wont change and Lowering the needle by raising the clip will mess up the mid range jetting.

However, since the needle will now sit 2mm lower at full throttle, it could have some influence on how much fuel can get by the needle at WOT leading to a lean condition if you were to be WOT for long enough to matter.
I think you would need to be racing or doing a top speed run to notice or seize. This would be a big deal on a snowmobile because they run at WOT a lot.
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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Re: PWK33 Slide Check

#9 Post by Smoker » Sat Jan 12, 2019 9:46 pm

brrrappp wrote:
Sat Jan 12, 2019 7:28 pm
However, since the needle will now sit 2mm lower at full throttle, it could have some influence on how much fuel can get by the needle at WOT leading to a lean condition if you were to be WOT for long enough to matter.
I think you would need to be racing or doing a top speed run to notice or seize. This would be a big deal on a snowmobile because they run at WOT a lot.
This is the question I'm asking.

I always keep the carb caps tight. :smt023

I'll test one of the carbs with a spacer and see if the cap seats all the way.

How does time make a difference if the needle has no effect at WOT?

I like WOT. Mostly, in a straight line.
And burnouts. Need a lot of WOT for burnouts.

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Re: PWK33 Slide Check

#10 Post by brrrappp » Sat Jan 12, 2019 11:42 pm

Smoker wrote: "How does time make a difference if the needle has no effect at WOT?"

The needle may have some impact at WOT IF it does not come all the way up at Full Throttle. The needle adjustment affects mid-throttle but because it is going to be 2mm lower at WOT it may restrict the normal fuel flow at WOT by not being fully extended upward.

An engine can withstand short bursts of lean condition without harm. In fact, the electronic power-jet on my 2-stroke KX250 actually shuts down at WOT+Hi-RPM to allow higher RPM (over-rev). The idea in MX racing, where you have short straights, is that sometimes it's faster to just over-rev than to up-shift. This is because if you upshift, you immediately need to brake and downshift for the next turn.
So, WOT with over-rev, even though you are well past the power peak, is still a few seconds faster per lap than up-shifting and being properly in the power-band.

If you run WOT for more than a few seconds with a lean condition, you can and probably will over heat and seize.

If the spacer you made doesn't allow the throttle cap to thread down fully onto the carb, you might consider using a stop screw at the top of the rectangular section of the slide.
Last edited by brrrappp on Sun Jan 13, 2019 12:21 am, edited 1 time in total.
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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Re: PWK33 Slide Check

#11 Post by brrrappp » Sun Jan 13, 2019 12:11 am

Here is a cut-away view of a generic carb. You can use your imagination to think of the needle traveling up/down in the needle-jet tube. Now try to imagine if the needle being 2mm lower at WTO would possibly restrict the normal WTO fuel flow. This is all based on a guess that the carb was in fact designed to have the slide move up as high as it does. I could be wrong- I obviously did not design the Mikuni Carb.

carbseethrough.jpg
carbseethrough.jpg (82.81 KiB) Viewed 116 times
A modest "high" can be obtained by listening carefully to the sound of a 2-stroke hitting the fat part of the powerband while inhaling cool fall air tainted by the exhaust scent of VP-C12 and Golden Spectro, mixed at 24:1.

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Re: PWK33 Slide Check

#12 Post by Smoker » Sun Jan 13, 2019 6:30 pm

MK wrote:
Sat Jan 12, 2019 6:36 pm
The RGV carbs have the same issue and reducing max. lift adds top end performance.
Thanks, Martin. :smt006

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Re: PWK33 Slide Check

#13 Post by JanBros » Sun Jan 13, 2019 8:10 pm

mount everything without the spacer, pull the slide all the way up and mark the needle.
do it again with the spacer.
now measure the different in needle diameter at the 2 marked points. If they are the same, you have nothing to worry ;-)
if they are different, it depends on how different. The question you need to ask then is : is the flow-through surface (surface needle-tube minus surface needle diameter at the mark) bigger than the flow-through surface of the main. If it is for both positions, the main is already the determining factor and the needle has no influence in both positions.
If the flow-through surface in the 2mm lower position is smaller than the main, and the all-the-way- up position is bigger than the main, you should put in a slightly bigger main.
if it runs, you can race it !

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Re: PWK33 Slide Check

#14 Post by Smoker » Mon Jan 14, 2019 1:19 am

Thanks Jan, for the great explanation!

I'll get out the carb and try it out. Guessing the mark on the needle should go as far down as I can get.

Will be interesting to see if it correlates with the following.

Info from Raz:
Each clip position is 0.9mm apart. So I put the numbers into kyajet.de and calculated the jetting as if the needle is 2 clip positions lower (so 1.8mm - close enough to my 2mm) and it looks as though the main jet will be about 1.5% leaner at full throttle.

Image

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