The PWK, jetting, and me
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Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 4:53 am quote
I must start by admitting I really hate jetting. It never seems to make sense.

I'm trying to get this Polini PWK 28 to play nice with the Malossi Sport 221. It does to varying degrees but I have difficulty working out whether it feels rich or lean.

I started out trying to follow this guide - http://www.shiny-red.net/guides/keihin-carburetor-jetting/ - and set the idle by the method they suggest. I ended up with a 48.
I had this carb fitted to the 177 for a bit, until the big end bearings felt they'd be happier without the restriction of the conrod. So now I've moved it to the 221. SDJohn suggested I start with a H needle (I have 3) and try the 30-hole atomiser... I thought I'd give it a go and see how it felt. It had a 122 main jet in there from the 177, it actually ran pretty well except for a patch in the 4-5000 area where it gurgled a bit. A 5 kilometre 100km/h run showed temps around 320f near the end, maybe they would keep climbing if I went further? Also I feel like 122 is pretty small, when I had an SI fitted to the same motor I was running a 155.

I looked for a richer main and the only one I had with me at the time was a 132 so i gave it a go. Quite similar but the gurgling area was now much broader.
I took it home and tried the 20-hole atomiser. Felt better at very low revs but very choppy until it gets up to the main.

So you can see I'm all over the place. I did follow SDJohn's thread (all 30-something pages) but the jetting advice never made much sense to me. I feel like what I need is a system. The link above has suggestions for individual carb bits, but the whole thing is based on the premise that the carb you buy is already pretty close to right.

What would be really nice is a guide that takes it step-by-step, but complete so you know where you're heading.

keihin pwk chart.jpg

Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4599
Location: San Diego, CA
Tue Sep 11, 2018 6:52 am quote
Here is my summary of the 30 pages

1) pick atomizer
2) set pilot jet to be kind of lean (slower return to idle)
3) move through needle diameters with huge main or no main (170 maybe) and test 1/4 throttle temps only, looking to stay below 300 on the plug on a long run
4) once you like (3) test 1/8 throttle temps for the same. If change is required here it may be a slide
4) if you've got (3) and (4) then you can fuss about with taper and main. they affect each other but you've got to start with such a big main it can't rev out. drop it in steps of 2 jets (4 or 5 numbers) until it can rev out. if you can't get past 1/2-3/4 throttle then you're probably too far off on taper, go leaner and try again with the main starting at the top
5) after engine can rev out only use small steps (one jet size) on the main. 1/2 throttle temps must stay below 300 still
6) Clip can be used to get the 3/8 - 1/2 area fine tuned but extends a bit further, so watch out if you move it. Use 1/2 clip washer to move small amounts at a time.

If you want to learn the difference of feel of rich to lean, put in a huge ass main and try to drive. I agree though that once you are into fine tuning it can be hard to figure which way to go, but if in doubt go bigger and see if it goes worse or better for you.

The key to this process is sorting all the low throttle stuff and leaving enough margin to be able to tune the high throttle stuff without causing danger when you roll off the throttle.
Ossessionato
76 Sprint V, 63 GL, 62 VBB, 05 Stella
Joined: 16 Jun 2011
Posts: 3126
Location: Nashville
Tue Sep 11, 2018 7:11 am quote
I just tried to do PWK tuning with no main jet and could never get it to work.

I've come to think of jetting as being similar to orienteering: Make sure you're off from your target one way or another, then work your way back to the goal. For jeting, I always go rich, then dial it down.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5754
Location: So Cal
Tue Sep 11, 2018 8:43 am quote
I just throw darts at a chart
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 1:21 pm quote
sdjohn wrote:
Here is my summary of the 30 pages

The key to this process is sorting all the low throttle stuff and leaving enough margin to be able to tune the high throttle stuff without causing danger when you roll off the throttle.
That's excellent John. It does give me a clear path that I hadn't seen.

An importatnt question though - as I did mark my throttle and try some 1/4 and 1/8th throttle runs - how hard are you running it during the test? Is it important to be pushing it with high revs in 2nd? Whichever gear I tried it in the temps were low (<220 approx?), and it did not run smoothly at all.
SoCalGuy wrote:
I just throw darts at a chart
This is my method as well. However the chart is in the next room so you have to get very lucky...
Ossessionato
2015 GTS300, 1974 Primavera, 04 Ninja 250
Joined: 04 Apr 2013
Posts: 4599
Location: San Diego, CA
Tue Sep 11, 2018 2:45 pm quote
Yeah sometimes depending on the atomizer, you will need more RPM just to run it cleanly at 1/4, which depends a lot on how big you made your big main.

That's why I like to do those at about 170 on my machine, because the jetting seems to keep landing in the high 150's to low 160's. This way I'm definitely rich but not crazy rich so that I can't drive.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 2:49 pm quote
So it has to run cleanly? And which gear do you use?
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5754
Location: So Cal
Tue Sep 11, 2018 2:52 pm quote
If the darts don’t work, here’s what I do:

1. Get the pilot right. It’s easiest.
Go with the leanest jet that responds to turns of the mixture screw at idle. If turning the mix screw all the way in doesn’t cause the idle to noticeably bog, go with a leaner richer pilot. If turning the mix screw out more than three turns doesn’t increase revs at idle, go with a leaner pilot. Done.

2. Clip the needle 3 down.

3. Put a huge main in and go for spin. Wring it out in every gear, watching throttle marks and temps.

5. Note where in the throttle it bogs/stutters/misses/ or runs rough. Also note temps.

6. Bogging or missing + high temp = jetting’s too lean. Bogging or missing + ok temp = jetting’s too rich.

7. 0-1/8 throttle = adjust pilot or tweak mix screw. 1/4-1/2 = adjust needle or clip. 3/4-WOT = adjust main.

This is the dummie’s guide I go by. Thereafter you can go round and round and round and round with endless tweaking as desired.

In the interest of full disclosure, none of my kitted scoots runs “perfectly”. However, all move respectably and none have ever seized (knocking on wood).

So there you go ... probably nothing here you didn’t already know, but maybe someone may find it useful.

Last edited by SoCalGuy on Tue Sep 11, 2018 7:44 pm; edited 4 times in total
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 7:02 pm quote
SoCalGuy wrote:
So there you go ... probably nothing here you didn’t already know, but maybe someone may find it useful.
Please... when you're talking to me, never assume I already know something!

I like the sound of this system, just a couple of questions.

What's the idea of the lean pilot? It's in yours and John's. And you change it afterwards, right?

How long does the wringing out through the gears last? I wouldn't expect to see much in the way of temp changes in such a short time.

Thanks for your advice SCG!
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5754
Location: So Cal
Tue Sep 11, 2018 7:34 pm quote
No sweat Ginch. Sorry if I wasn’t clear. I’m not really aiming for a lean pilot. I edited and corrected my post. The point is to find a pilot that responds well to the mixture screw. A little lean is ok because idle can be enrichened with the mix screw. A slightly lean pilot also makes adjusting the mid-throttle range easier because it minimizes bleed over.

As for wringing it, the engine has to be fully warmed up to get an accurate feel for jetting changes. So when I do jetting I run the scoot for of a couple miles at least. Don’t just go down the block, and don’t just putt around. Do some “real world” riding.

Last edited by SoCalGuy on Tue Sep 11, 2018 8:42 pm; edited 2 times in total
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Tue Sep 11, 2018 8:36 pm quote
sdjohn wrote:
Here is my summary of the 30 pages

1) pick atomizer
2) set pilot jet to be kind of lean (slower return to idle)
3) move through needle diameters with huge main or no main (170 maybe) and test 1/4 throttle temps only, looking to stay below 300 on the plug on a long run
4) once you like (3) test 1/8 throttle temps for the same. If change is required here it may be a slide
4) if you've got (3) and (4) then you can fuss about with taper and main. they affect each other but you've got to start with such a big main it can't rev out. drop it in steps of 2 jets (4 or 5 numbers) until it can rev out. if you can't get past 1/2-3/4 throttle then you're probably too far off on taper, go leaner and try again with the main starting at the top
5) after engine can rev out only use small steps (one jet size) on the main. 1/2 throttle temps must stay below 300 still
6) Clip can be used to get the 3/8 - 1/2 area fine tuned but extends a bit further, so watch out if you move it. Use 1/2 clip washer to move small amounts at a time.

If you want to learn the difference of feel of rich to lean, put in a huge ass main and try to drive. I agree though that once you are into fine tuning it can be hard to figure which way to go, but if in doubt go bigger and see if it goes worse or better for you.

The key to this process is sorting all the low throttle stuff and leaving enough margin to be able to tune the high throttle stuff without causing danger when you roll off the throttle.
Seems so easy when written as steps.

Here's a few pointers; The PWK atomiser is selected at 3/4 throttle and adjusted to the needle fitted.
PWK air screws are 1.5 turns for the correct pilot jet. Accurate setting needs full engine temperature, so stop and adjust while out and about.
Revving up on the stand in the garage can get you a long way to decent jetting before even going out on the road. If you don't have neighbours.

Where is your timing set to?
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 9:49 pm quote
Jack221 wrote:
Seems so easy when written as steps.

Here's a few pointers; The PWK atomiser is selected at 3/4 throttle and adjusted to the needle fitted.
So here's where the trickiness steps back in... John says pick an atomiser and Jack says select it at 3/4 throttle. Can you please enter that process into John's steps?
Jack221 wrote:
PWK air screws are 1.5 turns for the correct pilot jet. Accurate setting needs full engine temperature, so stop and adjust while out and about.
Revving up on the stand in the garage can get you a long way to decent jetting before even going out on the road. If you don't have neighbours.

Where is your timing set to?
No real concerns about revving up on the stand, how does that one work?

17-18
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 10:39 pm quote
SoCalGuy wrote:
No sweat Ginch. Sorry if I wasn’t clear. I’m not really aiming for a lean pilot. I edited and corrected my post. The point is to find a pilot that responds well to the mixture screw. A little lean is ok because idle can be enrichened with the mix screw. A slightly lean pilot also makes adjusting the mid-throttle range easier because it minimizes bleed over.

As for wringing it, the engine has to be fully warmed up to get an accurate feel for jetting changes. So when I do jetting I run the scoot for of a couple miles at least. Don’t just go down the block, and don’t just putt around. Do some “real world” riding.
Thanks for the explanation Mike. As I said, you can never assume I know what I'm doing!
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Tue Sep 11, 2018 11:25 pm quote
Ginch wrote:
So here's where the trickiness steps back in... John says pick an atomiser and Jack says select it at 3/4 throttle. Can you please enter that process into John's steps?

No real concerns about revving up on the stand, how does that one work?

17-18
It is tricky. You have to pick an atomiser to start but deciding if it is the right size or not requires working through to step 4. Then when everything previously is all set and running sweet, check the 3/4. If there is any issue at 3/4 that can't be sorted with the main jet while keeping WOT ok. Then back to step 1 and pick another atomiser (or needle taper), which messes up everything and start jetting from the beginning.

By doing a lot of the stages by holding the throttle open to position in the garage can get pretty close without actually doing any riding. If ever in the pits at 2 stroke bike racing it is all you hear. revving and jetting.

Timing probably ok. How tight is your squish?
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Tue Sep 11, 2018 11:54 pm quote
Jack221 wrote:
Ginch wrote:
So here's where the trickiness steps back in... John says pick an atomiser and Jack says select it at 3/4 throttle. Can you please enter that process into John's steps?

No real concerns about revving up on the stand, how does that one work?

17-18
It is tricky. You have to pick an atomiser to start but deciding if it is the right size or not requires working through to step 4. Then when everything previously is all set and running sweet, check the 3/4. If there is any issue at 3/4 that can't be sorted with the main jet while keeping WOT ok. Then back to step 1 and pick another atomiser (or needle taper), which messes up everything and start jetting from the beginning.

By doing a lot of the stages by holding the throttle open to position in the garage can get pretty close without actually doing any riding. If ever in the pits at 2 stroke bike racing it is all you hear. revving and jetting.

Timing probably ok. How tight is your squish?
Alright, so:


1) pick atomizer
2) set pilot jet to be kind of lean (slower return to idle)
3) move through needle diameters with huge main or no main (170 maybe) and test 1/4 throttle temps only, looking to stay below 300 on the plug on a long run
4) once you like (3) test 1/8 throttle temps for the same. If change is required here it may be a slide
4a) if you've got (3) and (4) then you can fuss about with taper and main. they affect each other but you've got to start with such a big main it can't rev out. drop it in steps of 2 jets (4 or 5 numbers) until it can rev out. if you can't get past 1/2-3/4 throttle then you're probably too far off on taper, go leaner and try again with the main starting at the top

4b) Then when everything is all set and running sweet, check the 3/4. If there is any issue at 3/4 that can't be sorted with the main jet while keeping WOT ok. Then back to step 1 and pick another atomiser (or needle taper), and start jetting from the beginning.

5) after engine can rev out only use small steps (one jet size) on the main. 1/2 throttle temps must stay below 300 still
6) Clip can be used to get the 3/8 - 1/2 area fine tuned but extends a bit further, so watch out if you move it. Use 1/2 clip washer to move small amounts at a time.


Is this correct now?

Could you clarify this bit? "...any issue at 3/4 that can't be sorted with the main jet while keeping WOT ok" Not sure if that refers to holding it at 3/4, or, as you go past 3/4 to get to wot.


On the stand, are you suggesting that you can check your temps while holding it at say, 1/4 throttle? How long roughly would you run it to get a useful figure?
Jet Eye Master
PX221 Malossi, O tuned PX200 and some motorbikes
Joined: 14 Jun 2017
Posts: 2705
Location: London UK
Wed Sep 12, 2018 2:18 am quote
The only adjustment for 3/4 throttle without affecting the whole carb is to fudge the main jet. And fit one that allows 3/4 and WOT to both be near ok. If there is no compromise with the main jet then changing the taper will mess up 1/4, 1/2, 3/4 and WOT and all will need attention. If a slightly different diameter or Atomiser is needed for the taper to be better, even the pilot jet will be affected ie. start again from near zero.

On the stand you can just feel how it revs and how high and make a rich or lean assessment. Often how the revs fall to a throttle position is a better indicator of how that position is working. Similar as on the road. Takes me ages. Rough jetting in an afternoon and fine jetting in months is usual.
bodgemaster
63 GL, 76 Super (x 2), 74 Primavera (x 2), 06 Fly 150
Joined: 26 Sep 2013
Posts: 5754
Location: So Cal
Wed Sep 12, 2018 6:05 am quote
Ginch, except for idle, you can’t really trust engine temps on the stand. The bike has to be under load IMO.

You don’t need to be in 4th gear to test 3/4 to WOT. I use 3rd. Find a flat stretch of road, ride at 3/4 throttle for a 1/2 mile or so, note temp, then take it to WOT. Engine should keep pulling and the temp shouldn’t shoot way up.

If it doesn’t pull and stumbles, but temp stays about the same, main is rich. If it doesn’t pull and temp goes up, main is lean.

Jack is the tuning guru so feel free to ignore everything I’m saying. But trust me on this - you’ll spend inordinate amounts of time in the garage and drain your wallet rapidly if you dick around too much with atomizers and needle tapers trying to squeeze the last 1% of performance out of your carb.

Use common sense. If the engine sounds good, feels good, revs easily, accelerates reasonably smoothly all the way to WOT, and doesn’t heat up ... call it good! Your sanity will thank you.
Veni, Vidi, Posti
74 Super, 75 Super, PX project, LML off-roader and new to 2018, '66 Blue Badge Smallframe
Joined: 30 Nov 2011
Posts: 8066
Location: Victoria, Australia
Wed Sep 12, 2018 4:44 pm quote
Thanks guys, very much appreciated. I have enough info to go on and make informed decisions now. Cheers!
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