LCGB Forums

The ability to post messages is restricted to LCGB members. Any questions contact us at lcgbadmin@googlemail.com

Polini 34mm (Keihin PWK)... Jetting?

Want to know which carb to choose? Having problems setting up the jetting details on your scooter? Wheter you are using a standard or tuned carb, ask your question here.

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Tue May 26, 2020 1:41 pm

Andy_S wrote:This is from the Polini CP Carb instruction page, which I presume will also apply to the PWK type.

The atomizer is a tube that connects the main jet to the venturi and it is called “two-stroke type” when it doesn’t have any hole. Otherwise if it has a series of holes along its entire length it is called “four-stroke type”.(Attention: to call the atomizers “2 or 4 stroke” is popular but technically incorrect because you can safely use atomizers with holes for the 2 stroke too. I.e the Minibikes have atomizers with “4 stroke” type holes). Its working is very easy: due to the depression in the venturi then, from the atomizer tube the liquid fuel is drawn, metered by the main jet and by the conical needle, while a certain airflow comes from the channel. Air and fuel mix together and the result is a pulverized spray sucked by the engine. To intervene in this process, in addition to the atomizer hole, there are the air channel diameter, the height of the part of the atomizer protruding into the chamber and the height of the step located in the nozzle. In addition to the atomizer’s hole diameter, the variables are therefore the diameter of the air channel, the height of the atomizer’s side that protrudes in the chamber and the “step” of the delivery nozzle. The “four stroke” type produces leaner mixture and more controlled; in fact since it is equipped with a series of holes the emulsion is done inside the tube, before the mixture reaches the nozzle in the venture. Obviously the arrangement of the holes and their diameter influences the delivery. Holes machined in the lower part of the atomizer are bathed in the fuel of the bowl so, by increasing the numbers of holes the flow of fuel increases and goes to emulsify itself with the air. The holes in the upper part of the atomizer are exposed to air and by increasing them you obtain weaker carburation at low rpm.
The POLINI CP carburetor can be adjusted so that it works with both the types, 2 or 4 stroke, changing the brake air jet.

Close air brake jet: 2 stroke atomizer type working as an atomizer without holes
Open air brake jet: the atomizer holes work and make the engine weaker.

URL for the page this was taken from - https://www.polini.com/en/technical-are ... tructions/


I have read similar information produced by other manufacturers, regardless of the type of carburettor.

The manufacturers have the knowledge & resources to make virtually any of their range of products work on a two or four stroke. In fact, Dell''Orto refer to the 'step' (in Polini speak) as being the norm for two strokes (a fact I'm certain we can all agree with) yet acknowledging that some racing two strokes may not have them fitted. Interestingly, as far as I am aware, the PWK's that we discuss here have no needle shielding in the venturi, yet the larger PWK's fitted as OEM to 'crossers do. I am not about to even try to question that, but I would vouch for the clean carburation in any case of the PWK's we discuss here to suggest that they probably do not need it. That conclusion is drawn by the fact that I have a converted VHB Dell''Orto that has a two stroke conversion via the PH needles, atomisers & venturi needle shield. IMHO, no doubt due to the flat guillotine slide, it works better than the round slide product that donated the bits (I hope that one day there will be a market for converted VHB carbs! Ha!)

The point I'm trying to make, is that given unlimited resources & a product with two stroke 'friendly' slides, then a good combination of atomiser, needle, slide cutaway & body insert could be found. Let's not forget the effect of atomisation upon the fuel air mix in a two stroke, which gains larger surface areas as the revs rise, resulting in the need for less ignition advance. What will occur if that efficiency were to be increased via 'extra' atomisation of the mix down the scale? Further retardation @ low revs?

Personally, I can't see much point in going down that route with the PWK's that I rate highly enough to work with what I have. The only augmentation likely is the adjustable power jet that I have grafted on. Currently it adorns a reed valve engine so can wait until it is back on a piston port engine.
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Sat Jun 06, 2020 11:24 pm

Right, atomisers turned up today. I've not had chance to do anything with them but now the tests can start...
Adam_Winstone
 
Posts: 966
Joined: Tue Jun 03, 2014 1:52 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Tue Jun 09, 2020 11:10 pm

8-hole marginally better with H needles but I believe that G needles may be where I go next.
Adam_Winstone
 
Posts: 966
Joined: Tue Jun 03, 2014 1:52 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Sun Jun 14, 2020 5:44 pm

I was hoping that the G needle would have arrived before the weekend, however, the H needles + 8-hole atomiser have certainly helped. Sorry that I've not got more to report at the moment but the bike is certainly becoming more rideable.

Cheers for now.
Adam_Winstone
 
Posts: 966
Joined: Tue Jun 03, 2014 1:52 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Sun Jun 21, 2020 10:38 am

Right I’ve bitten the bullet( hopefully not my arse again) and bought a cheap Chinatown pwk copy for the princely sum of £24.I’m gonna blue print it with advice taken from here but where can I purchase needles from? Only bits that are marked are the main n idle jets. It is certainly a weird carb on breaking it down for inspection :shock: oh it’s a 28 power jet model :D
User avatar
ULC Soulagent
 
Posts: 1345
Joined: Sun Apr 03, 2016 3:26 pm
Location: Milton Keynes

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Sun Jun 21, 2020 2:05 pm

Is it for a piston ported engine? It's just that I bet the power jet has no reference to a size! That's why I found a needle valve to adapt to an adjustable power jet, but it's turned off ATM whilst fitted to a TS.

As for needles, I bought some @ about 30 bob each from Trials UK (Sherco OEM spare parts) in 2018 but they may have cottoned on to their costs by now! Worth a browse but the website takes some manoeuvring.......

It's definitely worth shopping around, though the JJH seems to be a universal base line set up. I have accumulated five or six spare, so happy to lend out to 'try before you buy' if the costs are steep nowadays. I also have other needles, but can only find JFH's ATM.

Worthy of note is that the choke compatibility with Dell'Orto. My notes are a bit vague, but the plunger is definitely the same size.
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Sun Jun 21, 2020 3:57 pm

Warkton Tornado No.1 wrote:Is it for a piston ported engine? It's just that I bet the power jet has no reference to a size! That's why I found a needle valve to adapt to an adjustable power jet, but it's turned off ATM whilst fitted to a TS.

As for needles, I bought some @ about 30 bob each from Trials UK (Sherco OEM spare parts) in 2018 but they may have cottoned on to their costs by now! Worth a browse but the website takes some manoeuvring.......

It's definitely worth shopping around, though the JJH seems to be a universal base line set up. I have accumulated five or six spare, so happy to lend out to 'try before you buy' if the costs are steep nowadays. I also have other needles, but can only find JFH's ATM.

Worthy of note is that the choke compatibility with Dell'Orto. My notes are a bit vague, but the plunger is definitely the same size.

Gonna try it on my Mb tuned Ts1, boysen reeds, modified Bgm V3 pipe
I’ve also managed too get a proper Keihin Pwk 28/29mm on a straight swap with a good friend for one of my 30mm Delly
User avatar
ULC Soulagent
 
Posts: 1345
Joined: Sun Apr 03, 2016 3:26 pm
Location: Milton Keynes

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Sun Jun 21, 2020 5:08 pm

ULC Soulagent wrote:
Warkton Tornado No.1 wrote:Is it for a piston ported engine? It's just that I bet the power jet has no reference to a size! That's why I found a needle valve to adapt to an adjustable power jet, but it's turned off ATM whilst fitted to a TS.

As for needles, I bought some @ about 30 bob each from Trials UK (Sherco OEM spare parts) in 2018 but they may have cottoned on to their costs by now! Worth a browse but the website takes some manoeuvring.......

It's definitely worth shopping around, though the JJH seems to be a universal base line set up. I have accumulated five or six spare, so happy to lend out to 'try before you buy' if the costs are steep nowadays. I also have other needles, but can only find JFH's ATM.

Worthy of note is that the choke compatibility with Dell'Orto. My notes are a bit vague, but the plunger is definitely the same size.

Gonna try it on my Mb tuned Ts1, boysen reeds, modified Bgm V3 pipe
I’ve also managed too get a proper Keihin Pwk 28/29mm on a straight swap with a good friend for one of my 30mm Delly


Do you have base line settings to use?

Before I make any suggestions (as per ported TS 70 x 60 through foam filter 34 mm choke & an expansion chamber) I just wanted to know if you had anything in mind :D
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Mon Jun 22, 2020 5:51 am

No base settings as of yet,
Full state of play will be
Ts1 230 mb stage 3 tuned
That carb with filter
Modified Bgm V3 ( 25 mm diameter exit pipe)
Mb5 speed (17/46)
Just want a fast touring engine :lol:
User avatar
ULC Soulagent
 
Posts: 1345
Joined: Sun Apr 03, 2016 3:26 pm
Location: Milton Keynes

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Mon Jun 22, 2020 12:05 pm

ULC Soulagent wrote:No base settings as of yet,
Full state of play will be
Ts1 230 mb stage 3 tuned
That carb with filter
Modified Bgm V3 ( 25 mm diameter exit pipe)
Mb5 speed (17/46)
Just want a fast touring engine :lol:


Reed valve engines will need a bigger pilot jet than piston ported where we have gone down to 32. So a 50 would be my initial choice.

Main jet around about 150.

(You may well come down five numbers on both those jet sizes)

Power jet de-activated (for the time being @ least. Perhaps worth playing with later, but there seems scant information on them being used, let alone sizes used (!) when I have posted the question on 'our' Forums :roll: )

The ubiquitous JJH needle.

Plus all the other modifications & checks: float bowl modification for removal of internal float stops & slotting of drain plug hole for pilot jet access (you can go fairly big as the plug has an 'O' ring seal); float positioning relative to correct fuel level; any drill ways modified to optimum size in the main body (you might need Torx bits to do a proper job); worth fitting an in-line fuel filter to any carburettor; you might consider my own TightWad modification of centre drilling the drain plug & fitting a 3 mm pipe drain valve (pennies from China) just to save ruining your Gucci gloves...... :D
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Mon Jun 22, 2020 1:42 pm

Cheers WK1,
I’ll get those done in due course, no rush as we’re not riding very far these days :lol:
Slotting ? Is that the long brass pipe sticking into the float bowl?
User avatar
ULC Soulagent
 
Posts: 1345
Joined: Sun Apr 03, 2016 3:26 pm
Location: Milton Keynes

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Mon Jun 22, 2020 1:48 pm

No. The slotting of the drain plug hole in the bowl enables easy removal of the pilots with a screwdriver. A round file or Dremel type tool will see to it.

Thus, the bowl itself need never be removed again with no risk of disturbing "the long brass pipe sticking into the float bowl"
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Mon Jun 22, 2020 1:51 pm

Ahh ok ;)
User avatar
ULC Soulagent
 
Posts: 1345
Joined: Sun Apr 03, 2016 3:26 pm
Location: Milton Keynes

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby a.lo » Wed Jun 24, 2020 10:41 am

While on the subject of atomisers another variable is the height into the venturi, which I have seen mentioned as a difference between 2 and 4 stroke types (not so sure) the Dellorto tuning manual fig 23. (sorry cannot work out a link). says it is an enrichment device, this part on these PWK is separate from the atomiser sometimes called a needle jet. while trying to sort out some low down issues I thought this may be worn so thought it would be good to change. they are only one size so thought the one in a rebuild kit I had would do. wrong! found it was too short so the atomiser did not push it home. I proceeded to remove the ones from my other sized carbs and found them all to be different heights and varied in the projection into venturi
IMG_2047.JPG
IMG_2048.JPG
these are all aligned to the top lip which determines the projection into carb although the internal machining seems to vary from carb to carb i have ended up using the one on the right which came with the original 28/30 in my 26mm with good results as u can see it is adustable with a removable ring, god knows how i would get another! so any replacement could be any variation of these good luck!
You do not have the required permissions to view the files attached to this post.
a.lo
 
Posts: 102
Joined: Sat May 31, 2014 3:56 pm

Re: Polini 34mm (Keihin PWK)... Jetting

Postby Warkton Tornado No.1 » Wed Jun 24, 2020 11:33 am

Ah yes. The Dell'Orto tuning manual.

It mentions what I suppose is the 'nozzle' that tends to signify the fundamental obvious difference between their own two & four stroke products. If my memory is correct, there is also mention that there may be no such requirement for the nozzle projection in a racing two stroke.

The thing is, with the device (nozzle or whatever) in place in Dell'Orto's, it has it's open side toward the engine, acting somewhat like a windbreak on a beach. Do you remember beaches in this current lock down? :roll:

So, in the case of a Dell'Orto two stroke carburettor, the device is a shield like a windbreak to suit the two stroke.

However, in the case of the PWK's we are discussing here, there are no such shields fitted, but it appears variable height nozzles may be fitted/available.

TBH until now, I was unaware of the existence of any form of choice.

I guess 'we' need to find out more, because the nozzles seem distinctly different to Dell'Orto shields. Saying that, the Keihin 39 mm PWK I have, does have a conventional two stroke shield.

It would be great if somebody has the further knowledge to clarify selection of these nozzles & their availability. :D
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Wed Jun 24, 2020 11:28 pm

I've been looking at these different length shields too as nothing I do allows my 34mm to run cleanly enough. I'm currently considering these as well as drilling atomiser side holes (gone back from 8 hole to 30 hole) and a change to the 66mm long *48 needle series and then re'profiling needle to suit.

And I thought this would be easy!
Adam_Winstone
 
Posts: 966
Joined: Tue Jun 03, 2014 1:52 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Thu Jun 25, 2020 9:21 am

You have to wonder why Polini's version of the PWK seems to be so difficult to set up, whereas the KOSO, OKO & other versions seem comparatively easy :?:
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Thu Jun 25, 2020 3:32 pm

... constantly!
Adam_Winstone
 
Posts: 966
Joined: Tue Jun 03, 2014 1:52 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby SmudgeDog » Tue Jul 28, 2020 8:53 pm

So far for me these cheap PWks are a joy to work with but obviously I am no expert. It just seems like follow the OKO jetting instructions and you'll get there though I am still in the testing phase of my build. Powerjets are additive and usually they are set to operate at 7/8th of WOT I've read. You can though add more than 1 powerjet by drilling and placing the pick up deeper into the venturi. IE, PJ 1 is close enough to the centre of the venturi to start delivering extra fuel at 60% WOT and PJ2 is nearer the outside edge and delivers at 85% maybe. Figures just for an example, obviously not tested.
I've just read this stuff on non scooter tuning sites but struggled to find info on powerjets...mainly the size. So I have read that you block the powerjet and set up as normal then open the powerjet and deduct the powerjet size from the mainjet. Like a 130 main is equal to a 120 main and 10 powerjet. Can obviously alter the ratio to clear lean/rich spots?
Am I on the right track?
SmudgeDog
 
Posts: 1
Joined: Thu Jun 25, 2020 3:09 pm

Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Tue Jul 28, 2020 11:13 pm

You are on the right track, but information as to the sizes used & why etc was not forthcoming on this Forum when I asked.

That could be due to the efficiency of the carburettor in general, or the fact that it may be fitted but the owner be unaware of the size. In a way, it could be that a variable needle valve as I've gone for is a good solution.

As regards the standard position of the PWK power-jet feed into the venturi, for certain you can augment the standard 12 o'clock position with others @, say, 9 o'clock, but I'd be tempted to try variable height tubular extensions so that it comes into play sooner before adding extra positions..... ;)
Warkton Tornado No.1
 
Posts: 1582
Joined: Sun Jan 11, 2015 1:27 pm

Previous

Return to Carburettors

Who is online

Users browsing this forum: No registered users and 1 guest