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

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

Postby coaster » Mon May 04, 2020 9:35 am

Warkton Tornado No.1 wrote:................As you are running piston ported, you might later wish to try a power jet. As my appeal for recomendations on Forums met with little response about sizes of jets etc, I have grafted on an adjustable miniature needle jet valve atop the opening to the Venturi (bell-mouth you might refer it as) but it's not yet in use as the carb is temporarily adorning a reed valve motor.


That's interesting' the last PWK I bought (BGM 30mm) was fitted with a power jet but the advice on the forums ws to blank it off as it was for 4 stroke usage :shock: I plugged mine up but you now have me wondering 8-) The poxer jet only comprised of a brass jet tube in the top of the venturi connected via a small plastic pipe to an outlet in the flaot chamber.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Mon May 04, 2020 10:22 am

coaster wrote:Its a long time since I played with PWK's but Im sure I usually had a problem with a boggy spot somewhere in the mid throttle range and was advised to try a JJQ which did make a huge difference. I could be wrong but I THINK I remeber it only differing from the JJH in the taper length. All my info is up in Norfolk and Im locked down in London so cant check for you Im afraid :(


... I'm not going anywhere in a hurry, thanks.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Tue May 05, 2020 5:02 pm

MickYork wrote:KEIHIN N427-46 JET NEEDLES
These Keihin jet needles are given part numbers according to their dimensions
The syntax is N427-46-(TAPER)(L1)(DIAMETER)-(CLIP POSITION) – Eg: N427-46-JJH would be a taper of 3°00′, an L1 of 25.40mm and a diameter of 2.435mm
Clip positions are not required for ordering but for the sake of notation, 1 is the top clip position. (Near the blunt end of the needle.

pasted from this site, bit more info if you go to the site: http://www.allensperformance.co.uk/keih ... t-needles/


The nuts thing about this is that the Keihin illustration/table doesn't even allow you to figure out how long the tapered section is (unless I'm missing something) as the overall length 57mm does not start at the same point (datum) as L1, and the distance between the top of L1 and top of needle is not given. As such, how the hell do you know what length the taper is? Without the length of taper you cannot calculate the tip diameter from angle and unknown length.

If L1 and the overall started at the same point, or if they gave you the distance from top of L1 to top of needle, then calculation would be easy.

My conclusion... nobody seems to know what they're selling and/or fitting... me included :lol:

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

Postby phil23fair » Wed May 06, 2020 12:12 pm

Is this any use? It looks to be the same table as above but with a needle annotated A to E
https://www.rrd-preparation.com/fr/piec ... eihin.html
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Wed May 06, 2020 1:05 pm

Adam_Winstone wrote:
MickYork wrote:KEIHIN N427-46 JET NEEDLES
These Keihin jet needles are given part numbers according to their dimensions
The syntax is N427-46-(TAPER)(L1)(DIAMETER)-(CLIP POSITION) – Eg: N427-46-JJH would be a taper of 3°00′, an L1 of 25.40mm and a diameter of 2.435mm
Clip positions are not required for ordering but for the sake of notation, 1 is the top clip position. (Near the blunt end of the needle.

pasted from this site, bit more info if you go to the site: http://www.allensperformance.co.uk/keih ... t-needles/


The nuts thing about this is that the Keihin illustration/table doesn't even allow you to figure out how long the tapered section is (unless I'm missing something) as the overall length 57mm does not start at the same point (datum) as L1, and the distance between the top of L1 and top of needle is not given. As such, how the hell do you know what length the taper is? Without the length of taper you cannot calculate the tip diameter from angle and unknown length.

If L1 and the overall started at the same point, or if they gave you the distance from top of L1 to top of needle, then calculation would be easy.

My conclusion... nobody seems to know what they're selling and/or fitting... me included :lol:

Adam


I've got some new, unused JJH's which I will measure.

I agree that the information should be to hand on the most common of the needles that are in use, even on 'proper' bike set-ups. Not that it concerns me unduly as they just work well on whatever engine IMO & experience, which reinforces my view that the concept of PWK's is pretty well spot on. Icing on the cake is that no needle shrouding within the Venturi is required, as is usual with two strokes, so even less "Kerching @ the till" when variations are normal with, say, Dell'Orto :lol: Don't get me wrong, some Dell'Orto's are excellent two stroke carbs. Just not the type with a round slide IMHO.....

Quite why Polini have bought in to the PWK manufacture yet omit the JJH full dimensions is surely a schoolboy error :?:
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Wed May 06, 2020 2:19 pm

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

Postby Warkton Tornado No.1 » Thu May 07, 2020 1:56 am

I measured three new JJH needles that I acquired from splatshop prior to their realisation that their prices were too low ;)

They appear to be genuine Keihin & the dimensions that I could check would appear to verify that with their own chart. Give or take a Nat's. That chart confuses me, referring to 3°as leaner & 3.75° as richer :?

J (first letter) would appear to be to specification ( 3°) but not something easily checked. However, the tip diameters on all three were in the region of Ø 0.995 mm. I cannot be arsed to apply trigonometry now as Horlicks awaits....

J (second letter) would appear to be less than 27 mm, so 25.4 mm (for whatever L1 specifically denotes) could well be spot on.

H (third letter) is stated as Ø 2.435 mm. All three of mine measured more like Ø 2.45 mm.

Likewise, the new JFH was identical to the above apart from the non tapered portion L1 (it being an 'F' second letter) which was shorter & could well have been Ø 22.25 mm as per the chart.

Interesting that the two types that I possess seem to have the same tip diameter....
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Thu May 07, 2020 2:01 pm

phil23fair wrote:Is this any use? It looks to be the same table as above but with a needle annotated A to E
https://www.rrd-preparation.com/fr/piec ... eihin.html


Sadly, no, but thanks. No because the JJH needle is not in that list, which is what I find odd.

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

Postby Adam_Winstone » Thu May 07, 2020 2:21 pm

WT1

Thank you for your response and measurements.

Your measurements reflect mine, which is nice to have a second opinion on as I struggled to measure mine (micrometer and digital vernier for ballpark). Confirmation of these figures is good as I assumed the difference in shaft diameter and large, by comparison to the others quoted in the table above, gave me cause for concern about my accuracy. In a list with 0.96mm being the largest tip quoted for my needle range (JJK-L, N68E) the 0.99'ish mm (from memory as I discounted it and didn't write it down) for JJH seemed likely to be my measurement error .... but perhaps not then (?)

I still find it odd that the chart quotes needle tips for so many of the JJ* needles but not the JJH, even though SCK, VE and others recommend its use.

Thanks again.

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

Postby vegansydney » Fri May 08, 2020 1:17 am

I've been toying with the idea of fitting a 24mm Polini PWK on a GT186 after I found a TM24 (my carb of choice) fouls the engine mount using the manifold, and my welder suddenly moved out New York with the pandemic. (FYI a Polini PWK 24mm won't foul on a GT 25mm manifold).

In my research I found that GSF has a wealth of information, including that the Polini needle table differs from the KEIHIN needle table, genuine Keihin and KMT jets appear to be the only ones made to spec, not even Polini and BGM are correct. Like other PWK clones there have been reports of fuel flow issues with the the Polini.

Good luck, and please keep this thread updated. For now, I'm going to find a new welder and stick with the TM24, which is a shame because the Polini come in such a diverse range of sizes, and they're not an antiquated roundslide that UK tuners seem so infatuated with :lol:
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Fri May 08, 2020 12:15 pm

vegansydney wrote:I've been toying with the idea of fitting a 24mm Polini PWK on a GT186 after I found a TM24 (my carb of choice) fouls the engine mount using the manifold, and my welder suddenly moved out New York with the pandemic. (FYI a Polini PWK 24mm won't foul on a GT 25mm manifold).

In my research I found that GSF has a wealth of information, including that the Polini needle table differs from the KEIHIN needle table, genuine Keihin and KMT jets appear to be the only ones made to spec, not even Polini and BGM are correct. Like other PWK clones there have been reports of fuel flow issues with the the Polini.

Good luck, and please keep this thread updated. For now, I'm going to find a new welder and stick with the TM24, which is a shame because the Polini come in such a diverse range of sizes, and they're not an antiquated roundslide that UK tuners seem so infatuated with :lol:


There is some good information there, although Polini would appear to have issues with quality control. That is extremely disconcerting, especially when their manufacture of complete Paramotors is taken into consideration. The fact that Polini are a major supplier, predominantly to the race scene, with Polini only race series, must surely ensure that any quality issues will be addressed.

I've had PWK's from Koso, OKO, Stage6 & even a Chinese copy. It's fair to say the first three are very good out of the box whilst the Chinese replica needs more attention. But for ten quid?

Personally, I endeavour to follow my own advice in trying to select the most sensible guides from various sources.

Primarily, regardless of manufacture, any carburettor should be stripped to it's component parts & rebuilt, blueprinting as necessary. That instils both knowledge & confidence.

With any PWK, the float angle is critical, though not difficult to set.

The fuel feed to the float needle should be checked to the recommendations available on line. Kerist, the PWK has been around so long now that that Scooter-Tuning-Legend-in-his-own-Lunchtime (you know! The one that invented the wheel!) has PWK fuel feed modification data on his website. Other websites are available such as that from Mid-Atlantic Trials etc

The bowl has a float bottom stop which I remove with an electric sanding file.

As for jetting & needles, there will be variances between manufacture, but I reason that if they are bought in a set of ranges from one source, it doesn't matter a great deal because they will be used as comparators with one another, not with jets, say, of another manufacture. In the case of needles, I am fortunate to have Keihin, but even they do not conform with their own charts. But Hey Ho! I use them as comparators so see even that as fly sh1t.

The German owner/rider/tuners are heavily into the technology of tuning & their 'warts & all' responses are refreshing, with some knocking Polini, rather than the "Hail PHBH round slide four stroke derivative carburettors modified for two stroke use & all who use them!" mantra that predominates here. Incidentally. Google Translate came up with the word 'wischi-waschi' which amuses the likes of me. A lot like (so I'm told! Don't shoot the messenger!) the Welsh word for computer is 'computor'

As I said earlier, I doubt Polini's problems will be around for long. What has inspired the Germans is the principle with some buying into the Keihin PWK's. Hardly surprising, I suppose, as they have been OEM supplied on the best Japanese 'crossers for years. In fact, OKO PWK are also original equipment on quite a number of trial bikes, I believe.

Change is often quite frustrating. There will be people that want the individuals that try alternatives to fail.

The Scootering fraternity does tend to take comfort in many of them having a lot of the same things, & I can see the logic in that. But that ethos is sometimes flawed. Take the example of electronic ignitions fitted because 12v points based systems are 'unreliable'
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby rossclark » Sat May 09, 2020 12:18 am

Wishy - Washy is a well known west of Scotland expression of disgust at an equivocal answer ...
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Sun May 10, 2020 9:47 am

WT1, thanks for the response.

With regard to the fuel intake modification info on said website, I found a mention of it but the link didn't take me anywhere. I go and have another look as that could be a worthwhile mod. I've made similar modifications to VHSB carbs and would happily do the same to this Polini, although a read of such meds would be reassuring, before I drill through something that I shouldn't (lol).

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

Postby TVSi » Wed May 13, 2020 10:04 pm

Funnily enough i'm looking at fitting a PWK30 or 32 to a TS1 motor shortly so i have done a bit of searching and found this on Lambretta club USA site

" recently picked up a GP200 that had a OKO/PWK 30mm fitted with an old PM Style ScootRS pipe. Although it ran, it was jetted poorly, not to mention that whoever last rebuilt the carb stripped most of the housing and jet threads. So I picked up a few 30mm powerjet OKO/PWK carbs cheap off of ebay and decided to try various setups until I had it jetted perfectly.

These carbs seem to come from various manufacturing facilities. There are 5 modifications that you must perform.

Check and drill the float needle area to 5mm.
Check and drill the float needle center fuel inlet to 2.5mm.
Check and drill the float needle area horizontal feeder holes to 2mm.
Check and drill the idle/slow jet outlet in the venturi to 1.2mm. To do this you will need to unscrew the jet holder from the bottom of the carb. Start at .5mm or .6mm and work your way up to 1.2mm
If the OKO/PWK carb you have uses round jets, swap out the atomizer/diffuser for a Keihin style hex jet one https://www.treatland.tv/OKO-diffuser-a ... 8-hole.htm

One of the main problems people encounter with these carbs is that they can't get them to idle without turning the idle adjuster all the way in. This issue is because of two reasons:

The first is that the idle circuit needs to be drilled out.
The second is that people are running way too lean idle jets. People always think that if it is lean it will cause the engine to race, which is true for main jets and slightly smaller than required idle/slow jets, but if you don't give it enough fuel to begin with it will not run at all and will mimic a too-rich condition at idle. To compensate for the too lean idle jet, people turn the idle adjuster all the way in which uses the slide cutaway to add more fuel allowing it to somewhat idle at a higher rev. Another side effect of a way too small idle/slow jet is that it will idle with the choke on.
The jetting I used was:

slow/idle jet: 48
slide: stock 3.5
needle: JJH
needle position: 3
main jet: 125
air mixture screw: 1.5 turns out
filter: stock air bellow w/ oiled foam UNI pod filter.

I used a set of Dormer metric drill bits for the 2, 2.5, and 5mm holes, and the Harbor Freight micro drill bit set for the 1.2 mm hole.

At WOT I can see the Powerjet being utilized. There are no flat spots, and the throttle response is crisp. The plug is a nice chocolate brown. I did these mods to three individual carbs, and they all respond the same on the scooter"


I haven't got round to modifying mine yet as i still require the drill bits to try the above mods so i cant offer first hand feedback as yet but this info may help you.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Thu May 14, 2020 9:44 am

TVSi wrote:Funnily enough i'm looking at fitting a PWK30 or 32 to a TS1 motor shortly so i have done a bit of searching and found this on Lambretta club USA site

" recently picked up a GP200 that had a OKO/PWK 30mm fitted with an old PM Style ScootRS pipe. Although it ran, it was jetted poorly, not to mention that whoever last rebuilt the carb stripped most of the housing and jet threads. So I picked up a few 30mm powerjet OKO/PWK carbs cheap off of ebay and decided to try various setups until I had it jetted perfectly.

These carbs seem to come from various manufacturing facilities. There are 5 modifications that you must perform.

Check and drill the float needle area to 5mm.
Check and drill the float needle center fuel inlet to 2.5mm.
Check and drill the float needle area horizontal feeder holes to 2mm.
Check and drill the idle/slow jet outlet in the venturi to 1.2mm. To do this you will need to unscrew the jet holder from the bottom of the carb. Start at .5mm or .6mm and work your way up to 1.2mm
If the OKO/PWK carb you have uses round jets, swap out the atomizer/diffuser for a Keihin style hex jet one https://www.treatland.tv/OKO-diffuser-a ... 8-hole.htm

One of the main problems people encounter with these carbs is that they can't get them to idle without turning the idle adjuster all the way in. This issue is because of two reasons:

The first is that the idle circuit needs to be drilled out.
The second is that people are running way too lean idle jets. People always think that if it is lean it will cause the engine to race, which is true for main jets and slightly smaller than required idle/slow jets, but if you don't give it enough fuel to begin with it will not run at all and will mimic a too-rich condition at idle. To compensate for the too lean idle jet, people turn the idle adjuster all the way in which uses the slide cutaway to add more fuel allowing it to somewhat idle at a higher rev. Another side effect of a way too small idle/slow jet is that it will idle with the choke on.
The jetting I used was:

slow/idle jet: 48
slide: stock 3.5
needle: JJH
needle position: 3
main jet: 125
air mixture screw: 1.5 turns out
filter: stock air bellow w/ oiled foam UNI pod filter.

I used a set of Dormer metric drill bits for the 2, 2.5, and 5mm holes, and the Harbor Freight micro drill bit set for the 1.2 mm hole.

At WOT I can see the Powerjet being utilized. There are no flat spots, and the throttle response is crisp. The plug is a nice chocolate brown. I did these mods to three individual carbs, and they all respond the same on the scooter"


I haven't got round to modifying mine yet as i still require the drill bits to try the above mods so i cant offer first hand feedback as yet but this info may help you.


Thank you for posting this information. It's fair to say that any carburettor should be checked out prior to bolting on, & the PWK's may need 'blueprinting' to perform as they should on a Lambretta.

Personally, of all the PWK's that have been in my grubby hands, one or two needed no modifications whatsoever to conform. IMHO the float check is mandatory, as with any make of carb, but I also grind out the alloy float stops in the bowl & add a slot in the drain plug for pilot jet access. There is a knack to removing the bowl, but the drain plug modification removes the need. Oh, & the last PWK that I 'built' had a very small fuel cock added to the drain plug as I envisaged much time spent jetting in, which wasn't the case! The TS has a JJH, 45 pilot & 145 main, all running through an air filter. The 145 main compared to the 125 in the example above is not surprising as the TS has been modified quite extensively.

Despite what some may think, I have found the PWK's to be completely fuss free with air filters fitted, though they must be adequate. They really allow an engine to perform well & on other two stroke Forums are considered to be equivalent to other manufacturer's carbs of larger venturi.

The only downside is the need for an inline fuel filter, what with the poor standard tap filtration we have. That's not too bad a price to pay, as they are considerably easier to check visually & clear, for that matter, & many of us use them regardless of carburation.

A mate has just bought his third PWK & sent me an image. It has the Keihin-esque screw on top & the main body/bowl are a graphite colour. Very nice & like all PWK's comes as standard with float bowl overflow. You can buy two for less than the cost of one 12975 PHBH Float bowl with overflow & drain...... :lol:
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Thu May 14, 2020 2:01 pm

Thanks for the last couple of responses, interesting and informative reading.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby TVSi » Sun May 17, 2020 9:08 am

I have found this a good tool http://www.kyajet.de/?lang=en

You can put in your needle say JJH and then by adjusting the the needle sections you can see how it fuels and the needle required to meet these changes.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby coaster » Sun May 17, 2020 10:08 am

TVSi wrote:I have found this a good tool http://www.kyajet.de/?lang=en

You can put in your needle say JJH and then by adjusting the the needle sections you can see how it fuels and the needle required to meet these changes.


That looks very useful thanks for posting
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Sun May 17, 2020 5:23 pm

^... I like the way that you can simply press + or - buttons to see which needle you should be changing to. Thanks

My first tests yesterday show that I need to buy a range of needles to test so this should be a great help.

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

Postby Adam_Winstone » Mon May 18, 2020 10:28 pm

Hmmmm, interesting...?

Well, my road tests have suggested that the progression becomes too rich too soon as I work up the progression stages / throttle positions. Tickover and takeoff can be set fairly easily with the pilot slide, slide and initial relationship of needle shaft diameter and fixed needle jet, however, I then find that it gets too rich too soon unless I start to restrict it by fitting a restrictive main jet (not a good idea!). Yes, i can delay this richness by dropping the needle (raising clip), however, it is coming onto the taper too quickly.

OK, I can increase the L1 distance so that it doesn't come onto the taper so quickly, however, this then puts me outside the range of needles that I commonly see others quoting. Also, I could reduce the taper angle (first identifying letter of the Polini/PWK/Keihin needles) but this too takes me away from the range of needles that others are quoting. Considering that the commonly quoted JJH, JJK, JJL needles all have the same 3 degree taper, which appears steep when compared to the Dellorto and Amal needles that I am accustomed to... and had me thinking...

Taking the Amal MKII carb first, as it is the simplest and has just one common 2-stroke needle (2D1), the needle shaft diameter / taper length / tip diameter gives a taper angle of 1.9 degrees. Considering that Amal used to pride themselves on the simplicity and ease of setup of this one needle design, you would think that the 1.9 degree angle should probably be a pretty good starting point for any 2-stroke carb that relies on needle + atomiser function. OK a big difference from the 3 degree JJK that is currently working best on my Polini / Keihin carb.

So, let's then look at the common X7, X13 and X2 needles. These work out as X7 = 2.0 degrees, X13 = 1.8 degrees and X2 = 1.7 degrees. Now for anyone who will have played with this range of needles, you'll know that this is not necessarily so much about the taper as it is about how long the taper is; how quickly it starts to richen up. All have the same 2.5mm shaft, the same 1.8mm tip but vary on the taper length 20, 22 and 24mm respectively.

So, where does this leave me? The too much too soon suggests that I should be looking at needles that do not come onto the taper so quickly (longer L1 - middle letter) and possibly a shallower taper (first letter - angle of taper). The shallower taper would be much more in keeping with the taper and progression speed of all of the other needle /carbs above.

Hmmmmmm, food for thought!

Adam

PS - I'll eventually get this dynoed, if I don't blow it up first (LOL).
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