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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 Adam_Winstone » Tue May 19, 2020 11:41 am

LOL... the penny has just dropped that I may (may) be able to find suitable Dellorto needles to experiment with, rather than having to try to source PWK / Polini / Keihin needles to suit. OK, there are a few elements that I'll need to confirm are appropriate for use first (clip size, clip seat, overall length, then the various aspects of needle variation) but it may just be that my Dellorto (perhaps even Amal) needles can be put to good use.

Considering that I have PHBL, PHBH, VHSA, VHSH and VHSB needles, surely something might be viable....?! If not, I have a few Keihins on order.

I'll let you know how I get on.

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

Postby Warkton Tornado No.1 » Tue May 19, 2020 4:03 pm

Adam_Winstone wrote: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).


What pilot jets have you tried? IMO & experience you need to try & get near to what might be required prior to progressing through the rest of the range. After all, the lower part of any carburettor's range will always come into play, regardless of the rest.

With the piston ported set-up, you will need something possibly as small as 30's. If you haven't already, do try an air filter. I think you might be pleasantly surprised @ just how well the PWK's function with one ;)

For a range of ten pilot jets etc, you may as well get a set from any of the sellers that do them that way, unless you intend to swap jets carb to carb & require the consistency/accuracy @ this stage. I've ended up with multiples of some pilot jets & main jets, but the origin of them hardly matters to me, as long as a test & plug chop is carried out. As I have recommended on previous occasions, a kill switch near to the throttle twist grip to augment the standard OEM switch is essential for 'chops & need only be fitted whilst testing. The standard horn button can be used as it has a high amperage rating. It just needs a changeover switch to select the function then: 'Horn' or 'Test'
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Tue May 19, 2020 5:53 pm

^... I bought the set of pilots, mains and JJ* needles when I bought the carb. I'm happy with tickover and initial progression 0-1/8-1/4 and can ride around cleanly at those throttle positions, having started rich and worked down. I can make the bike semi-rideadble, but horrible, by dropping the needle to its weakest clip setting but this is simply bodging it to get by and not satisfactory or safe.

I do take on what you're saying about progression stage 'bleed over' of influence though, which is very valid.

Thanks

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

Postby coaster » Tue May 19, 2020 10:19 pm

The needle clips are the same Adam
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby coaster » Thu May 21, 2020 8:38 am

Adam_Winstone wrote:
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.


Sorry for the delay Adam but lockdown easment finally gave me a chance to get to my carb notes. The notes arent as comprehensive as Id thought but for the setup of my old Muggy 186 I was having problems with 4 stroking and with a JJH needle on top clip it was still 4 strocking badly around 30-35mph. I then tried a JJQ on recomendation (cant remember who now) and on the top clip it cleaned up all through the rev range but was a little lean around 1/4 throttle. I then tried a GJH which was ok but still a little rich at 1/4 throttle even on the top clip but only when the engine was warm.

Other notes that I found on line somewhere were:
Taper affects 1/2 to 3/4 throttle
Length affects 1/4 to 1/2 throttle
Diameter affects up to 1/4 throttle

Richness increases with the letter

Not very useful Im afraid, I gave up on the muggy and have concentrated on phbh on all my other engines since......But all this chat has got me thinking again, might have a play with my Grand Tourisimo 200 ;)
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Storkfoot » Thu May 21, 2020 9:50 am

I persevered with a couple of PWK 28s on a Mugello 186 back in 2008/9. I tried on an AF Ckubman and a PM25. Looking at my notes, I tried various clip settings on JJK, JJH, GJH and possibly GJK. I finished on a JJK but I was never happy with the way it ran.

I gave up on PWKs at this point and never returned. However, I never checked the port timings of the Mugello barrel (it was a Version 3). With hindsight, I should have and it is possible that my main issue lay elsewhere than the carbs themselves. That said, the ease at which the float height altered with the slightest touch (easy when your float bowl is on and off all the time) used to drive me demented.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Warkton Tornado No.1 » Thu May 21, 2020 11:13 am

I have tried differing carburettor's & engine configurations over the years, much of that time with race engines, though an awful lot with road tunes of my own plus other people's. What I am trying to say is that the amount of setting up that I have been involved with may be considered to be somewhat above the 'average' Lambretta owner.

Fundamental to the PWK is the float setting, & the more that it can avoid being disturbed, the better. IMO! It is easy to modify the bowl for pilot access & that negates one problem.

As for the pilot, do not be surprised if 32 or less is required for a piston ported engine, even without large inlet timing.

Get that cleaned up & then the rest of the carb settings will fall into place ;)

If I were to try & dial in an old Group Four barrel, I'd expect the pilot to be in the 20's.....
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Thu May 21, 2020 5:26 pm

^... thanks guys for the latest round of comments. I am continuing to test various needles and will report back when I have more to share.

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

Postby Adam_Winstone » Sat May 23, 2020 5:12 pm

OK, latest update.

The initial situation of the motor being extremely rich at 1/2 upwards is still giving issue, however, I've been experimenting with needle with various to try to resolve this. It is odd that the motor goes very rich so soon so I've been playing with needles of longer shaft/shorter taper and needles with less taper angle and fatter tip, yet still the system persists and isn't something that I can overcome with the needles that I have. However...

I've noted that my 34mm (34.D) Polini uses a 30 hole atomiser as standard, whereas the 24-30mm uses an 8-hole atomiser. 4 polini atomisers are available (8, 20, 30 and 40), which don't function in quite the same manner as I'm used to with Dellortos, however, it does seem as though the needles that most are quoting may be more suited to the atomiser fitted to the 28/30mm model. I've ordered the 8, 20 and 40 items so that I can test how the motor responds to those. I also note that OKO produce an 18 hole atomiser that typically works in the PWK carbs that many have tried on Lambrettas. NB: The holes in these are not the same size so it is not simply a matter of number of holes indicating richer or weaker.

There is a very good but extremely long thread (42 pages) on the American Modern Vespa site (http://modernvespa.com/forum/topic15333 ... =210+build) in which a guy struggles to get his carb to carburate cleanly on a 210 motor, with many of the comments about atomiser changes reflecting my own experience to date.

My first thought is to put the 8-hole of the 30mm in, to see how it relates to the needles commonly quoted / in use, but I'll do some testing on all. I'll keep my eye open for an OKO 18-hole atomiser too as that appears different again.

I may yet end up buying other needles but the jetting is currently so far out that I feel a fairly major transition is required to change what the atomiser is delivering and how fast.

Cheers all.

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

Postby coaster » Sat May 23, 2020 10:28 pm

I have a fairly extensive collection of needles and jets which I;d be happy to loan you if that might help, just post back when you are done. I might have another go with my pwk's at some stage but probably not this year 8-)

Pm me your address if you want then 8-)

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

Postby Warkton Tornado No.1 » Sat May 23, 2020 10:46 pm

I've just checked three atomisers that I have to hand. They all have just eight holes! You have a four stroke atomiser, I reckon. I would imagine the 18 holer is too ;)

That'll be your problem. I don't have the information with me, but two stroke & four stroke atomisers vary, as do the float weights.

There was an on-line pdf of Dell'orto's tuning manual. The information contained will still be relevant :)
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Sat May 23, 2020 10:57 pm

coaster wrote:I have a fairly extensive collection of needles and jets which I;d be happy to loan you if that might help, just post back when you are done. I might have another go with my pwk's at some stage but probably not this year 8-)

Pm me your address if you want then 8-)

Colin


Thanks for the offer. I'll keep testing with those items that I already have and have on order but may take you up on the offer if I struggle.

Funnily enough, I posted out a couple of Amal powerjets for another Lambretta fan to test... needing one back when he's done with it. Nice that we have this community spirit.

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

Postby Adam_Winstone » Sat May 23, 2020 11:16 pm

Warkton Tornado No.1 wrote:I've just checked three atomisers that I have to hand. They all have just eight holes! You have a four stroke atomiser, I reckon. I would imagine the 18 holer is too ;)

That'll be your problem. I don't have the information with me, but two stroke & four stroke atomisers vary, as do the float weights.

There was an on-line pdf of Dell'orto's tuning manual. The information contained will still be relevant :)


Bugger, I just lost my long response... fat fingers and small phone!

Basically, from info I can find, all can be applied to 2-stroke use, with them being fitted as standard to their respective size carbs; the Polini site states 8-hole standard fitment for 24-30mm and 30-hole for 32 and 34mm bodies.

I'll be interested to try the 8-hole in my 34mm with the more commonly quoted needle range.

Cheers

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

Postby Warkton Tornado No.1 » Sun May 24, 2020 11:28 pm

Adam_Winstone wrote:
Warkton Tornado No.1 wrote:I've just checked three atomisers that I have to hand. They all have just eight holes! You have a four stroke atomiser, I reckon. I would imagine the 18 holer is too ;)

That'll be your problem. I don't have the information with me, but two stroke & four stroke atomisers vary, as do the float weights.

There was an on-line pdf of Dell'orto's tuning manual. The information contained will still be relevant :)


Bugger, I just lost my long response... fat fingers and small phone!

Basically, from info I can find, all can be applied to 2-stroke use, with them being fitted as standard to their respective size carbs; the Polini site states 8-hole standard fitment for 24-30mm and 30-hole for 32 and 34mm bodies.

I'll be interested to try the 8-hole in my 34mm with the more commonly quoted needle range.

Cheers

Adam


I've found the Dell'Orto comprehensive instructions that relate to all carburettor's, & like Polini, they say that an atomiser intended for four stroke can be made to function on a two stroke.

That's fine & dandy, but there is a distinction between the two types, so I can't imagine why anybody would like to try, apart from trying to prove something :?

I'm happy to lend you one of my atomisers (or needles etc) but I reckon you'll need to get one anyway ;)
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby ULC Soulagent » Mon May 25, 2020 11:42 am

Very interesting topic going on here, I purchased two delly vhst 28 when I worked in the scooter trade( bought for peanuts) and even though they were apparently designed for the 4-stroke engine but using some of the phbl d- range needle set. I set about converting it over fully, Blue printed it and along with adding 2-stroke atomiser bush and all the internals of the phbl card range bar the floats which was two single self floating units.I initially got good results from the acceleration side and top end part sussed but when it came too cruising on the mid section setup,The carb had me baffled and after destroying my 4 top ends, 3 imola’s and a Ts1 I gave up and went back to a phbl with no issues since. I’m keen on these pwk & Oko copies from China and obviously they need work too make them right but I’m reluctant too get bit on the arse twice :lol:
Sorry if I’ve taken it off topic.
Regards
Shane
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Mon May 25, 2020 2:13 pm

Warkton Tornado No.1 wrote:
Adam_Winstone wrote:
Warkton Tornado No.1 wrote:I've just checked three atomisers that I have to hand. They all have just eight holes! You have a four stroke atomiser, I reckon. I would imagine the 18 holer is too ;)

That'll be your problem. I don't have the information with me, but two stroke & four stroke atomisers vary, as do the float weights.

There was an on-line pdf of Dell'orto's tuning manual. The information contained will still be relevant :)


Bugger, I just lost my long response... fat fingers and small phone!

Basically, from info I can find, all can be applied to 2-stroke use, with them being fitted as standard to their respective size carbs; the Polini site states 8-hole standard fitment for 24-30mm and 30-hole for 32 and 34mm bodies.

I'll be interested to try the 8-hole in my 34mm with the more commonly quoted needle range.

Cheers

Adam


I've found the Dell'Orto comprehensive instructions that relate to all carburettor's, & like Polini, they say that an atomiser intended for four stroke can be made to function on a two stroke.

That's fine & dandy, but there is a distinction between the two types, so I can't imagine why anybody would like to try, apart from trying to prove something :?

I'm happy to lend you one of my atomisers (or needles etc) but I reckon you'll need to get one anyway ;)


Thanks WT1 but I've already ordered the other atomiser types to try but thanks for the offer.
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Adam_Winstone » Mon May 25, 2020 2:29 pm

ULC Soulagent wrote:Very interesting topic going on here, I purchased two delly vhst 28 when I worked in the scooter trade( bought for peanuts) and even though they were apparently designed for the 4-stroke engine but using some of the phbl d- range needle set. I set about converting it over fully, Blue printed it and along with adding 2-stroke atomiser bush and all the internals of the phbl card range bar the floats which was two single self floating units.I initially got good results from the acceleration side and top end part sussed but when it came too cruising on the mid section setup,The carb had me baffled and after destroying my 4 top ends, 3 imola’s and a Ts1 I gave up and went back to a phbl with no issues since. I’m keen on these pwk & Oko copies from China and obviously they need work too make them right but I’m reluctant too get bit on the arse twice :lol:
Sorry if I’ve taken it off topic.
Regards
Shane


No, perfectly on topic.

I was interested to read that WT1's 34mm had the 8-hole atomiser in, which mine did not. Although the Polini website says that the 30-hole is fitted as standard to the 32 and 34mm, with the 8-hole being fitted by someone along the way, which is standard fitment for the 24-30mm.

I have been down the similar route of changing the atomiser holders over on Dellortos, changing a PHBH 'T' atomiser holder to an AV/AS longer nosed 2-stroke type. Thankfully, I didn't have any further issue with the carb. Funnily enough, a clubmate ran his iron 175 on the same type with T atomiser for years and that was both quick (for a 175) and reliable, with us doing a number of long distance rallies together. However, that also ran a needle that I'd never heard of anyone else running for Lambretta applications.

As WT1 commented above, I have no doubt that the 30-hole could be made to work, whether it transpires to be 2 or 4-stroke, but that would lead me down the path of buying a whole range of other needles to try, where shaft diameter, taper length and tip diameter could / might be able to resolve my current carburation issues, however, I have no intention to buy the world's stock of needles to determine which one might work. God knows that I've gone down that rabbit hole with other carbs! As such, I think that a swap to the 8-hole for testing is probably my best step (8, 20 and 40 on order) so that the work already put in by the majority can be put to good use, starting with those needles that I see regularly listed.

I'll keep people posted as there is very little that I can find online about atomiser use for this range of carbs and copies (Keihin, OKO, PWK, Polini). The Modern Vespa link that I posted previously is about as good as the info available gets.

Ta

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

Postby Warkton Tornado No.1 » Mon May 25, 2020 7:45 pm

ULC Soulagent wrote:Very interesting topic going on here, I purchased two delly vhst 28 when I worked in the scooter trade( bought for peanuts) and even though they were apparently designed for the 4-stroke engine but using some of the phbl d- range needle set. I set about converting it over fully, Blue printed it and along with adding 2-stroke atomiser bush and all the internals of the phbl card range bar the floats which was two single self floating units.I initially got good results from the acceleration side and top end part sussed but when it came too cruising on the mid section setup,The carb had me baffled and after destroying my 4 top ends, 3 imola’s and a Ts1 I gave up and went back to a phbl with no issues since. I’m keen on these pwk & Oko copies from China and obviously they need work too make them right but I’m reluctant too get bit on the arse twice :lol:
Sorry if I’ve taken it off topic.
Regards
Shane


I like the PWK's because they have been so refreshingly easy to set up. I also do like some Dell'Orto's other than the round slide versions, though that will be due to familiarity with the VHBC.

Although they are used extensively for racing, even they can catch you out & probably more so with road going, tuned scooters. I think they can be 'fussy' as I had problems @ Cadwell Park, not due to the float needle valve, but the actual volume of the float chamber. Once a bigger one was fitted, it had enough juice for WOT :roll:
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Re: Polini 34mm (Keihin PWK)... Jetting?

Postby Andy_S » Tue May 26, 2020 12:27 pm

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

Postby Adam_Winstone » Tue May 26, 2020 1:11 pm

Hi Andy,

Funnily enough I'd already read that but it is for a different model of Polini carb to the ones that the PWKs and others are based on. Ironically, none of the Keihin, PWK, OKO or Polini versions come with a closed atomiser, only being available with 8, 18, 20 or 30-hole versions. If that was to be applied to these carbs that we're all buying, none of them would be suitable for 2-stroke applications.

It does follow though that the fewer holes, the closer we are to closed function, which may explain why people are getting better 2-stroke results with the 8-hole atomiser. That said, the 18 hole that is supplied with the PWK/OKOs also seems to work, and the holes in that are considerably bigger than the holes in the Polini 20. The Polini 30-hole has large holes again, then they are very small in the 40. Quite which has the largest cross-sectional area of holes is not that straight forward.

As no ZERO-hole atomiser is available for this range of carb (unlike the model that your quote refers to) so it has to be a case of one or more of these 8 / 18 / 20 / 30 / 40-hole being for 2-stroke applications. Although not needle type carbs, the SH1 and SH2 carbs fitted as standard to SIIIs all run atomisers with various holes in (different number, heights and diameters) to control the rate of fuel delivery across the rev range so it is not the case that no 2-stroke atomisers use cross drillings, however, they are less common on needle type carbs. I found that same info on the Polini site to be confusing and frustrating as it seems to speak for all Polini carbs, yet 'our' common model does not have said zero-hole option. "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."... none too helpful.

Thanks for looking this out though, and posting it, as the more we can find out about these carbs and their components the better.

Adam
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