LCGB Forums

Hi all,

Is anyone running a 34mm Polini (Keihin PWK) carb on a piston port motor? If so, got a starting point for jetting please?

Once we're allowed back out onto the roads then I can start to evaluate any setup that I start with, however, knowing what others may be using would give me a place to start.

Thanks in advance.

Adam

You would need to let us know what cylinder and exhaust you've got before anyone can give you jetting advice

On the Polini carbs, are the jet numbers flow-rated (like Amal) or sized in millimetres (like Dell’orto)?

Sized

The motor is one of my own configuration so I'm looking for any piston port motor jetting that people are running reliably and will relate their settings to mine. Because these carbs are totally new to me I'll be interested to hear of any combos that others are using.

Ideally I'd be hearing from people that are running a 30mm or 34mm version of this carb on a piston port motor. Capacity, state of tune and exhaust type will obviously influence jetting/carb setup but rather than people trying to recommend what might suit my engine, I'd rather hear what others are running reliably on their setups.

Please note that I'm specifically interested in the Polini 34mm, although Polini 30mm setups might be close enough to help.

Thank you.

I'm going to check my notes, but a blueprinted 34 mm PWK was the carb of choice for a mate that selected that size, simply because it was the norm when he last had a SX200 in the 70's/80's. Actually, the 34 mm was not oversize on his 200 motor (stage IVish with an expansion chamber), or fussy, as you might expect. It also ran fantastically well with an adequate DIY filter. The experience has really got me into being a convert, especially when the same carb was plonked on a TS 70 x 60 I recently built & started with a half hearted kick first time.

The needle for the piston ported engine was the common choice, JJH, pilots were low 30's (being piston ported) but I cannot remember main jet size.

I hope that helps so far....

Thanks WT1 all/any info helps at the moment as these carbs are completely new to me.

Funnily enough, I have bought a needle selection kit that has a range of needles that includes the JJH but it appears to be just about the only one not detailed in the common needle charts, whereas the others in the kit are. I have asked elsewhere if anyone can provide me with details of the JJH spec so that I can desk review how it fits into the needle range.

The motor/bike in question is up and running so I'll be able to do limited tests/evaluation on the stand but knowing what others are running might allow me to start somewhere closer for evaluation. I'm all marked up for throttle positions and am itching to get testing, however, I'll not do so until the situation with travel changes.

Thanks again

Here you go Adam, hope this is what your after

JJH Needle
Taper = 3°
Dia = 2.435mm
Straight length = 25.4mm
Total length = 57mm

Mark

Great, thanks Mark. Yes, that really does help me when considering against other needles in the range and where to go re: changes to my jetting.

Out of interest, did you manage to find it in a chart somewhere?

Ta

Actually, that's not quite the same as some of the other Polini charts as the one thing that it misses is tip diameter. I've noticed this on some data sheets but it isn't quite a like for like comparison as the common charts that seem to quote for all bar the JJH have tip diameter. After decades of using Dellorto charts, quoting tip diameters, I'm going to struggle to compare the JJH by taper angle.

I guess that I can work it out but it is odd that the others (JJL, JJN, JJM, etc) are all quoted by tip dia in the common chart, but not JJH.

Looks like I'll be doing some trigonometry to work it out, which at such fine tolerances may give me an error if they've rounded off to 3 degree angle.

Hmmmm, finding it quoted would have removed any doubt.

Ta

Adam

Is this of any use...



I'm taking column B as tip size and the JJ- range seem to increase/decrease by 0.01

Can't remember exactly where I got it from but kept it as I'm playing with my PWK 28 at the mo

The JJH needle full geometry is listed somewhere, but I do not have details to hand. However, I managed to buy a few genuine Keihin type from a dealer for Sherpas (no! Not the van ) that seemed unaware of their true value. Ha!

Also got a few drain plugs the same way because I wanted to fit a miniature drain tap, envisaging that there would be the requirement for lots of jet changes. That's simply not been the case, but a useful addition IMHO if only to avoid a mess on our silky smooth Queen's Highways.

The pilot jet on my 34 mm PWK has been made accessible without bowl removal via slotting the threaded portion where the drain plug fits. It is simply to allow access with a slotted screwdriver. I saw that many fans of PWK's advocate this simple modification, & it is really worth doing. The drain plug has a 'O' ring fitted so there is no loss to the sealing ability.

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.

BTW. This may be teaching grannie how to suck eggs, but a kill switch fitted in easy reach of the right hand whilst holding set throttle openings is extremely useful. It can, say, be a toggle switch grafted on or proprietary kill button. My favourite is to use the horn button via having a changeover switch fitted elsewhere (my SX has a key operated threeway switch with adequate amperage in the legshield toolbox top) so that I can select 'Legal' or 'Not so Legal but testing, Officer'

I'll get main jet setting on here later.

Oh. I should mention that I think it worthwhile fitting a good fuel filter, in line if necessary, as the screen mesh of Lambretta type are fairly crude plus the majority have a hole in the top! Probably good practice whatever carb is used....

Solid Air wrote:Is this of any use...



I'm taking column B as tip size and the JJ- range seem to increase/decrease by 0.01

Can't remember exactly where I got it from but kept it as I'm playing with my PWK 28 at the mo

Yes, this is the common (Dellorto'esque) chart that is readily available on lots of sites... just odd that it doesn't list the JJH needle, which actually seems to be quite commonly quoted! I'd rather not make any assumption re: what the JJH might be as it would be just my luck that it doesn't fit the pattern.

Thanks

Warkton Tornado No.1 wrote:The JJH needle full geometry is listed somewhere, but I do not have details to hand. However, I managed to buy a few genuine Keihin type from a dealer for Sherpas (no! Not the van ) that seemed unaware of their true value. Ha!

Also got a few drain plugs the same way because I wanted to fit a miniature drain tap, envisaging that there would be the requirement for lots of jet changes. That's simply not been the case, but a useful addition IMHO if only to avoid a mess on our silky smooth Queen's Highways.

The pilot jet on my 34 mm PWK has been made accessible without bowl removal via slotting the threaded portion where the drain plug fits. It is simply to allow access with a slotted screwdriver. I saw that many fans of PWK's advocate this simple modification, & it is really worth doing. The drain plug has a 'O' ring fitted so there is no loss to the sealing ability.

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.

BTW. This may be teaching grannie how to suck eggs, but a kill switch fitted in easy reach of the right hand whilst holding set throttle openings is extremely useful. It can, say, be a toggle switch grafted on or proprietary kill button. My favourite is to use the horn button via having a changeover switch fitted elsewhere (my SX has a key operated threeway switch with adequate amperage in the legshield toolbox top) so that I can select 'Legal' or 'Not so Legal but testing, Officer'

I'll get main jet setting on here later.

Oh. I should mention that I think it worthwhile fitting a good fuel filter, in line if necessary, as the screen mesh of Lambretta type are fairly crude plus the majority have a hole in the top! Probably good practice whatever carb is used....

All noted and thanks for mentioning.

MJ setting would be great, thanks.

Adam

Adam_Winstone wrote:

Solid Air wrote:Is this of any use...



I'm taking column B as tip size and the JJ- range seem to increase/decrease by 0.01

Can't remember exactly where I got it from but kept it as I'm playing with my PWK 28 at the mo

Yes, this is the common (Dellorto'esque) chart that is readily available on lots of sites... just odd that it doesn't list the JJH needle, which actually seems to be quite commonly quoted! I'd rather not make any assumption re: what the JJH might be as it would be just my luck that it doesn't fit the pattern.

Thanks

No problem

hi

have you tried the mb tech site, there is some pwk jet / needle info on there .Also theres a bit of help on the kyajet tool page , interesting .

cheers.

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/

Again, odd that it doesn't quote tip diameter, whereas other charts for this range do... just not for the JJH.

Ta

Main jet was eventually set @ 115

^... ta

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