LCGB Forums

I'm looking for advice on the sizes that are likely to be used, if anybody on here uses them, for OKO etc PWK carburettors on a piston ported engine, please.

We have tried non power-jet PWK's temporarily, & were impressed, but now want to be a bit more 'scientific' about our next attempts. I have in mind a couple of engines, both with expansion type exhausts, one a mildly tweaked 200 & the other a ported, 61 mm stroked x 70.5 Suzuki piston Rapido.

Any sensible advice would be appreciated, including the needle (usually JJH, it seems....) main & pilot jets (we get the impression that even 35 may be too big due to piston port set-ups 'pulsing' & picking up extra fuel @ lower revs) regardless of whether knitted or not. Even if power-jets have been tried & taken off, we'd still like to hear about it, please

All the PWK carburettors we have to hand have been checked over for conformity with all of the best advice on other Forums about fuel flow/air passages/float stops etc & range from 30 up to 38-ish.

We are looking for ball park figures before we venture further & both engines run through adequately sized filters & not the restrictive Lambretta bellows &/or airbox.

Thank you in anticipation....

I've always used PWK's.
My engine is an RT195, BGM Clubman, 58mm crank. Was originally built with a 24mm PWK and jetted by Paul Baker.
This ended up with a JJN needle at T4 (one clip from bottom), 108 main and 32 idle jet. The jets were the smallest I had and it was still a little rich, at approx. 6.5 CO reading , where 6.0 is ideal.
Was always a little boggy at lower revs, so managed to get a 25 idle jet off eBay.

Ran engine like this for a couple of years until I tried a 28mm OKO bought from eBay to try.
I used settings from MB pages as a basis, which were Main 126-118, Needle JJM/JJL/JJK at T3, Idle 40-35.

Tried these settings, 125/35 jets but I couldn't get it to pick up at all, tried with a JJH needle which is richer that the 3 suggested and was immediately better.
On one ride out the engine felt quite zingy on some of the quicker sections, at home the plug was lean, so set the main at 128 and moved needle to T4.

I did find the engine would starve at long periods at full throttle, MB suggests that these carbs need 'blueprinting' to improve fuel flow.
i drilled out the float valve to 3.0mm but the bloody drill snagged and damaged the brass float jet body.
Tried a 30mm OKO from a guy in the club, but no matter what i tried i couldn't get this set up, i think it was just too big for the engine.

Got another 28mm off Ebay, drilled out float valve to 3.0mm successfully and used all previous jets, and engine was good again.

i have fettled a little bit and ended up with 125 main/42 idle and JJH at T3, mainly to improve lower end pick up and top end richness. Moving needle to T4 previously to improve the weakness caused to be a bit boggy mid range.

One thing i did find that transformed the engine is using a DC CDI from Anthony Tambs. Runs great throughout the range with the improved spark.

24mm was non power jet, all the 28mm's i've used have power jets.
24mm was open mouth, I've used breathe sweet filters on an elbow on the 28mm carbs, mainly to stop the oil spitting.

Paul_from_Thornbury wrote:I've always used PWK's.
My engine is an RT195, BGM Clubman, 58mm crank. Was originally built with a 24mm PWK and jetted by Paul Baker.
This ended up with a JJN needle at T4 (one clip from bottom), 108 main and 32 idle jet. The jets were the smallest I had and it was still a little rich, at approx. 6.5 CO reading , where 6.0 is ideal.
Was always a little boggy at lower revs, so managed to get a 25 idle jet off eBay.

Ran engine like this for a couple of years until I tried a 28mm OKO bought from eBay to try.
I used settings from MB pages as a basis, which were Main 126-118, Needle JJM/JJL/JJK at T3, Idle 40-35.

Tried these settings, 125/35 jets but I couldn't get it to pick up at all, tried with a JJH needle which is richer that the 3 suggested and was immediately better.
On one ride out the engine felt quite zingy on some of the quicker sections, at home the plug was lean, so set the main at 128 and moved needle to T4.

I did find the engine would starve at long periods at full throttle, MB suggests that these carbs need 'blueprinting' to improve fuel flow.
i drilled out the float valve to 3.0mm but the bloody drill snagged and damaged the brass float jet body.
Tried a 30mm OKO from a guy in the club, but no matter what i tried i couldn't get this set up, i think it was just too big for the engine.

Got another 28mm off Ebay, drilled out float valve to 3.0mm successfully and used all previous jets, and engine was good again.

i have fettled a little bit and ended up with 135 main/42 idle and JJH at T3, mainly to improve lower end pick up and top end richness. Moving needle to T4 previously to improve the weakness caused to be a bit boggy mid range.

One thing i did find that transformed the engine is using a DC CDI from Anthony Tambs. Runs great throughout the range with the improved spark.

24mm was non power jet, all the 28mm's i've used have power jets.
24mm was open mouth, I've used breathe sweet filters on an elbow on the 28mm carbs, mainly to stop the oil spitting.

Thank you for the response

Most of the feedback from two-wheel Forums that I have read about PWK carburettors is good, specifically from those that adopt a methodical approach in getting them dialled in. My belief is that they have very advantageous features in respect of the venturi & slide which is why fans claim that they “punch above their weight” to such an extent that a Ø 30 mm will be the equivalent of a Ø 34 mm from other manufacturers. The fact that Polini have bought into the concept with their own licensed manufacture, & fits them on their own engines, signifies how convinced such a forward-thinking company must be of their benefits to their product range.

As mentioned, all of the carbs we have check out as far as being “blue printed” (orifice sizes etc) & although I have modified the only one metal float bowl I have, the float stops have been ground out with my trusty Makita file. I am aware that some of the Chinese copies lack the quality of properly, licensed manufacture to Keihin standards, but we have not bought into that type, fortunately.

The pilot jet sizes we have range upwards from 35 but it appears we may need smaller.

The needles we have include JFH & JJH so that should get us in the correct range.

The main jet sizes already acquired covers every likely requirement.

However, as yet, we still have no information about likely power jet sizes, whereabouts they influence the throttle opening or their influence on the main jet size, though presumably their number (size) should be subtracted from the main jet number……

I have liked these carbs for a long time, have a 26mm on a rapido, moved down from 28/30 to gain mpg these are non powerjet but I also have a 24mm powerjet version for future use, all the 24's I have seen seem to be PJ. the powerjet does not reach into the venturi at all so I presume would only work at very high throttle opening. I have seen powerjets for them on sale but when I removed mine there was no marking to tell what it was (they are very small) so would be a bit unsure how to dial one in

I'm sure I read that the power jet fitted to these needs to be blanked off for 2 stroke use, no idea why though. I blocked mine off when I tried one on a TS1, it ran pretty well but I was nervous about the small float bowl and put a PHBH on instead.

"As mentioned, all of the carbs we have check out as far as being “blue printed” (orifice sizes etc) & although I have modified the only one metal float bowl I have, the float stops have been ground out with my trusty Makita file. I am aware that some of the Chinese copies lack the quality of properly, licensed manufacture to Keihin standards, but we have not bought into that type, fortunately."

where are you sourcing your original Keihin/PWK carbs as they are not cheap?

Paul_from_Thornbury wrote:"As mentioned, all of the carbs we have check out as far as being “blue printed” (orifice sizes etc) & although I have modified the only one metal float bowl I have, the float stops have been ground out with my trusty Makita file. I am aware that some of the Chinese copies lack the quality of properly, licensed manufacture to Keihin standards, but we have not bought into that type, fortunately."

where are you sourcing your original Keihin/PWK carbs as they are not cheap?

Although I do have a genuine Keihin PWK bought secondhand, all of the others are made under license to their standards by the likes of OKO etc, & none of what we have are unbranded Chinese copies. My mate obtained his brand new 34 mm for about £20 from eBay & it is fine, with it's hard plated slide, an item which cannot be bought separately for that kind of money!

For the tuning of the carbs, sets of jets have been obtained, which, although they may also not be Keihin, the fact that they are from the same manufacture, means a fair degree of consistency IMHO.

However, needles are genuine Keihin, I believe.

Which leads me to ask where you obtained your power jets & what sizes do you use, please?

There's a shop in Chesterfield called Splatshop who stock genuine Keihin parts , interesting how you say you are using pilot jets in the 30's , I have just fitted a genuine Keihin PWK 28mm on my Rapido 225 after using OKO's before and I'm using a 48 pilot jet and may well go higher to get a regular tick over , mine isn't power jet though ?

drivera wrote:There's a shop in Chesterfield called Splatshop who stock genuine Keihin parts , interesting how you say you are using pilot jets in the 30's , I have just fitted a genuine Keihin PWK 28mm on my Rapido 225 after using OKO's before and I'm using a 48 pilot jet and may well go higher to get a regular tick over , mine isn't power jet though ?

Neither of us have yet ventured into trying power jets, which was why I asked about them on here. In any case, I don't think they have any effect or overlap within the lower range.

As far as pilot jets go, I have heard of Lambretta owners with a 32 fitted.

Much depends on the engine inlet timing as far as piston ported cylinders go, though I don't have my own figures to hand (uncut TS 250 piston x 61 mm stroke) but likely less than yours! Which should mean you could be running pilots in the 30's too, but we are talking about Lambrettas here, that seem to vary wildly, even if there ought to be a consistency in comparison

I do endeavour to set all the engines I build the best that I can, as I reason that you always have to ride through the lower range, even if you are WOT a lot of the time, where you simply cannot risk any tendency to running lean. Based on my own experiences, a lot of Lambrettas are set up richer than they need be, but that is just my findings. Don't forget that our use of air filters does have quite an influence on jetting, principally to stop the left leg of our baggies(!) getting soaked in fuel....