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PWK jetting, running through airbox

Posted: Sat Aug 08, 2020 1:40 pm
by Lukemx82
Couple of Lambretta setups and my experience using PWK / OKO style carbs through the airbox.

Couldn't find many setups like mine on forums when I was trying to jet them, yet I think running carbs though the airbox is becoming more popular so thought I would share my experience.

Chose this carb because of:
- its compact size and my need / want to run them through the airbox,
- good reviews and wide use on modern motocross 2 strokes, and trials bikes of similar capacity,
- availability of the Kyajet app to aid me in jetting them and gaining understanding, bit of a beginner so a visual tool is nice!

Found the CMD Kyajet tool on website very useful, with only 3 caveats (not sure if the later version of iphone app cover these differently than the website version)
- when jumping from different carburettor sizes on the same bike the kyajet chart shows no effect on main jet size - I found I needed to drop 3 main jet sizes going to the 28mm vs the 30mm? This ties up with carburettor theory that a smaller diameter throat will speed up the air and increase venturi effect, requiring a smaller main to balance it out.
- the effect of needle tapers, in my case from a H-- to a J-- shows quite small on the tool, I found it to have a bigger effect on 3/4 throttle and main jet size? - I found my piston port setup would have needed a main jet 2-3 sizes smaller to make the J taper work, however my reed valve setup liked the J taper a little more.
- the website tool doesn't cover pilot jets below 35 - all of my setups required pilots below this, so I entered incorrect pilot sizes on the tool to still compare effect of raising/dropping pilot jets.

Pilot jets - despite finding some very convincing articles saying pilot jets of 40 and above need to be used on these PWK carbs, I found pilot jets in the low 30's were required - despite trying the larger pilots and giving it a fair shot on all of my setups I couldn't get them to work like this, I needed pilot jets in the low 30's on both piston port and reed valve setups.
The main problem I found with big pilot jets was that I had to turn the idle adjuster in quite far just to get it to tickover and to stop it loading up - this had the knock on effect of when running and backing off the throttle the revs would hang, as the slide was being held open by the high idle adjustment, and this hanging would mimic a too lean setting and throw me off the scent. The high idle adjustment actually put me more onto the needle at 0 throttle rather than the pilot.
Arriving at a pilot jet in the low 30's through normal pilot jettting logic, using a good ear and counting the amount of airscrew turns required, I found I could set the idle adjuster nice and low, and then when running and backing off / shutting the throttle the revs dropped correctly, and still a nice blue haze coming from the exhaust at tickover when looking back in the mirror.

Blueprinting - I followed some other articles on this and did blueprint the pilot jet outlet hole into the venturi opening to 0.8mm, so I could be sure it lined up with jet block and flowed enough pilot jet mixture. I also checked the small 1mm hole in the jet block just infront of the brass needle jet neck, some carbs this was blocked/ not drilled all the way through. I also opened up the float inlet hole diameter to 2.5mm, side holes to 2.0mm and did a fuel flow check as many others have advised for all PWK style carbs.

Float height - after setting it to 19mm as per the PWK norm, I bought a clear float bowl and saw that when set to this it actually gave a really high fuel level in the bowl, above the float bowl gasket itself and pretty much at overflow spill point.
- setting it to 21mm gave me a fuel level closer to the float bowl gasket height.

SR 190 small block
suzuki ts piston
64 x 58 116 rod
Piston port
Running through wide neck airbox with stomach removed.
BGM free flow filter
BGM pwk wide airboot
Taffy style ss pipe
32:1 Motul 800
PWK 28
32 pilot air screw 1.5 turns
HKG needle 2.5 clip (washer)
122 main
Float height 21mm
Power jet blanked off not used.

SR / KBA 200 large block
suzuki ts piston
66 x 58 116 rod
MB shorty reed valve (derbi 6 petal, polini 360 inlet manifold), with 15mm wide boost port and 360 inlet timing
Running through wide neck airbox with stomach removed.
BGM free flow filter
BGM pwk wide airboot
Taffy style ss pipe
32:1 Motul 800
PWK 28 mm
30 pilot
Air screw 1.0 turns
JJH needle 3rd clip
125 main
Float height 21mm
PWK 30 mm
32 pilot
Air screw 1.5 turns
JJG needle 2nd clip
132 main
Both with Power jet blanked off not used.

Re: PWK jetting, running through airbox

Posted: Sat Aug 08, 2020 2:35 pm
by Adam_Winstone
Interesting read, with nice detail, thanks.

My 34mm Polini Keihin copy is still giving me issue, with it not running happily on the standard needle range from the smaller body size, as per the 28 and 30mm PWK/ OKO / Polini / Keihin models. I'm putting my issues down to the different body, which should use the same needles, to being somewhere between this and the next body size up, which uses a different series of needles. Thankfully, I've got to the stage where the carb is jetted sufficiently well to allow me to take it to be dynoed later this month, which should be interesting as it'll tell me much more about how the motor and carb are functioning, with the hope that it'll leave with decent setup across the throttle range.

Thanks for the detail post / info.


Re: PWK jetting, running through airbox

Posted: Wed Aug 12, 2020 3:20 am
by vegansydney
A very informative post and the most interesting thing I've read on here recently. Thanks for sharing!

I'm also running a couple of TS185 conversions though the air box with BGM filters and BGM 'armadillo' air hoses, albeit on Mikuni TM24s. I also found most of the widely available knowledge on the TM24 to be unsuitable for piston ported applications running Clubmans. According to my experience (and few others, all at sea level) pilot jets need to be smaller (10, 12.5), atomizers need to be richer (454 Series Q3 or Q4) and slides leaner (4.0+) than what the TM24s come with from the factory. This is at odds with most of the available recommendations online, but verified through multiple jetting runs with a wideband UEGO sensor and complemented with plug chops.

Out of interest, what manifold did you use to align the PWK with the airbox? BGM flange type? Mito flange type? Something else?

Re: PWK jetting, running through airbox

Posted: Thu Aug 20, 2020 10:29 am
by Lukemx82
Thanks, good to know that you found similar and was also able to back it up with the wideband sensor readings.
Would be good to get mine on the dyno at some point just out of interest to see the A/F ratio, but like you I have done plenty of plug chops and therefore happy that im in the safe zone.

I used a scootrs type inlet manifold on the small block piston port setup, that positioned the carb low and horizontal to align with the BGM air boot nice and easy.

I had more difficulty aligning to the airbox on the large block MB shorty reed valve setup. For that I had to buy the Polini 360 reed manifold for the Derbi/AM6 to be able to twist the angle of the manifold so as to point the carb back towards the airbox at the right angle.

The one piece MB viton rubber manifold that come with the shorty reed, depending on which way I fitted it either pointed the carb to far forward or up at too steep an angle to mount the airboot, so I guess it is designed more for an open mouth setup.
Ive seen now that MB sells the shorty reed valves in different combinations, so anyone planning to run through the airbox could buy just the shorty reed aluminium block and derbi reed valve without the rubber manifold, and then buy a manifold of their own choice. Both Malossi and Poloni do 360 type rubber manifolds to fit the Derbi/AM6 reed that MB uses, and you can get them in lots of different carb mouth diameters giving you plenty of choice.

Re: PWK jetting, running through airbox

Posted: Sat Aug 22, 2020 11:08 pm
by Smudge
Thanks OP, that is informative. I got a cheap 28mm knock off and blueprinted it, put the jets suggested by the BGM RT kit manual and had to have the tickover screw fully in to get it to do that. (nearly quarter open!) I'd already sussed that PJ had to come down which I had started to do but then I snapped the last original part of the indian engine..kickstart shaft. Thing is that whilst I was waiting for genuine Jets/needles to come I gave the £20 carb a go anyway and it was sharp to accelerate, dropped to tickover fairly...I could not really fault it at all but went with the perceived wisdom when it comes to jetting. I'm just pondering that maybe I'll just put the cheap chinese jets/needle back in and leave it at that, seemed fine with the front end lifting and starting first kick. Maybe we are fools led by the blind!

Re: PWK jetting, running through airbox

Posted: Sun Aug 23, 2020 12:03 pm
by Warkton Tornado No.1
An excellent Post & with regard to the sizes of pilot jets, very similar to my own experience but for one thing.......

Piston ported engines inevitably shuffle the mix back & forth through the venturi, picking up more fuel making the carburation richer until the revs get to a level where only one way traffic exists. Consequently, a small pilot jet makes sense.

Reed valve engines should virtually eliminate that scenario.

Indeed, when a PWK was fitted onto a freshly built TS engine, my guestimate of 50 was about right. Previously ithe carb had a pilot of low 30's on a piston ported top end.

I would expect similar of any carburettor, so why your reed valve engine only uses a 32 would appear to indicate something I would not be totally happy with.....

As regards jet manufacture, I have no doubt that Keihin are the best, but if a block of ten pattern jets to cover a range can be bought so cheaply, then it could be argued that it is likely to enable more precise setting simply because there is more opportunity to do so, regardless of the number stamped on the jets. In other words, the cheap set of jets can be used as comparators, rather than even trying to be accurate OEM Keihin clones. As long as testing throughout the rev range is carried out (seat of the pants dyno with plug chops) then all should be well.

Re: PWK jetting, running through airbox

Posted: Wed Aug 26, 2020 9:04 am
by Lukemx82

Yes this is the dilemma I found myself in when trying to jet the reed setup. Most widely recognised reed jetting knowledge comes from everyones ts1 experience, yet the ts1 inlet is on the opposite side of the scooter and therefore doesn't run through the airbox.

So comparing a 50 pilot on a ts1, and reading everyones justification for it, got me into a mess with the reed setup and made it more difficult to jet than the piston port.

Like everyone else, I atributed the large pilots people are using on ts1's to the reed valve stopping the piston port backwash pilot inrichment, and not atributing it instead to the ts1 running an open mouth rhs inlet, vs me running lhs through the airbox.

I wasted a few weekends, a couple of fouled plugs, and a ton of fuel trying to make pilot jets in the 40-50 range work on my reed setup, reading too much into ts1 jetting and ignoring all the seat of the pants and jetting from scratch logic that I learnt a year ago when I setup the piston port scooter.

Thats what lead me to adding my experience into this thread.

When I threw out all of the ts1 / reed assumptions of big pilots, I started from scratch, and arrived at a 32 pilot again through correct pilot jetting practise of air screw adjustment and ear.

And to be honest I arrived at a 32 when I wrote the original post, but as the weather has warmed up on the days I have ridden her since, I have since gone down to a 30 pilot on it, as when its warm she could still choke up on tickover using the 32, signature being a dropping tickover and black smoke out of the exhaust.

Therefore looking at the similarities in my piston port vs reed setups I can only assume that the increased intake signal of the extra 15cc's on the reed valve motor, and maybe the larger intake duration, has a greater effect on reducing the pilot size verses the effect a reed valve cutting out any piston port enrichment would have on increasing it?

Even after arriving at similar pilot sizes, like you I had my doubts and had the reeds out a couple of times to make sure they are sealing properly and they are.
Ive even completed a second leakdown test on the reed motor, including the reed valve and manifold, and she is still airtight.

Through all of this I would tend to steer away from the assumption that small jetting highlights a problem, and instead go the other way to say large jetting would worry me more. With a well built motor with no leaks, the intake signal should be nice and strong and pull good amounts of fuel through smaller jets?

Agree on the pilot jets from different manufacturers, ive got quite a mix in my toolkit. Also Keihin don't make genuine pilot jets below a 35, so anything smaller and you have to go elsewhere. Im using polini jets in the 30 and 32 sizes.

Re: PWK jetting, running through airbox

Posted: Thu Aug 27, 2020 2:05 pm
by Warkton Tornado No.1
Thank you for your response.

However, your set up is not the norm as in needle & slides to what I have tended to work around. Standard slide & atomisers have always yielded results for me, though there is the element of Kerching that I am avoiding. Me? Tightwad? Perhaps.....

Neither solution is necessarily 'wrong' but just alternative ways @ arriving with a solution.

That is unless the proximity of your reed & lower rev 'efficiancy' is not as capable as, say, the TS geometry.

I avoid the reed valve add ons because of my doubts about their location relative to the piston, but they do work to a great extent. Purely my own gut feeling, possibly without foundation.

It would be interesting to compare PWK set ups of a Quattrini, which I would anticipate to be quite similar to A N Other reed only cylinder kit, rather than an add on.

That information may be on the German Forums, but finding it means reading much of their banter & frankly I can't justify the time! :lol:

Re: PWK jetting, running through airbox

Posted: Fri Sep 18, 2020 9:19 pm
by Smudge
I have the full set of BGM needles, PJs and MJs and blueprinted my £20 cheap copy PWK but have only ever got it to run near proper with the provided set from China or wherever. I will probably change this carb in the next few days anyway because the float needle valve will only seal for a week or so and then I have to clean and reset float height. I could maybe find a Viton tipped one but hey, it was a cheap experiment.