PWK jetting, running through airbox
Posted: Sat Aug 08, 2020 1:40 pm
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
Or
PWK 30 mm
32 pilot
Air screw 1.5 turns
JJG needle 2nd clip
132 main
Both with Power jet blanked off not used.
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
Or
PWK 30 mm
32 pilot
Air screw 1.5 turns
JJG needle 2nd clip
132 main
Both with Power jet blanked off not used.