Jetting a PWK 28mm carb for a Montesa 315R

[Chuck.]

Based on some favorable reports I thought I would replace the Dellorto carb on my 315R with a Keihin PWK. Curious what jetting others are using with the PWK on a Trials bike.

[pitbulldoug]

Chuck This should be interesting,I'm curious as to how this shakes out for you?How much $ etc.new airbox,intake boots replacement?keep us posted

[laser17]

Chuck This should be interesting,I'm curious as to how this shakes out for you?How much $ etc.new airbox,intake boots replacement?keep us posted

The jetting used on GG's around here is:

122 main

42-45 pilot

3.5 slide

JJH needle - middle clip position

The biggest difference on a GG is the bottom end is smoother - less pop and surge on the pilot jet and has more mid range and top end. Very linear feel overall and very light slide spring. My only complaint is that the 28mm is a bit big on the 250 and while the low end is really clean running, its alittle soft on response - but that is splitting hairs. A must have on the 280's and 300's IMO.

You might also consider the dellorto flat slide VHST. 26mm on a 250 - hence better low end throttle response and smooth running like the keihin on the bottom. Not as much up top. Probably alittle easier to fit though. About 40% of the cost of the Keihin. Might also check out OKO's if you like the keihin design and want a low cost solution.

[liviob]

After the install if you decide that you want more bottom response you can add a Boyeson power wing for a KX 85 to the intake of the carb. I did this to my 290 EVO and got a nice boost in bottom response.

[Chuck.]

My thought was to clean up the bottom end, but improvement in mid range and top end would be nice. I have Boyeson reeds and based on some things I read on Trials Central I thought this would be the next step.

Maybe an OKO 26mm is a better solution?

The Power Wing is a nice idea, thanks.

[laser17]

After the install if you decide that you want more bottom response you can add a Boyeson power wing for a KX 85 to the intake of the carb. I did this to my 290 EVO and got a nice boost in bottom response.

Good to know on the power wing:thumbsup:

[jeddclampette]

The jetting used on GG's around here is:

122 main

42-45 pilot

3.5 slide

JJH needle - middle clip position

Would you happen to know a good jetting baseline to go w/ the Keihin 28mm PWK (from '06 YZ85) for an '02 Sherco 2.9 ?

[laser17]

I think the above would be close, but I suggest either calling RYP or doing a search on TrialsCentral to get more exact info.

[ccullins]

The jetting that Laser gave is the same as we use on the Sherco's with a Keihin.

[Chuck.]

I checked the Kawasaki and Yamaha sites for 80/85 jetting (PWK 28 carbs) and they use much larger jets but then I realized that they run much more oil.

[Chuck.]

I started a thread on the Intake-Jetting forum asking about jetting https://www.thumpertalk.com/forum/showthread.php?t=1036036 but not much activity yet.

After checking with Boyesen on X-Wing fitment I bought a PWK28 from a 03 KX85, it has stock jetting:

Main: 140

Pilot: 45

Needle: NAPE (multi taper)

Clip groove: 3

Slide: 3.5

A recommended jetting for a Gas Gas conversions:

Main: 122 main

Pilot: 42-45 pilot

Needle: JJH (straight taper Sudco)

Clip groove: 3

Slide: 3.5

My research indicates the Gas Gas jetting is stock aftermarket Sudco PWK jetting and is much leaner than KX85 jetting. This could be because of the difference in oil ratio between the KX and Trials bikes, just don't know.

My research also shows that carb tuning is more complex than I assumed. The Sudco needles have a unique ID system and are straight tapers, the OEM needle that I've found use a four letter designator and have multi tapers. There is also a thread on RM tuning that suggests using different needle tapers to fine tune throttle response at different engine speeds. https://www.thumpertalk.com/forum/showthread.php?t=978866

Here is a thread on 2T part throttle ping https://www.thumpertalk.com/forum/showthread.php?t=1033583

IMO a big question is the degree that oil mixture ratio has on jetting. The reason is the KX85 runs 32:1 while the Trials bikes run 80-90:1. Is that enough to explain the big difference in jetting?

[ccullins]

The KX and Montesa are completely different motors that run at completely different RPM ranges. It's like comparing apples to oranges. One is a low revving 250cc and the other a very high revving 85cc. Jetting differences have nothing to do with oil mixtures.

Go with the recommended trials jetting, I think you'll find it very close to what you'll need. From my experience all the different brand trials bikes are jetted in the same ball park.

I buy the KX carbs myself when putting a Kehein on the trials bikes. I pull out the stock KX jetting and go with the recommended jetting and it's always within a jet size or two.