Power jet tuning in Keihin PWK

yes, it's basically a 2nd main jet that is open and closed by a solenoid that is controlled by the ECU.  As you said, it is open during high throttle openings  up to a certain RPM and then closes.  This allows the mixture to be rich under heavy load acceleration and then lean out on top so that you get nice clean overrev.  People generally block it off when they are running the carb on a bike that does not have the electronics or connections to support it.  You can disconnect the wires and try the bike and see if you even notice a difference.   I believe it is "normal-open" so when disconnected it is flowing all the time and does not close on the extreme high-rpm.

You can play around with tuning it.  You have to remove the bowl to change them so sort of a pain.  The actual jets themselves for the Power Jet circuit are the same jets Keihin used in the FCR carbs on the 4-strokes for the leak jet.    However, they work opposite.  On the 4-stoke, smaller jet was richer since it was a bleed off (leak) and on the 2-stroke smaller jet is leaner since it is direct in the fuel path to the carb throat.

What are you trying to accomplish, cleaner throttle response, more over rev, etc ?

I bet if 95% of the guys using a carb with a power jet were to disconnect it and block/ plug the jet itself, they would never know, except for how much cleaner the bike ran. 
If you you’re going to keep it,put the smallest jet you can in there. I usually use a 38 or 40 power jet in, and jet from there on YZ250’s. 

I have to admit, I've never really played around with oxgenated fuels other than VP 2t, which is 6% O2, and even then, I always used it as a blend with pump fuel. I've always looked at it as an insurance against detonation, rather than a performance enhancement.  The vast majority of time I ask people to use non oxygenated race fuels, simply because they are more accessible. 

I use the leanest power jet. FGR is correct in saying the power jet and Leak jet are the small jet physically. When I need them, I usually purchase them under a genuine Keihin leak jet part #, as opposed to a manufactures power jet part #. They are typically cheaper that way. The leaner power jet lessens the effect of the power jet overall, which in turn makes the bike overall easier to jet. typically.

The newer YZ125's that use the shared PWK's with the 250, are using a different philosophy on how to jet them, that I'm not really a fan of. They're using huge pilots and small mains, with an odd needle to compensate for the lean slide. It's a total cost saving measure to share the carb with the 250, similar to what KTM did a few years ago on their carb bikes.

With the #60 power jet in there, the bike is going to run richer overall. The #35 power jet that is in there currently(oem) is the leanest power/leak jet that Keihin makes, as far as I know. Yamaha has already done some of the work for you by specing the smallest power jet that Keihin makes. They know, but I'm sure the bean counters at Yamaha are making them share that carb with the 250. Make no mistake, I'm a fan of the Keihin in the Yamaha 125, just not the setup they have in there now.

Digger, on the new gen YZ125 I've had good luck with the JD needles.  But if you are going to do it right and not try to work around the lean slide, what slide are you using?

Ultimately, it depends on which needle I’m going to use. Everything gets tuned around the needle. It is the most important tuning aspect in the Carb, IMO. 
Yamaha has a very good duel taper needle in their N3C/Dx needles, but the reason they can’t use it in the new Keihin setup in the 125 is solely because of the slide, which is the most expensive part to replace, spec in the carb. I’m sure I could get that series to work in the 125 starting out with a #6 slide and working it leaner. I would probably start out with a G starting diameter and again work my way leaner as necessary.  It may take me a little time, but I’m sure I could get to work with a Keihin needle. 

How is JD going about jetting the newer yz125 ? Are they using a more traditional setup, smallish pilot and larger main ? 

Yes, JD kit with their needles calls for 50 pilot and 170'ish main.

IMO, It's a compromise needle for the lean slide. The way Yamaha went about jetting the bike, huge pilot, small main, rich needle, can be made to work, but I'm not a fan of going about jetting that way. I'm more of a traditionalist when it comes to jetting. I would be getting rid of that oem(NYxx) series needle and installing something that I am more familiar with, in terms of how I would expect the bike to run. I have certain needle series that I like to use in certain displacements, depending on how I want the bike to run. The Keihin equipped Yamaha 125's bikes seem to like the single taper DCL or K needles, along with the leaner pilots and richer main. 

Digger , if you were to start from scratch for 6000 feet altitudes what would you do?

Im Also considering an Apex head with an xc dome 

I worked on a 23’ 125x during the fall this year, 60 deg temps, 500’ of elevation, and I wound up with  a,CCL needle in #3, a #6 slide, 52 pilot, but I had the air screw cranked in, 1 turn  out, with a 182 pilot and 35 power jet, but he was using the stock head.  I forgot the specs, but that XC insert has a small combustion chamber than the stock head.  
So, for 6000’ and similar temps, I wouldn’t change a lot.  The only things that I would change are the the main, drop it down to a 178, and the air screw setting, back in out to 1.75, And That’s it. The slide is a must change item thought. It allows you to have way more jetting options than the oem 7.0 and 6.5 slides that come in the YZ and YZx bikes. 

thanks Digger i will go that way along with dremeling the tabs on the airboot. maybe a tooth larger rear sprocket.

This is a really good forum for jetting YZ 250 and other bikes with keihin carbs with the power jet.  https://www.thumpertalk.com/forums/topic/1237673-power-jet-40/ 

I recently picked up a 2000 YZ 250 and the PO told me he was fouling plugs running pump gas but not with race fuel.  I just took apart the carb and the power jet is a 55 with the stock needle on the 3rd clip.  From what I understand the power jet and needle work hand in hand together just like the pilot jet and air screw.  The current jetting specs on this carb are likely the main reason the bike was fouling plugs.

So the solenoid valve is supposed to close power valve circuit when powered.

Today out of curiosity I measured YZ125 2022 as following:

~24 Ohms resistance, 12V click test ok.

Input voltage 0,5V at idle and 23V after 1/8 throttle. Connector plugged I got ~7V at rpms

Isn't it working the opposite as it should? Has anybody else measured input voltage at idle/rpm's?