CR250/CR125 PWK Air Striker Conversion – Throw Out The TMX

Tired of having to re-jet your Mikuni TMX carb after fouling plugs over and over again? It’s no secret that Honda’s Mikuni carbs on their CR two-strokes in the 2000 era are poorly jetted from the factory. Not only are they jetted pig rich, but they are finicky when there’s a temperature or altitude change. In the morning you may have a 45 pilot and the needle at the third clip, but in the afternoon you may have to put in a 40 pilot jet with the clip in the second position, otherwise it will cough and blubber.

The new popular mod for 2000 and newer CR125’s and 2001+ CR250’s is swapping out the TMX carburetor for a Keihin PWK Air Striker carb from another bike. These carbs are relatively easy to find, as many other bikes had them. The good news is that they are cheaper than most bolt-on aftermarket parts. Even a brand new one can cost you under 200 bucks. For the difference it will make, many late model CR owners says it’s night and day better. The bad news is that not all of them are the same as far as fitment goes. Read on to find out which will fit your dirt bike.

What Makes The PWK A/S Carb Better?

To simply put it, the PWK Air Striker is just a better overall carburetor compared to the stock TMX, or even the older Keihin PJ. It has the same bore as the previous models, but it’s not the same carb. The Air Striker has quad vents, as well as two fins on the inlet side. There are some other minor differences, but those two are the major ones. It’s designed to prevent bogging when the bike goes over whoops or jumps. The throttle-response is crisp from idle to redline when jetted correctly. CR owners that converted to the A/S rave about how much cleaner it runs from idle to half throttle.

Some say that you can get the Mikuni to run just as well, but it’s not going to be as consistent. Most people that do the swap say that it’s a set it and forgot it modification. So unless you’re going from sea-level to the rocky mountains, you’re not going to be chasing your tail with jetting changes throughout the riding season.

There’s More Than One Style PWK?

I have spent many hours researching this PWK carb conversion, so I decided to write this article to save YOU the time and money of doing it yourself. Since most people are only converting their CR250’s and CR125’s (some put it on the CR500 as well), it’s a little bit easier to determine which ones will fit. This guide will save you the time and headaches you could be having while trying to re-jet your Mikuni TMX Carburetor…

Before I confuse you with all of the different Keihin PWK carburetor models, I’m going to show you how to tell the difference between them. Scroll down to see the picture, and I will go from left to right, describing the differences in length and electronics they have.

  1. The first one on the left has the black screw top, and is the older style “long-body” PWK Air Striker. The body length is the distance from the tip of the inlet to the tip of the outlet, and on this one is 91mm. There are no electronics/TPS on this one.
  2. The second PWK is the newer style short-body Air Striker. It has two allen screws on top, and has a TPS (Throttle-Position-Sensor). The short body length is 75mm from inlet to outlet.
  3. The third one is the same as the second, except it does not have the actual TPS; just the spot where where it would be. The allen head screws on top also mean it is the short-body style.
  4. The fourth one on the far right is the standard Keihin PWK carb and NOT the Air Striker. The Air Striker is identified by the two “fins” on the inlet side, roughly at the 5 and 7 o’clock position. While it is still a step above the Mikuni TMX, it’s not as good as the PWK Air Striker.

Also, all of these PWK carburetors have the same size inlet and outlet diameters. Although their lengths may vary, they will all fit over the same size intake boots. The older and long-body style has the screw-on cap, and the newer and short-body style has the 2 allen screws cover.

All Versions of the Keihin PWK Carbs
All Versions of the Keihin PWK Carbs. Photo Credit: hallsy on ThumperTalk

First:

  • Screw cap (black)
  • Long body (91mm)
  • No electronics/TPS
  • Air Striker Quad-vent

Second:

  • Allen head cap (2 screws)
  • TPS (Throttle-Position-Sensor)
  • Short-body (75mm)
  • Air Striker Quad-vent

Third:

  • Allen head cap (2 screws)
  • No TPS, but has the spot for one
  • Short-body (75mm)
  • Air Striker Quad-vent

Fourth:

  • Allen head cap (2 screws)
  • Standard, NON-Air Striker Carb (No “fins”)
  • No TPS
  • Short-body (75mm)

Now to find out which PWK A/S will fit your motocross bike, you must determine the year of your Honda CR250 or CR125. The 2000-2003 CR125’s have the older, long-body style with no TPS (85mm length). They will need the longer A/S with the screw cap, which is the first one on the left in the image above. The 2001-2003 CR250 also uses the same carburetor (the ’00 already has the PWK A/S). The 2004 and newer Honda CR two-strokes have the newer, short-body (75mm). If you can’t remember that, just remember that if your bike doesn’t have a TPS carb, then you need the older long-body with the screw cap. If it has a TPS, then you want the newer short-body with the two allen screws and TPS PWK carb. However, there are people that have put the long-body on the ’04-’07 CR250 had it worked fine. You just won’t be using the TPS.

There are some people using TPS Air Strikers on their bike that originally did not have it with no problems. They simply do not use it, and it does not seem to affect it. If you just need one cheap and that’s the only thing you can find, you can make it work as long as the length is close to what the stock carburetor is.

What Bike Can I Find One On?

Unfortunately, I cannot go through every single year of each model that had a PWK air striker and which bike it will fit. I’ll just give you a quick run through of the bikes that could have it, and then you will have to determine if it will fit yours. Don’t worry, just follow the guidelines above and you will be fine. Remember that just because the seller says it’s a PWK Air Striker carb does not mean it is. The easiest way to tell is if it has the two fins on the inlet side. So, onto the list of dirt bikes that have it (some years had variations of it, so pay close attention before buying it):

  • ’01 and newer Yamaha YZ250 w/TPS
  • ’98 and newer Kawasaki KX250 w/TPS
  • ’98 and newer Suzuki RM250 w/TPS
  • ’99-’00 Honda CR250 long-body w/out TPS
  • ’02 and newer KTM 2-strokes (some are the Air Striker, while some are the standard PWK, as well as 36mm)

Starting Point For Jetting

CR250:

  • Elevation: 1000ft
  • Main: 175
  • Pilot: 45
  • Slide: #7.0
  • Needle: Third clip position (Cxx/R13xx)
  • Air Screw: 1.5 turns out

CR125:

  • Elevation: 1000ft
  • Main: 180
  • Pilot: 50
  • Slide: #5.5
  • Needle: Third clip position (Dxx/R14xx)
  • Air screw: 1.5 turns out

The 1999 CR250 A/S is the easiest starting point for the CR125 because it has the #5.5 slide. It is richer, and 125’s require richer jetting than a 250 with the same carburetor. The best starting point for CR250’s are with the ’00 PWK A/S, as it has the leaner #7.0 slide.

Do I think a Mikuni can be jetted to run as good as the PWK? Probably, but with a small temperature or elevation change, you’ll be needing to swap jets or messing with the needle again.

I tried to cover everything you need to know and make it as easy to follow as possible. If you feel there is some information that is missing or should be added, feel free to comment or send me an email.

-Tom Stark

Why Does My 2-Stroke ‘Load Up’ After Sitting A Few Minutes?

“Loading up” is a common problem for 2-stroke dirt bikes and snowmobiles. If you don’t know what it means or haven’t heard the term before, it’s usually when you let the engine sit or idle for more than a minute and when you go to rev it, it coughs and sputters. This feedback makes for terrible throttle-response, and will eventually foul the spark plug.

To temporarily fix this, assuming the bike is fully warmed up, give it a few quick bursts of WOT (Wide-open-throttle). This will clear it out, and should run like normal again. However, for those of you that want to fix this problem long-term, you’re going to have to spend a little more time, as well as a couple bucks…

The reason your two-stroke ‘loads up’ is because not all of the gas gets burned through the cylinder, so it just sits in the crankcase. When you give it some throttle, the fuel/air ratio is too high, causing it to blubber and sputter until it clears out this excess fuel. When there’s too much fuel going through the cylinder at the same time, the spark plug cannot burn all of it, thus resulting in a fouled plug. A spark plug is fouled when the jetting is too rich, and the tip of it will come out black and often wet (Click here to read more about Jetting).

Honda CR125 is a common bike that loads up.

So how do I prevent this from happening? The pilot jet is almost always the culprit. Many two-strokes (Honda for the most part) come jetted rich from the factory. This is probably to prevent engine failures, but it often causes more problems and headaches. A properly jetted dirt bike will have more power, much better throttle response, and no plug fouling.

Since the pilot jet is too rich, pull the float bowl off of the carburetor and unscrew the pilot jet. It will usually have a smaller number stamped on it, such as 42 or 55 (may require a magnifying glass). Go to your local bike shop and buy a couple jets smaller for it. Don’t worry, they’re only a few bucks each. To get an idea of what size you need, first check your manual if they have any jetting specs or recommendations. The internet is incredibly resourceful, so check there as well. In the end, every bike is different, so you may just have to try a few jets to see what works the best. Just remember that a rich pilot jet will cough and hiccup, while a pilot that’s too lean will bog off idle. Either one will have poor throttle response, so having the right jetting will go a long ways.

If re-jetting won’t get rid of the loading up problem, there’s probably something else wrong with your bike. Check and make sure the air filter is clean, the reeds are not cracked, the engine has good compression, and you are using fresh gas that has the right Pre-mix.

Have fun, and ride safe!

-Tom Stark