WR250 (ZD) Carb boot/mount/flange options

I have a 1992 WR250 motor that I am swapping into another frame and am running into some carb fitment issues. This is the stock Mikuni vm 38 carb and with the stock boot, which seems to be about a 45 degree angle, the back corner of the carb hits the shock mount. This is without an air cleaner. I am hoping there some aftermarket/factory interchangeable options for different angled boots. Possibly different years YZ250/YZ125 or other models with the same boot mounting and same inner diameter flange.

Not finding much from my Google searches, likely due to the age of the bike, hoping to tap you brains for solutions.

Worst case, are there options for a thinner carb (possibly flat side) that I could swap for to get me room without cutting/moving the shock mount?

The vm38 is 4.57" inches in full length.

Should have pictures in the next few days.

Thanks

Edited by Doubledown

Here is the picture.. Any ideas for solutions aside from cutting and moving the shock mount are appreciated.

65d645e9.jpg

gonna have to give me some time to play around but I say a 79 yz 400 carb boot (shorter sharper angle I beleive) and a 38 mm flatside(shorter end to end) might be the ticket I have a few different carb boots and mikuni carbs remind me for the weekend to see if I can kinda measure out how each one would set

That would be awesome. Really hoping something will work out. I wouldn't mind spending a couple hundred bucks on a carb that would fit and would actually be an upgrade like a flat side rather than cutting frame and swing arm to move the suspension back.

My next post is to find which swing arms (possibly yz) might fit this thing.. From what I can find the yz125 and yz250s have used the same swing arms spanning different years. I am looking for options that will fit or cut down to about a 6 5/8" pivot point width. Any direction where to find dimensions for the different years of swing arms (and if they are shared between different models e.g. yz125 and yz250) is appreciated as well.

gonna have to give me some time to play around but I say a 79 yz 400 carb boot (shorter sharper angle I beleive) and a 38 mm flatside(shorter end to end) might be the ticket I have a few different carb boots and mikuni carbs remind me for the weekend to see if I can kinda measure out how each one would set

here is a ebay ad/picture of the 79 yz400 boot that I found. (ad# 350456945934)

it does look like it is a much more drastic downward angle.. is there a compensation with the floats I would have to make for something like that? I am guessing those engines had a more forward slanting head the these would keep the carb level? wondering if this would make the carb, or a flat side, not work correctly?

also.. are these all really $100+ seems crazy for such a part.

gonna have to give me some time to play around but I say a 79 yz 400 carb boot (shorter sharper angle I beleive) and a 38 mm flatside(shorter end to end) might be the ticket I have a few different carb boots and mikuni carbs remind me for the weekend to see if I can kinda measure out how each one would set

Did you happen to get a chance to get out to the garage?

Thanks

a Keihin PWM is much shorter than the VM38. You can also find shorty PWK's off of newer YZ250's (i think yz's, i bought on a while back). These use the PWK slide, but are the same dimensions of the PWM, not to mention they are MUCH cheaper than PWM since you can find used ones on ebay off of parted MX bikes. Check SUDCO out for exact dimensions.

Also, the easiest way to verify intake flange interchange is to search up reed spacers, find some on ebay and you should get a list of bikes they fit that corelate with your WR. You can also get a RAD valve for the WR which has a short tube off the intake and you can put an angled hose on that, might not be ideal for power but would give more options.

a Keihin PWM is much shorter than the VM38. You can also find shorty PWK's off of newer YZ250's (i think yz's, i bought on a while back). These use the PWK slide, but are the same dimensions of the PWM, not to mention they are MUCH cheaper than PWM since you can find used ones on ebay off of parted MX bikes. Check SUDCO out for exact dimensions.

Also, the easiest way to verify intake flange interchange is to search up reed spacers, find some on ebay and you should get a list of bikes they fit that corelate with your WR. You can also get a RAD valve for the WR which has a short tube off the intake and you can put an angled hose on that, might not be ideal for power but would give more options.

Those are definitely thinner.. vm38(116mm/4.57") pwk(83mm/3.27" pwm(75mm/2.95")

So I think either of those will get me at least enough room to get a flexible foam filter on there.

A few questions on those carbs though.:

most important, do these keihins work with the mikuni carb boot? (I would assume use my current carb boot since I would have plenty of room.

Would I need new throttle linkage?

The YZ250 PWK i am seeing all TPS, is this removable for use with my bike?

VM38: Your dimensions are way off i think, i just swapped out a VM38 for a standard PWK38 this week and the PWK is about 1/4" shorter at most. I dont know where those VM numbers came from but i have seen them posted on various sites for a while, and they are off.

TPS: you can remove all of this easily. Very simple to do. All that is left is a small hole in a little cover near the cap that you fill with epoxy, or you can tap it and lock-tite a machine screw in it. I used epoxy. You also have the fuel soleniod opposite the choke. You can just leave it since it fails closed, power to open. Or, you can remove the whole thing and epoxy the hole, if you do that you also need to plug a small port in the bowl flange that pulls the fuel up into the solenoid valve. Easy stuff.

WR250 INTAKE BOOT: The PWK/PWM outlet is about 46mm i think, and the VM is

43mm. It will fit (others have done it and been fine). I took about 2mm off the O.D. of my PWK to make it easier. Easy to do in a small lathe, doesnt have to be centered or zeroed in the lathe, just clamp a three jaw on the big end.. LIGHTLY, put a steady rest against the outlet(side you want to machine), i cut it at about 250rpm, slow for aluminum and light cuts but safer!! It fits great on the stock intake boot on my 94

WR250.

THROTTLE LINKAGE: All depends on your carb top. My PWK had a brass peice on top that the cable just sat in, no threads. THe fitting was pressed into the cap, no threads.

To use the WR250 cable i had to take the circlip off the VM cap and pull the little 45* elbow out, the stock WR cable threads onto this elbow to adjust the free play. I then drilled out rivet on the brass PWK fitting, then i had to remove good portion of the spring guide inside the cap so i could put the circlip back on the VM fitting in the PWK cap. I also drilled out the PWK cap just a tad so the VM fitting would slip in the hole. I had a few rubber throttle cable caps laying around(small, shaped like a bullet) and i slipped one of those over the VM eblow, this seals it up and keeps the elbow circlip tight against the cap. If you have a threaded top on your PWK(some standard PWK's do, not all, the TPS pwk and all PWM have threaded tops) then just go to your local moto shop and get a threaded elbow, threads on both ends and you are good to go! I should have just bought a threaded cap but i tried to do it with what i had. I think you can use the large rubber boot that the mikuni used to go over the whole thing when your done.

VM38: Your dimensions are way off i think, i just swapped out a VM38 for a standard PWK38 this week and the PWK is about 1/4" shorter at most. I dont know where those VM numbers came from but i have seen them posted on various sites for a while, and they are off.

TPS: you can remove all of this easily. Very simple to do. All that is left is a small hole in a little cover near the cap that you fill with epoxy, or you can tap it and lock-tite a machine screw in it. I used epoxy. You also have the fuel soleniod opposite the choke. You can just leave it since it fails closed, power to open. Or, you can remove the whole thing and epoxy the hole, if you do that you also need to plug a small port in the bowl flange that pulls the fuel up into the solenoid valve. Easy stuff.

WR250 INTAKE BOOT: The PWK/PWM outlet is about 46mm i think, and the VM is

43mm. It will fit (others have done it and been fine). I took about 2mm off the O.D. of my PWK to make it easier. Easy to do in a small lathe, doesnt have to be centered or zeroed in the lathe, just clamp a three jaw on the big end.. LIGHTLY, put a steady rest against the outlet(side you want to machine), i cut it at about 250rpm, slow for aluminum and light cuts but safer!! It fits great on the stock intake boot on my 94

WR250.

THROTTLE LINKAGE: All depends on your carb top. My PWK had a brass peice on top that the cable just sat in, no threads. THe fitting was pressed into the cap, no threads.

To use the WR250 cable i had to take the circlip off the VM cap and pull the little 45* elbow out, the stock WR cable threads onto this elbow to adjust the free play. I then drilled out rivet on the brass PWK fitting, then i had to remove good portion of the spring guide inside the cap so i could put the circlip back on the VM fitting in the PWK cap. I also drilled out the PWK cap just a tad so the VM fitting would slip in the hole. I had a few rubber throttle cable caps laying around(small, shaped like a bullet) and i slipped one of those over the VM eblow, this seals it up and keeps the elbow circlip tight against the cap. If you have a threaded top on your PWK(some standard PWK's do, not all, the TPS pwk and all PWM have threaded tops) then just go to your local moto shop and get a threaded elbow, threads on both ends and you are good to go! I should have just bought a threaded cap but i tried to do it with what i had. I think you can use the large rubber boot that the mikuni used to go over the whole thing when your done.

Now you have me worried. haha I pulled my measurement from this site http://www.procycle.us/carb/mikuni_dimensions.htm as the bike is at my "shop" about 30 minutes away. What are your measurements of the VM38?

Here are the PWK dimensions I found http://www.motorcyclecarbs.com/Keihin_PWK_Jetting_and_Dimensi_W71.cfm

and PWM http://www.motorcyclecarbs.com/_W73.cfm

Pulled this from another forum thread.medium.jpg and this is supposedly the key.

PWM -Left.

"Long" PWK- Middle

"Short" PWK- Right.

I am looking for the 38mm PWK "short" which is narrow as possible and from the VM38 number I used from the site mentioned earlier. I thought by going to a PWK thin model I would be gaining 1.3" which would be plenty to squeeze in an air filter and have no frame contact.

That is good news on the TPS, just missed a cheap carb on ebay I was worried about that TPS on..doh

Search up on here for "wr250 pwk swap". A guy changed his VM out and he list the dimensions im pretty sure. I know that i have to put some pressure on the airbox boot to get it to cover the clamp area on my PWK, and i fudge an 1/8" on the motor side so about a 1/4" shorter in the bike. ALso, several filter manufactures sell angled boots for their filters, K&N does for sure. Look into that. Also, for a performance standpoint i have had mixed results with clamp on filters right on the carb, some motors seem to like a longer intake tract. When i bought my shorty PWK i didnt even know it had the TPS. It shares the same slide as all other PWK's though, the TPS is a gear/lever arangement that the slide pushes on when it rises and the gear shaft turns a small pot/reostat that bolts on the outside with a couple small screws. I had it torn off and cleaned up in about 5 minutes after pulling it out of the shipping box. BTW, supposedly KTM uses the shorty PWK a lot. They have some 36mm shorties though so be careful.

Edited by sucmah
Search up on here for "wr250 pwk swap". A guy changed his VM out and he list the dimensions im pretty sure. I know that i have to put some pressure on the airbox boot to get it to cover the clamp area on my PWK, and i fudge an 1/8" on the motor side so about a 1/4" shorter in the bike. ALso, several filter manufactures sell angled boots for their filters, K&N does for sure. Look into that. Also, for a performance standpoint i have had mixed results with clamp on filters right on the carb, some motors seem to like a longer intake tract. When i bought my shorty PWK i didnt even know it had the TPS. It shares the same slide as all other PWK's though, the TPS is a gear/lever arangement that the slide pushes on when it rises and the gear shaft turns a small pot/reostat that bolts on the outside with a couple small screws. I had it torn off and cleaned up in about 5 minutes after pulling it out of the shipping box. BTW, supposedly KTM uses the shorty PWK a lot. They have some 36mm shorties though so be careful.

I think this is the thread you are referring to: http://www.thumpertalk.com/forum/showthread.php?t=782718&highlight=1997+wr250+pwk

I have found a couple different sites listing different dimensions on these carbs also though the above post is the dimensions I will use for the vm38 carb at 97mm/3.82". I read in other posts about the shorty version of the PWK that is was the same depth as the PWM which is listed at 75mm/2.95". Even with this shorter vm38 dimension I would be expecting about 3/4" gain in space with the difference in the shoty PWK and the vm38 at .87".

So you are saying you have a shorty version of the PWK and are ending up with only a 1/4" in over all depth difference in the bike?

Can you confirm the measurement of 75mm depth for the shorty?

Was PWK measurements from later model YZ carb you picked up??

I also read about the intake boot issue, I found a post of a guy who milled down the diameter of the carb, but I figured I could make it work like you did. Possibly use a head gun on there to loosen things up a bit.

I posted http://www.thumpertalk.com/forum/showthread.php?t=983672&highlight= hoping to get some direction in finding what factory bikes have the shorty PWK 38mm, preferably without the tps and powerjet but I am all for filling in some wholes for a cheaper carb.

The other post I found http://www.thumpertalk.com/forum/showthread.php?t=978139&highlight=pwk+conversion may also be one you were referring too. This one is saying a 06 sx250 would be my ideal candidate though doesnt list the bore diameter and I am having some issues finding information on the bike.

I did NOT, repeat NOT use a shorty PWK on my WR. I have a shorty, and it is sitting on a shelf, doing nothing. I used a standard PWK38 on my WR since it fits best in the stock carb to airbox gap. I only mentioned the shorty and my experience with it to help you make the best choice for what you are doing, sorry about the confusion. I dont think the tight fit on the carb outlet will be an issue since others have done it with success, just a new clamp is all. I machined mine since i have access to a lathe and thought it would be fun. I will take some measurements when i have time.

sorry for dropping the ball on you....... the difference between manifolds are pretty impressive between a 2k7(78yz250/79yz400) and a 4da(?) and I can't slip a flat side mikuni into them (meant for the round slide mikuni) the 2k7 manifold appears to be made for a cylinder that is tilted forward more than the 4da

I was going to try and take pictures of the three manifolds I have 2k7,4da and boyeson rad valve meant for a 96 yz 250 with a 38 round slide mikuni and then a 38 flat side mikuni on how they set within a project I have been letting collect dust but I couldnt find my camera........there is also another manifold with a very simlar 4 bolt pattern and that would be a yz 465.....

again sorry for the delay on anything truly comparable

sorry for dropping the ball on you....... the difference between manifolds are pretty impressive between a 2k7(78yz250/79yz400) and a 4da(?) and I can't slip a flat side mikuni into them (meant for the round slide mikuni) the 2k7 manifold appears to be made for a cylinder that is tilted forward more than the 4da

I was going to try and take pictures of the three manifolds I have 2k7,4da and boyeson rad valve meant for a 96 yz 250 with a 38 round slide mikuni and then a 38 flat side mikuni on how they set within a project I have been letting collect dust but I couldnt find my camera........there is also another manifold with a very simlar 4 bolt pattern and that would be a yz 465.....

again sorry for the delay on anything truly comparable

No worries, I am not expecting this project to be done anytime soon haha. Just want to get my ducks in a row. Even with this "Short" version of the keihin pwk38 I may still need to look into other boot options.

What are the difference in the mikuni flat side and round slide boots?

If you come across your camera and feel like snapping those shots it would still be helpful.

Thanks

well, that shorty PWK might be getting pressed into service now :smirk: I have an 09 CR125 husky i race and the TMXX mikuni was way off last night with a 15* swing from last week. It went from ok to deadly bog, kinda scary going to a jump face wondering if your going to get on the pipe or not?? Anyway, the point is that a mikuni TMXX is pretty much identicle dimension wise to a PWM or short PWK. Just another option, and from what i can gather the mikuni runs harder on top than the PWK, gives up a little on the bottom but is more change sensative than the Keihins.

well, that shorty PWK might be getting pressed into service now :smirk: I have an 09 CR125 husky i race and the TMXX mikuni was way off last night with a 15* swing from last week. It went from ok to deadly bog, kinda scary going to a jump face wondering if your going to get on the pipe or not?? Anyway, the point is that a mikuni TMXX is pretty much identicle dimension wise to a PWM or short PWK. Just another option, and from what i can gather the mikuni runs harder on top than the PWK, gives up a little on the bottom but is more change sensative than the Keihins.

Well I pulled the trigger on the shorty PWK from a 2003 KTM sx250 so its the 38 but doesnt have all the tps or powerjet to remove and fill. That should be here in the next few days. Right now I am working on the engine placement to get the sprocket lined up to the rear.. I dont know enough about these 2 strokes and their oiling system so I am not sure how detrimental the angle of the engine is. Where I have it, unwelded/test fit, it is pretty level. I will tack it for now though depending on if this new carb fits right in with a decent filter or not I am wondering if I could tilt it a bit forward, making more room for the carb, without any ill affects for oiling or anything else for that matter.

I will probably make a post on the topic though wonder what your knowledge is.

Thanks

Good thing about the two stroke is you can fix oil level issues easily, just add more to compensate for the high end. The only trade off is the possiblilty of slightly higher windage due to the low end being submerged more. I wouldnt lose any sleep over the angle. What is the motor the PWK is going on? I am trying to figure out what needle to run ASAP in mine, i have a 5,6 and 7 slide and a pile of needles but not sure if any of those needles would work on a modern 125/144??

Good thing about the two stroke is you can fix oil level issues easily, just add more to compensate for the high end. The only trade off is the possiblilty of slightly higher windage due to the low end being submerged more. I wouldnt lose any sleep over the angle. What is the motor the PWK is going on? I am trying to figure out what needle to run ASAP in mine, i have a 5,6 and 7 slide and a pile of needles but not sure if any of those needles would work on a modern 125/144??

well I would say if I were to drop it down a bit it wouldnt be more that about degrees forward. The PWK is for the 92' WR250 engine I bought for my ysr50.

if at all possible I would try to line up the front sprocket to where the original one was.... the oiling system of a 2 stroke is incidental and haphazard compared to chain torque on the rear suspension in my mind not that I would understand either but I can see the difference in applied technology(?) between the two

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