Keihin PKW 39mm or 38 PKW Striker for RMX 250

KDX200Kev · Oct 7, 2005

I read in Eric Gorr’s book that the Keihin PKW 39.5 mm carb is a good mod for the RMX 250. I have a few questions. First, would the Keihin PKW 38mm striker carb be a better choice? Second, is there any issue for fitment on the rubber boot of the engine or air box for these carbs? I assume the 39.5 mm carb that I have seen references for the RMX is the Keihin PKW 39 mm carb. My RMX is a 1996 model, and it has the air lid removed, FMF pipe, FMF S/A, and I am looking at doing the cylinder, and cylinder head performance gasket mod. I used the RMX just for casual Desert riding. Thanks for your input.

KDX200Kev · Oct 7, 2005

I read in Eric Gorr’s book that the Keihin PKW 39.5 mm carb is a good mod for the RMX 250. I have a few questions. First, would the Keihin PKW 38mm striker carb be a better choice? Second, is there any issue for fitment on the rubber boot of the engine or air box for these carbs? I assume the 39.5 mm carb that I have seen references for the RMX is the Keihin PKW 39 mm carb. I just bought a stock 1996 RMX and I am modifying it. I plan on removing the air lid, installing a FMF pipe and FMF S/A. I am also considering doing either the 89 cylinder head gasket replacement or the Cometic cylinder and cylinder head performance gasket replacement? Not sure what the difference in performance will be between the 89 RM gasket and Cometic gaskets. Any thoughts on it would be appreciated. I will use the RMX just for casual Desert riding. Thanks for your input.

rmxtwofitty · Oct 7, 2005

I just installed the 38mm pwk on my rmx. This is by far the best mod I have done yet. I could never get the mikuni carb to respond or Idle very well. I bolted on the carb and the response and overall performance is better. As far as the 39mm, I think it will fit in the air boots.With the 39mm you will get better high end performance.
The other mod I found that helped my rmx a lot was double stacking the base gaskets, and cutting the head the same thickness as the gasket. It really woke that motor up. :) Good luck.

elf · Oct 7, 2005

You won't find much difference between the 2 carbs. The 38 will give you a little more low end power compared to the 39. The 39 might make a bit more peak hp though.

If your stock carb is the pwk 36mm the 38 or 39 will fit right in, the 3 carbs are the same size on the outside and use the same slide and jets.

placelast · Oct 8, 2005

There is no performance diff. between the Cometic HiPerf top-end gasket and the OEM '89 RM250. It's been too long ago for me to remember what the outcome of mic'ing was but I would suspect they were the same thickness. I can measure them for you if you want.

Either PWK should be fine, though they both take up more room than the PJ, making it difficult but not impossible to change the needle position. The key is to rotate the regulator for a slight improvement in access.

The FMF pipe requires a ball peen hammer treatment to make space between the tank and top of the expansion bulge as there is contact there, and at the kickstarter tip. And their T/C spark arrester fails in time at the leading pipe inlet to the main body, plus clearance is not adequate for the tire; washers help push it outboard although that strains the fittings, perhaps leading to early the stress fractures at the inlet fitting others and I have experienced (3x) along with the tire rubbing. It is a pain to open for repacking - good it doesn't need it that often.

The PC setup has none of the above clearance/fitting problems. The bend and angle is such that coming out of the cylinder from the left side to the right is slightly higher off the ground by a fraction of an inch and does make a diff. in rocks. Pipe to S/A fitting is a simple slip fit without the better-designed FMF flange - this is the only place where the FMF may have an advantage, although both can leak; the PC has a perfect alignment.

Performance is similar though slightly diff. Seat-of-the-pants dyno sez FMF has seemingly thicker bottom response whereas the PC is maybe better all-around; they are close. I've never tride a DynoPort. Any system would be far superior to the stock double-walled setup and choked-off muffler, yet test at a reasonable 96db (by the USFS @ Kennedy Meadows, in my case).

FMF has thicker metal, fair plating. PC is thinner sheet metal with a better plating job. For either one use a pipe guard as who knows how much longer they will be available. I have CF on both.

You are welcome to try my FMF setup.

ulmanb · Oct 10, 2005

what year did they start coming stock with the larger diameter carb? I am considering a 1994 RMX and dont know anything about them.

KDX200Kev · Oct 31, 2005

Thanks for the responses. I ended up getting a PKW 39mm. The top of the carb is different then for the PJ. The PKW has a metal cable adjuster on top of the carb where the PJ does not. Did you use the PKW with your standard throttle cable, modify it or use another throttle cable? Thanks for your response?

placelast · Nov 1, 2005

KDX200Kev said:
The top of the carb is different then for the PJ. The PKW has a metal cable adjuster on top of the carb where the PJ does not. Did you use the PKW with your standard throttle cable, modify it or use another throttle cable? Thanks for your response?

Stock cable - no change. Just make sure you end up with the same adjustment or play (a tad or wee bit for when you turn the bars left to right.)

ulmanb: (you asked: what year did they start coming stock with the larger diameter carb?) All carbs were 38mm, be it Mikuni TMX ('89-'92) or Keihin PJ ('93-up).

KDX200Kev · Nov 1, 2005

placelast said:
Stock cable - no change. Just make sure you end up with the same adjustment or play (a tad or wee bit for when you turn the bars left to right.)

ulmanb: (you asked: what year did they start coming stock with the larger diameter carb?) All carbs were 38mm, be it Mikuni TMX ('89-'92) or Keihin PJ ('93-up).

Thanks. Any recommendations on jetting. I plan to ride the high desert area (2000 ft) 60 to 80 F with the following mods FMF pipe and S/A, aftermarket reeds, 39mm carb, and a thicker cylinder gasket and thinner head gasket. I am thinking 178 for the main, 55 for the pilot. I have a DHG needle currently that came with the carb. Just looking for a basis to get me in the ball park. Thanks

placelast · Nov 1, 2005

My jetting specs are off from yours as I have the KDX-ish carb divider.

Call FMF/PC for a base line or if you can wait - send me an e-mail (hit-n-miss@removeToSenddslextreme.com), I can look up some past articles but you are best served with the two pipe companies' recommendations for the baseline you are after.

If you can swing it get the BRP top clamp to move the out of your lap/side (forward and up), and if range isn't a concern, a '89-'92 RM gas tank, and tall seat foam for an improved cockpit.

KDX200Kev · Nov 2, 2005

I plan to get the RB KDX-ish carb divider, and tall soft seat foam and gripper seat cover from Guts. Have this on my KDX and really like it. I believe you sent me the comparison write up article of the 1997 KDX 220 and RMX 250 when I was considering buying a disassembled 1990 RMX for $500 a year ago or so. :cool: Anyway I finally found a 96 RMX for $1000 that had the original tires on it with almost no wear on them. :) Have to replace the tires since the front cracked from age and I believe the rear will soon. Anyway I stripped the bike all the way down to check it out. Its missing some bolts and washers but nothing major. Couple rust spots on the frame and boot wear so I will touch that up with paint. I changed out the fork and shock oil per manual instructions. Still have to have the shock nitrogen charged and change out the brake fluids in addition to new fluids for the trans and rad. The steering stem bearings I still have to replace. I order the BRP triple clamp with the Scotts steering damper and Pro Taper bars. Also new Ceet decals and shrouds, and Baja skid plate. The person I bought this bike from bought it from his friends father less then a year ago who didn't ride much. He said he put in a new piston and clean the power valves and rode it about 20 times but he's more a ATV rider. I don't think he rode it much, since the lower steering bearing was rusted bad and resulted in very jerky steering. He said the rear shock needed a stiffer rear spring. When I took the shock off I noticed it had air in it since when you compress the shock with out the spring on it it would go easy and then compression valving would kick in. Anyway after my oil change and air bleeding on the shock the compression worked from the beggining of compression. I was wondering if I will need stiffer springs front and back. The Racetech site says I do since I weigh 193 without riding gear. I'm not sure? Thinking when I get the bike back together setting the shock sag and check the static sag to determine if it is too much preload. Any recommendations on springs and tires would be welcome. I only rode the RMX up an down the street a couple of times but I felt right at home on the bike. :ride: Since the bike was stock it felt slower then my modified KDX. Once the mods are done it will look new and be a lot faster than my kdx. To bad there is not an RMX forum. :(

placelast · Nov 4, 2005

Don't get the RB carb divider work done; even Ron (?) told me afterwards, when I was experimenting with jetting, said his work doesn't do much for 250s and my experience backs up his statement. His work is excellent but the results are benefitial for pretty much the KDX only. Plus the jetting throws me off of the mainstream.

You'd be better served zeroing in on correct jetting with a 38mm PWK Airstriker; at the most bore it to 39.9mm if you want more top.

If you find the rear brake grabby to the point of causing stalls, consider a Steahly FWW.

At 193 you definitely need stiffer springs, and later valving when you have the $; stock springs are good for Joe Avg. at 160-170 lbs.

Tires? Pick your favorites. I used to like the 756s since they work everywhere for me but wear too fast. S12s are good in the back but are prone to pinch flats (been there, done that 2x - even with Xtra HD tubes, and it cost me a class win). My last pair were 773s (like 'em, esp. for mud and sand); next I may try the 952s or M5B but do not know what to use with the IRC up front.

There is not an RMX fourm because the numbers of owners are dwindling and few. The Euro mfgs pretty much have picked up the ball with their enduro two stokers; Japan has concentrated on valves and cams as of late.

zerormx250 · Dec 20, 2005

KDX200Kev said:
I read in Eric Gorr’s book that the Keihin PKW 39.5 mm carb is a good mod for the RMX 250. I have a few questions. First, would the Keihin PKW 38mm striker carb be a better choice? Second, is there any issue for fitment on the rubber boot of the engine or air box for these carbs? I assume the 39.5 mm carb that I have seen references for the RMX is the Keihin PKW 39 mm carb. My RMX is a 1996 model, and it has the air lid removed, FMF pipe, FMF S/A, and I am looking at doing the cylinder, and cylinder head performance gasket mod. I used the RMX just for casual Desert riding. Thanks for your input.

It should be able to fit. I own a RMX 96' and i replace the original carb to keihin PWK 38mm. But the only weird thing is,
with the original airbox the feul consumtion was suprisingly high. So my fren who owns the same bike with me suggested to do a rocket scientist test on it... We replaced the original airbox with RM 250 96' model. It was a suprise! The feul consumtion reduced and much powerful and better respond then before. Anyone can explain? or advise?

placelast · Dec 21, 2005

I cannot see how you could swap them as the RMX is based on the '89-'92 RM250 (1/3 of a subframe). The '96 subframe is different, being complete.

I'd sure like to look into that, but there are few '96-'98 RMs around to compare.

zerormx250 · Dec 21, 2005

I know this should be the 97' model but in the vehicle registration card it stated that "RMX 250S 96" ??? I'm quite confused and the other strange thing is our model is different or missing from country like Japan, Australia etc.for example the water pump and several accessories such as rear brake calipre guard...

I can post the pic some other time, maybe can anyone advise what model actually it is...

Another thing to share with, not only the airbox i manage to swop the RM 250 00' tank as well but certain modification has to be done. The tricky part is the tank mounting if you manage to figure out how to fit it on, i can be sure that the rest (RM subframe 96-00, airbox 96-00, seat 96-00) can fit in nicely. It makes a trail bike with MX feel!!! I'll post the pics if anyone interested.

placelast · Dec 22, 2005

Yes, please post pix.

For an MX cockpit I installed a '89(-'92) tank, BRP top clamp (forward mount bars) and tall seat foam. Better than stock although not quite up to modern standards.

joshp · Dec 22, 2005

I have some questions about the Keihin carb. I have a 93 RMX and according to the manual this year should have the Mikuni but maybe the previous owner swapped it out for the Keihin.

Anyway the manual shows the idle adjustment not being a typical screw but being the choke knob. Are you supposed to turn the choke knob? The manual doesn't give any details, it just shows a picture of choke knob for the idle adjustment. If it's the choke knob how do you get it to stay in place? I would think it would change everytime you pull the choke up as it would slowly rotate. I swapped out some jets and the bike is idling too fast.

Let me know thanks.

placelast · Dec 23, 2005

The '93 was the 1st year with the Keihin.

I never rode mine with the stock carb, but had it for a short time on my son's '90. The manual says to rotate the choke knob to adjust the idle; no sooner than it was adjusted it would drift and need readjusting.

If you can swing it, buy a new or used 38-39.5mm PWK (not the Air Striker.)

apb · Dec 24, 2005

why not the air striker?

Placelast, I'm just curious why you recommend avoiding the air striker pwk on rmx250's. I thought the air striker was supposed to have improved throttle response over the standard pwk? At any rate, I just installed the air striker 38mm on my rmx, and it seems to run a ton better than with the old mikuni... But theres a still a little dialing-in to do on the jetting to get it just right. Just wondering if there's something I should be aware of setting it up. Got it off a '97 kx250 for $37, so it was a pretty cheap mod! Thanks as always for the help.

placelast · Dec 24, 2005

The AirStriker is certainly better than the stock PJ - it's what I have on my '97. But I was wondering why it is more finiky than the regular PWK I had on my son's '90 RMX (a real good running bike) and on my KDX - at first I suspected the RB carb mod done to mine.

KDX200Kev recently talked to Sudco and they are recommending the regular PWK saying it is less fussy than the AirStriker, so maybe that's my problem plus my bonehead jetting skills.

Jay25 · May 26, 2006

38mm/39mm pwk

Hi guys,

Where can I find a 38mm/39mm pwk carb for my RMX and will the year of my ride make any diff?

Oh and what would the cost me?

Regards
Jason

apb · May 26, 2006

Jay25 said:
Hi guys,

Where can I find a 38mm/39mm pwk carb for my RMX and will the year of my ride make any diff?

Oh and what would the cost me?

Regards
Jason

You can buy a new one from either sudco or i believe carb parts warehouse, or save a bunch of money and get a used one off that online auction site like I did -- I paid about 38 dollars for a pwk 38 airstriker off a '97 kx250. Works great. I'd recommend trying to get one of the pre-solenoid power jet carbs like I did for the RMX (I know the kx250 switched to the solenoid model in '98; not sure when honda switched exactly on the cr250..). Some people also say the rmx works best (cleaner low speed jetting) with the non-airstriker pwk, but mine works great for me in my application (I also have a boyeson rad valve and fmf exhaust). Good luck with it.

Keihin PKW 39mm or 38 PKW Striker for RMX 250

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