and so the saga continues............ PWK carb!

bclapham

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#1
OK, call me obssesed, or just call me a freak, i am still messing with the carby on this rm144.

to cut a long story short, the stock mikuni carb was just too rich at the lower to middle throttle positions and i still havent managed to get it leaned out- it runs well, but i think it can run better and the spooge embarases me also. so, ive swapped the stock 38mmTMX for a brand new Keihin PWK 38mm airstriker...but i cant get the bike to run properly at all.

ive tried every thing, from 40-55 pilots, and about 10 clip positions using the stock needles from rms that ran the PWK and also some of the richer needles from a ktm and also from 160-180 main jets and the floats are set properly too. no matter what i try, if i crack the throttle open real quick the bike bogs real bad. it seemed to be better with richer needle settings, but it was still bad and also blowing oil out the back.
i am using the #6 slide, and it did seem to get slightly worse when i ran a #7 slide. the older rms ran a #6 slide.

ive also tried an older used 38mm PWK that has been sleeved down to 36mm, same deal, it just bogs real bad. ive asked lots of people, and ive run the bike back to back with the stock carb and it runs ok so i dont think there is an air leak etc. the place where i have been testing is about 4000ft and 100F but the bike runs there OK with the stock carb (back to back)

i cant understand why the carb isnt working, i met someone at the track a while back and he has put the keihin on their 02rm125 with excellent results, murphys law that i havent seen him since to find out more!

so has anyone got any ideas? steve and eric think it might be the v-force and they could be right, but why would it run OK with one carb and not with the other? eitherway, i hope to be putting the stock reeds back in soon!

and i should mention, the only mods i did to the carb were just taking 2mm of the front spigot so it would fit in the boot properly!

so put your thinking caps on gentlemen.

thanks bruce
 
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#2
In 93 my kx 125 had a bog problem when the throttle was opened quickly as well in winter especially,after the bike seized i looked into rejetting and found the same sort of problem rich in one area like pilot , needle, main and lean on the cut away on the slide i ended up cutting .4 mm off bottom of slide and i then had to re-adjust all of the other circuits , no mo seize and carbed real good 'eh.I also had an03rm125 with odd brass inside no one would think it would run without seizing but it was almost perfect.(i'm at sea level @61deg F with good density)
 

bclapham

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#4
yep, i saw it, i think i may well try one of those narrower needles and maybe cut a bit of the bottom of the slide also. thanks, Bruce
 

steve125

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#5
Bruce sorry I just don't have any more advice than ive already given you. But jetting aside, because it is a big deal to performance. In my testing carby swaps, reed valves, power now's, all have had the least effect on performance than their worth. Of course there are a few exceptions, but i dont feel your Rm is one of them. The Mikuni your bike is speced with stock is an excellent carb. The good thing with all this is you are learning. Learning how in most cases just how good stock is these days. Besides proper jetting, it's best to leave the OEM carbs and reeds systems alone and search for power elsewhere.  Just my 2 cents. good luck!
 
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bclapham

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#6
thanks steve, to be honest, i wasnt searching for more power with the carb swap, i know the mikuni is a good carb (especially good to work on), but we have to kind of agree to disagree since i just cant get this carb jetted properly, and its not like i havent tried everything- since there is so much more info available for the keihin needles that was the main motivation for the swap. the irony is, its starting to cool here and Carlsbad has just reopened- it is around 70F there and sea level and i bet when i ride there it will be right on the money- the only good tracks here right now are glen helen and cahuilla and they are in the high areas and 100F and i think this is what is sending me over the edge jetting wise.

as you say, i think the v-force has been the problem, trouble was, its an irreversible mod, but as i told you ive located a stock reed setup to try out.

thanks again for your help

Bruce
 

jmics19067

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#7
Bruce,

what do you mean by an irreversible mod? did you have to cut the rubber stuffers off the stock manifold? If so it might be worth it to just go out and buy a new rubber manifold and start from scratch. It MIGHT be worth it to get a decent baseline of carb settings with a stock intake just to get a good feel of how the bike should act , it could get you closer to bringing the vforce in check. very easy for me to gamble though when it isn't my money but too me 50$ or so at this point would be minimal to the amount of aspirin eaten let alone needles and slide mods.

I was also wondering about your mikuni setup. If I remember correctly you end milled where the needle sets in the slide and then had to shorten the needle length. Could using a 36mm tmx needle open up your options a little more for your mikuni problems? So you wont have to fit the length as you did.
 

bclapham

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#8
jim: very good points!:) ive been hunting on ebay and just won a stock reed and manifold from a 98 rm and also a stock reed and manifold from a 2001yz125- they share the same holes so they should interchange. if i can get good jettting with the stock reeds then the v-force is for sale, LOL.

our current theory is that the slide is too lean, just a hunch since it seemed to bog worse with the 7 slide and big daddy had to run a real rich needle to get his cr125 working; the big bore 144's are known to have a wesk signal on the pilot but we will see what happens. i am shaving the bottom of the slide to make it richer and will get a real rich needle (ie 1464 vs 1469) and see what happens. if it goes in the right direction then i might lay down the cash for the richest slide they have.

thanks, Bruce
 

G. Gearloose

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#9
Hmm, a 14xx is a d-taper, no? and that sounds pretty steep...
have you tried a C taper? Or are you just trying different staight section diameters? A shallower taper can move the start of the taper down, richening the smaller throttle openings,( leaning the larger..)
Have you tuned in the WOT and mid throttle already and just working on the off-idle, or is it all still undertermined?
I say set your main, set half throttle with the clip, then fire up JD's calculator to tune in the rest with seat-of-the-pants observations.
But how exactly does shaving the bottom of the slide make it richer? I think tune it in with $5 needles instead of cutting $80 slides.
 

bclapham

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#10
gearloose: yes, the 14xx is the D series needle- the older RM's came with 1469 needles back in the day when they ran PWK, PWM and PWK powerjet 36mm carbs. i think some of the older kx125 and ktm125sx also. the 13 series needle may indeed be worth a try.

as for the slide, i have carefully cut 1mm off the bottom of the slide (not the opening) so it sits lower in the carb and is now essentially a #5 from a #6; its an old trick if you are cutting the opening to make it lean and go too far you can go back a bit.
my current theory is that this bike (and probably most 125s) doesnt have enough signal at slower engine speeds (often the case with bore bores), Big Daddy's finding that he has to use a mega rich 1464 needle kind of backs this up- as you know we can tune this area with needle diameter and slides

If this makes a difference to the cronic bogging then i will try and tune the rest of it out with more narrow ie richer needles and the 13 series also- good idea, thanks
 

jmics19067

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#11
Bruce,

not going to be of any help but just questions. If I remember correctly I thought that the mikuni you had was way too rich at low throttle settings. and now the Keihin is way too lean?

The points I am pondering is why the drastic difference between the two carbs. If the engine airflow decreased the signal wouldn't it affect both carbs or if the Vforce increased signal ditto? would the engine mods and Vforce compromise each other? Anyway you can make a Mikeihini carb :confused:

sheesh I am getting silly thinking about something 3000 miles away I can only imagine what you are going thru trying to get that thing running right. When you are ready I can lend you a sledge hammer for when you start back from scratch.
 

bclapham

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#12
you are right, two different carbs and they must work in total opposite; this make no sense at all, but i suppose it is testement to how good the TMX is and how they can get that compact 38mm TMX working so well on these stock bikes. I will look properly, but i think the slide opening on the PWK is significantly bigger than the TMX, only time will tell but WHEN i get this thing dialed in.
 
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#13
There is something I found out years ago on my PWK. The vent lines do not like to be hooked together on my bike. In addition, the carb worked best when the vent lines were short and individual.

That is probably not your issue, but you never know.

Also, I really hate to use the spooge factor as a jetting tool. I ignore that all together and concentrate on making it run well. Too many factors are involved in the spooge level.

As you know the cutaway is really only a factor when the throttle is just above idle.

The issues you have are similar to my RZ350 (which has 34mm Mic VM's). I ended up going really lean on the main jet (too lean for my liking) and very rich on the needle. That seems to give the best overall performance. However I expected this kind of issue with the older VM's.

Good luck!

Chris
 
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#14
BC, it is also possible that the fuel you are using is part of the problem. Race gas may help.

Chris
 

bclapham

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#15
Chris: i am running race gas, that is one of the problems since the C12 seems to run real rich. i havent had time to test the PWK with the different slide yet, i will report back when i do!