setting/adjusting idle on 98 RM125


26 replies to this topic
  • Bhavesh

Posted July 16, 2009 - 07:40 PM

#1

Just got a 98 RM125 with a 144 kit and stroker kit.

The previous owner told me that it wouldn't idle, but I'm sure it will with some carb adjustment.

Right now, if I'm off the gas, it'll stall.

How do I change this?

  • dirtynidge

Posted July 16, 2009 - 09:25 PM

#2

You should have the Keihin PWK36 on that bike, the idle screw is a black plastic screw on the left-hand side.

Item number 29 on this drawing, covers PWK36/38 carbs

http://sudco.com/Dia...s123/exppwk.gif

If for any reason you have Keihin PJ carb you adjust idle by rotating choke knob but I don't know of that carb ever being on the RM125

  • 2-Strokes 4-ever

Posted July 18, 2009 - 03:25 AM

#3

Even with proper jetting, we've found modern MX 125's not able to idle... maybe when first started, but not once warmed up. I think the issue is the big carbs used for peak horsepower... the little 125 piston and large ports don't allow enough vacuum at low RPM to draw fuel out of the big carb. 125's and 80's would need a smaller carb to enhance low RPM performance IMO.

  • Bhavesh

Posted July 18, 2009 - 11:56 AM

#4

thanks for the repsonses

Is there anyway to tell if I have the Keihin PWK36 carb?

I don't see a plastic screw on the carb, shown as #29, above where the airbox attaches to the carb

Posted Image

if I don't have the PWK36, do I just adjust the black plastic knob, that you pull to operate the choke?

the reason I ask is because it says "idle adjust" on the black rubber boot, above where it says "choke". I tried twisting the cap for the choke, but it doesn't affect the bike's ability to idle

  • Bhavesh

Posted July 20, 2009 - 05:41 PM

#5

I heard on another forum that the choke and idle adjust are part of the same mechanism...could that be what controls the idle speed?

  • FooMoto

Posted July 20, 2009 - 06:37 PM

#6

The owners manual for a 1998 RM125 says:

"The choke/idle adjust knob is used to work the choke system and to adjust the engine idle speed. The choke system works by pulling up the knob. The engine idle speed is adjusted by turning the knob. Turn the knob clockwise to decrease engine idle speed and turn the knob counterclockwise to increase engine idle speed."


With this in mind, I have a 1998 RM125 that I purchased new in 1997. It has very low hours and runs perfectly. No matter what I do I've never been able to get it to idle.

  • dirtynidge

Posted July 20, 2009 - 08:05 PM

#7

The owners manual for a 1998 RM125 says:

"The choke/idle adjust knob is used to work the choke system and to adjust the engine idle speed. The choke system works by pulling up the knob. The engine idle speed is adjusted by turning the knob. Turn the knob clockwise to decrease engine idle speed and turn the knob counterclockwise to increase engine idle speed."


With this in mind, I have a 1998 RM125 that I purchased new in 1997. It has very low hours and runs perfectly. No matter what I do I've never been able to get it to idle.


The carb you have pictured is a Keihin PJ. This is adjusted as described above (and in my first reply). Good carb for MX, POS on the trails due to almost impossible to set idle, If this doesnt work you most likely have have the wrong size pilot jet or a dirty carb.

It is nothing to do with being on a small bore bike as I had the exact same trouble on my RMX250, nearly everyone does.

PWK is a much better carb and pretty cheap secondhand, just keep and eye out for one. A PWK36 or 38 will both fit straight in your boots.

  • Bhavesh

Posted October 12, 2009 - 07:53 AM

#8

Sorry to bring up an old thread, but I want to make some clarifications based on what I've learned, so that it can be used for reference:

1.) The carburetor that came stock on the 1998 RM125 is a Keihin PWM36, per the specification sheet on this website:

http://www.suzukicyc....shtml~isoraami

Posted Image

(I uploaded the spec sheet to my photobucket in case his site goes down)

It is a short carburetor for great top end power delivery making it great for a motocross track, but less friendly in the woods.

http://www.keihin-us.com/pwm.htm

2.) The idle speed adjustment screw and the choke assembly are on the same mechanism pictured above. Spinning the grooved disc changes the idle speed and lifting it operates the choke

3.) The jets that came from the factory are 175 (main) and 55 (pilot). According to this thumpertalk thread:

http://www.thumperta...65&postcount=24

...and this chart (from the same thread)

Posted Image

Depending on your altitude and how hot it is outside, the main jet (175) was too rich from the factory.

  • Bhavesh

Posted October 12, 2009 - 07:58 AM

#9

Ok, onwards to my tuning:

Since I've been riding at 50-70 degree F at approximately zero degree elevation, the chart says I should roughly have a:
-170 main jet
-155 pilot jet
-stock needle, 3rd position
-the air screw 1.5 turns out

I pulled the carb, made sure the passageways were clear, installed new jets (so I know they weren't clogged), and surprisingly...the bike still doesn't idle.

The idle works like this: the slide is all the way down, so gas predominantly comes through the pilot jet. Air comes from a small, 1/4" tunnel that goes around the slide and connects the intake side to the air box side. By turning the idle adjust screw, you regulate the size of the opening of that tunnel. The bigger the opening of that tunnel, the more air comes through, and mixes with gas from the pilot.

Any ideas?

  • Tailender

Posted October 12, 2009 - 02:24 PM

#10

Have someone help you by holding the throttle open very slightly and turn the air screw to see if it changes the rpms. The pilot may be the wrong size. See the FAQ at the top of the forum for how to adjust. I had to change mine down 5 sizes and now it idles no problem. 02 RM 250. I went one size at a time to find the size that was within 2 1/2 turns. It's at 2 turns now.

  • Bumperlt

Posted February 08, 2010 - 08:37 PM

#11

I resurrect you!

Did you thread starter ever get the bike idling? I have a 96 model with apparently a 98 model carb (only one with adle/choke in one). Mine is running decent but will not idle no matter how many turns the air screw is out (set at 2) and no matter how far in or out the idle screw (choke) is. I am considering swapping it out for the keihin pwk36.

  • 2strokezuke

Posted February 09, 2010 - 01:05 PM

#12

bhavesh:
that website is wrong, 1998 rm 125 uses a PWK 36mm airstriker. If you get one off ebay or something make sure it is from a rm 125. The stock carb uses a solenoid that injects gas in the mid rpms. if you use a carb without the solenoid you gonna run into some jetting problems. to get it to idle, adjust the throttle cable to get some tension in the wire/ raise the slide valve. The fuel mixture isn't going to make it idle alone. you can get into the right position by looking at the color of the smoke, listening to the sound of the bike, checking the spark plug color.

also try a 52 pilot jet

hope this helped

  • Bhavesh

Posted February 09, 2010 - 06:38 PM

#13

I resurrect you!

Did you thread starter ever get the bike idling? I have a 96 model with apparently a 98 model carb (only one with adle/choke in one). Mine is running decent but will not idle no matter how many turns the air screw is out (set at 2) and no matter how far in or out the idle screw (choke) is. I am considering swapping it out for the keihin pwk36.


The thread rises again!

Unfortunately, the bike never got the point where it would idle, so it's still dead. I tried:
-cleaning out the pilot jet passageways by blowing carb cleaner through it until it blew through the other side
-replacing the pilot jet with sizes 55 (richer), 50 (leaner), and 48 (even leaner) with no success
-turning the air screw farther and farther out to lean out the mixture
-turning the "idle adjust" knob all the way out (= position with highest theoretical idle speed)

none of this works

the only time it will resemble an idle is when:
-I adjust the throttle cable slack so the slide is being raised, slightly...except the bike takes off like it's WOT when the handle bars are turned to the right
-the bike is almost out of gas

with that being said, my guess is that the pilot circuit is letting in too much gas, and that a leaner a/f mixture would allow it to idle. The dealer was all out of leaner jets, so I bought a total 8 jets (4 richer, 4 leaner) online and will find out in a week if a leaner pilot jet rectifies the situation!

bhavesh:
that website is wrong, 1998 rm 125 uses a PWK 36mm airstriker. If you get one off ebay or something make sure it is from a rm 125. The stock carb uses a solenoid that injects gas in the mid rpms. if you use a carb without the solenoid you gonna run into some jetting problems. to get it to idle, adjust the throttle cable to get some tension in the wire/ raise the slide valve. The fuel mixture isn't going to make it idle alone. you can get into the right position by looking at the color of the smoke, listening to the sound of the bike, checking the spark plug color.

also try a 52 pilot jet

hope this helped


hello 2strokezuke, thanks for the input

are you sure about the OEM carb being an PWK 36mm airstriker?

when I pulled the carb off the bike (and it looks like it fits perfectly well/came with the bike), there was indeed a PWM36 on there to begin with.

and sadly, after the 55 pilot jet, I tried a 50, and a 48 with no success

  • Bumperlt

Posted February 09, 2010 - 07:08 PM

#14

bhavesh:
that website is wrong, 1998 rm 125 uses a PWK 36mm airstriker. If you get one off ebay or something make sure it is from a rm 125. The stock carb uses a solenoid that injects gas in the mid rpms. if you use a carb without the solenoid you gonna run into some jetting problems. to get it to idle, adjust the throttle cable to get some tension in the wire/ raise the slide valve. The fuel mixture isn't going to make it idle alone. you can get into the right position by looking at the color of the smoke, listening to the sound of the bike, checking the spark plug color.

also try a 52 pilot jet

hope this helped


This is wrong. 1996-97 used a PWK36, 1999-00 used a PWK36pj, and the 98 is a PWM36. The 98 is the only one with a choke/idle adjust. The 99+ had an electronic powerjet.

That said, I got my 98 carb to idle today, A little bit high, but idling none the less. I adjusted the float as it was out of spec, and then I shot compressed air through all the openings. Hooked it back up and low and behold it seemed to operate like its suppose to. Of course now it's low on gas and it wont idle again haha. I'm getting gas tomorrow.

I'd recomend you see if the float hight is correct. I can give yout he specs if you need.

  • 2strokezuke

Posted February 09, 2010 - 07:34 PM

#15

the wot open throttle problem is an easy fix. reposition the throttle cable so it doesn't have tension when you turn right. Make sure it is the right side of the steering column, you might have to find completely different path from the carb to the throttle pully.

The correct carb for a 1998 rm 125 is becoming a bit of a mystery. The carb i pulled off mine was a PWK airstriker and I know it was specifically designed for my bike because of the solenoid going to the computer and it has 36E imprinted on it which is found on all the other intake components. The manual says it uses a pwk airstriker 38mm which lines up with my measurements. I know that some 1998 use upside down forks like mine, they might have changed the carb during that year also.

check to see if your carb is plug into the ignitor box or if the ignitor box has an extra input with nothing plug into to it. This would mean someone took off the stock carb and put on the PWM. If all that checks out, you might wanna call keihin or sudco to see what the deal is with having a PWM.

just because it fits doesn't mean anything. I have a carb meant for a cr 500 of my 125 and it fits like a glove

  • Bumperlt

Posted February 09, 2010 - 08:23 PM

#16

Clymer states that 1998 had the PWM. That definetly not end all be all, just citing my source.

PS if anyone has a PWK36 they want to sell cheap, pm me.

  • 2strokezuke

Posted February 09, 2010 - 09:05 PM

#17

bhavesh
how are your forks arrange
Upside down-> shinny part down
right side up_> shinny part up

bumperlt
if you gonna get a PWK make sure its an airstriker.

  • Bhavesh

Posted February 10, 2010 - 07:18 AM

#18

This is wrong. 1996-97 used a PWK36, 1999-00 used a PWK36pj, and the 98 is a PWM36. The 98 is the only one with a choke/idle adjust. The 99+ had an electronic powerjet.

That said, I got my 98 carb to idle today, A little bit high, but idling none the less. I adjusted the float as it was out of spec, and then I shot compressed air through all the openings. Hooked it back up and low and behold it seemed to operate like its suppose to. Of course now it's low on gas and it wont idle again haha. I'm getting gas tomorrow.

I'd recomend you see if the float hight is correct. I can give yout he specs if you need.


congratulations! :ride:

having a working idle can make all the difference on the trails when you have to clutch/stop

I thought my float was in spec, but I could be wrong. If you don't mind posting the correct float height, I can give it a shot.

the wot open throttle problem is an easy fix. reposition the throttle cable so it doesn't have tension when you turn right. Make sure it is the right side of the steering column, you might have to find completely different path from the carb to the throttle pully.

The correct carb for a 1998 rm 125 is becoming a bit of a mystery. The carb i pulled off mine was a PWK airstriker and I know it was specifically designed for my bike because of the solenoid going to the computer and it has 36E imprinted on it which is found on all the other intake components. The manual says it uses a pwk airstriker 38mm which lines up with my measurements. I know that some 1998 use upside down forks like mine, they might have changed the carb during that year also.

check to see if your carb is plug into the ignitor box or if the ignitor box has an extra input with nothing plug into to it. This would mean someone took off the stock carb and put on the PWM. If all that checks out, you might wanna call keihin or sudco to see what the deal is with having a PWM.

just because it fits doesn't mean anything. I have a carb meant for a cr 500 of my 125 and it fits like a glove


it just seems like the cable isn't long enough to be in a position that won't get tugged on with the bars turned...but I'll try re-routing it

I also agree that this is quite a mystery, because there isn't anything connected to the carburetor that connects to the ignitor. Just because my carb fits doesn't have to mean it was the correct carb...but the thing that confuses me is that your forks are inverted. Mine are conventional (shiny part up).

Posted Image

So maybe we have different bikes?

check the 10th digit on your vin...if it's a 'W', then you have a 1998 model

also, if you look at the picture on the microfiche, it sort of looks like a PWM (by looking at the idle adjustment/choke knob, #19)

Posted Image

  • 2strokezuke

Posted February 10, 2010 - 10:18 AM

#19

im effed, my bike is a 1999. I thought i had a 98 for all these years. ill have to rebuild the whole bike from the bottom up, tranny, crank, everything... no wonder it wasnt running up to snuff.

  • 2strokezuke

Posted February 10, 2010 - 10:20 AM

#20

can you tell me what the # on your engine is , they might use the same tranny, hopefully

if not, you wanna buy some nearly brand new shifting forks and gears, never shifted, just idled in nuetrel




 
x

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.