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2000 YZ125M Sputtering badly 0-1/4-1/2 throttle...

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Just picked up a used 2000 YZ125 with the 144cc Big Bore kit for my son.  Not sure which kit was used.  Bought it off two brothers who seemed to really know bikes and mechanics.  After a cold start it ran well but was flooding badly (I mean puking out the pipe once it finally started) if started warm after that.  Going through the carb I found the following:


1. 0-Ring at the base of the Float Valve Seat for the float plunger was compromised badly so it must have been feeding too much continuously to the carb. 

2. Carb is still set up with all the OEM jets/needle/slide for a 125 but the Jet Needle clip was on the second notch.  OEM is third notch. 

3. Pilot jet is at the airbox/intake bell and was out at least two full turns.  OEM is 1 1/2

4. Throttle cable was pinched and bent up in the throttle assembly and was slipping off the throttle tube cam.


I figure it was burning too rich from the carb flooding so they adjusted the clip and pilot leaner to compensate. I replaced the cable and the float plunger assembly after which it quit flooding.  Made a few adjustments to the slide stop screw and pilot jet to reset the idle and 1/4 throttle response.  Starts on the first kick every time and runs great.


Until you get on it in any gear.  It does two things now:


1. Seems like its sputtering in 1rst and 2nd especially at 0-1/2 throttle.

2 Once you hit half throttle it runs like a bat out of hell but a little too lean still.


I figured they probably raised the needle clip a notch and lowered the float level trying to compensate for the bad o-ring in the float needle assembly and now that the float was functioning properly the carb fuel level was falling too low.


1. Set the needle clip to the 3rd notch (OEM)

2. Raised the float settings.


No go. Still sputtering and coughing pretty rough at 0-1/4 throttle even when flying along and you throttle back to clutch and downshift. I thought it was too rich but it actually wants to cut out when you throttle back so now I'm thinking it needs a bigger pilot jet to compensate for bigger bore?


Any suggestions?  Any help would be much appreciated.



Jupiter, FL



These are the OEM carb specs and where it's at now:


Carb: Mikuni TMX36SS: Main Jet- 350, Jet Needle- 6DHY53-75, Clip- #3, Needle Jet- Unknown but assuming OEM, Slide Cutout- 6.5, Pilot Jet- 35, Float Valve Seat- 3.8, Fuel Level- 12.5-13.5mm & Clymer says there is a Starter Jet of 80 but I can't find one.

Edited by Mshep1

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