Idling so high!!

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3/16/2018 4:54 PM

Hey guys,

when I got this bike it would idle really high and shut off when choke disengaged. Cleaned carb & 2nd kick had it running. Was idling low so I turned throttle screw clockwise and you'll see what happens next in the videos below.

Bike: 1998 yz125, big bore kit 144cc?

Known problems to date (not sure of relevance to issue):

1) 2 nuts and bolts missing in the intake boot, (not sure if air leak could cause high idle)
2) Hose clamp for intake boot missing
3) Main jet is a 330, Revzilla calls for 340-350 (part coming today)
4) Pilot jet looks possibly bent.. have a new 47.5 coming today. (Revzilla calls for 45-50)
5) Tension spring + both o-rings missing from exhaust pipe
6) Throttle spring in carb is bent (can feel when opening up throttle on grip)
7) Air vent hoses on carb missing + fuel vent hose.. but just cleaned carb.

Videos of issue:

Note that at 00:09 of the second video, I hit the kill-switch and let go to take one more crack at the throttle screw. That's the reason for bogging down.

Other relevant concern:
I wasn't sure how much to tighten the throttle cable at the top of the carb.. I noticed that tightening it dropped the slide and loosening it, raised it. Where is the slide supposed to be at rest position?
(i.e: Halfway, all the way down, etc).


Any and all help is greatly appreciated!
I'm just starting to dive into the world of internal combustion engines!


3/16/2018 6:14 PM
Edited Date/Time: 3/16/2018 6:19 PM

I have had a high idle simply due to taking all the slack out of the throttle cable at the handlebar adjuster. I can’t see vid right now but if it’s not increasing and run away idle it could just be that adjustment. If the idle screw doesn’t lower it try going in (clockwise) on your air screw to riches the idle/pilot circuit. That should drop it if not you’ll have to chase air leaks or richen pilot jet. Start with the easiest/cheapest first!

Make sure everything is clean and tight in the carb and the boots are secure and all hardware is there. Also make sure the throttle cable and assembly are operating smooth and snappy and that the slide is able to snap back down with an audible thud/clunk.


3/16/2018 6:21 PM

Air leak....

Paw Paw


3/16/2018 6:46 PM

1st step is fixing your list of problems. Id bet missing screws in the boot is causing an air leak - causing idle issues.

Patch it up, spray carb cleaner around the boots and listen for idle changes.


3/20/2018 11:29 PM


* Replaced main jet & pilot jet (running 47.5 pilot, 340 main with 1 1/2 air screw)
* Replaced the nuts for intake boot
* Got some hose clamps from plumbing isle @ hardware store.. (unfortunately doesn't fit exactly in the little groves but working well for now).

Progress Videos:

Idling low with choke off

Getting closer with idle screw:

Went for a (light) ride to see if boggyness would subside. Noticed something:

See if you can hear blender noise i'm talking about:


Bike has no power.. feels like bike is running very hard . Heard weird blender-type noise coming from cylinder.. sounds like fragmented pieces spinning around (hope the cylinder is fine).. need to take it apart and inspect. 


Should I check reed valve? .. could that be another reason for loss of power? 

3/28/2018 11:23 AM

If cylinder is coming off . reed check would be in the same aisle silly those clamps need to be right. as others said air needs to be in the right spots


If I'm healthy enough to complain. I shouldn't be complaining.

3/28/2018 5:40 PM
Edited Date/Time: 3/28/2018 5:40 PM

I like the fact that you appreciate any and all of the help.


much ty. How to spot a paid forum poster/artificial forum traffic producer (see list of actions/phrases below):

Copius pattern amounts of phrases like “Anyone have”..., “Anybody know?”.... and their variations.

Any help is appreciated!
Thanks in advance!

3/29/2018 8:22 PM

First of all if it has low compression it's not going to do anything very well, it will be hard to start, won't idle well, won't run well (if at all). It looks like it could use some TLC from the videos. I'd do a top end, probably would send your cylinder to PowerSeal or Millennium and get them to send you a refurbished cylinder, also as mentioned check the reeds for any gaps or chips, clean your air filter etc. The bike will run better. Then after your rebuild work on the jetting, it seems way rich.


21.5 350 XC-F
96' XR600r