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Fouling plugs & cold start high idle

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So my 2001 yz 125 keeps fouling plugs. Fouled 2 plugs in the last week. My pilot jet was a 35 when I fouled those two plugs. So I thought I must be running real rich. Today I went to 30 pilot jet, 2 turns out on fuel screw, and now it runs bad in the pilot circuit. (lean bog, then it passes once revs pick up). Figure i'll turn fuel screw out a bit more and report back.

Now from what I've read people say that high idle on cold startup means lean pilot. Well I was fouling plugs with my 35 pilot jet and cold starts were revving to the moon. (intermittently pushed stop button until it warmed up a bit). 

So these two scenarios kind of contradict each other. Any ideas/thoughts?

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Google spanky's jetting guide and read through the part about air and idle screw adjustment  , check your reeds and do a pressure test 

 

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Your manual provides pretty good information on jetting.  Yamahas come from the factory jetted pretty well, and usually require only minor tweaks, assuming you are running the 30:1 (32:1 is close enough) ratio specified in the manual.  I usually start with the clip position, and depending on how that goes, decide if I need to change jets.

Also keep in mind that the plug itself can be changed to better suit conditions.  I have two YZ 125s that are used mostly for woods racing and trail riding.  I use "8" plugs in the woods on both of those bikes, but I will change it to a "9" for the track or deep sand conditions.

Lastly, no need to buy expensive plugs.  The bike will work just fine on NGK "ES" plugs.  Lots of guys use them in smokers.  I started using them back when I was doing a little two stroke jet ski racing.  They are $2.50 each at any auto parts store. I use a  BR8ES for the woods, and a BR9ES for the track.

 

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Your manual provides pretty good information on jetting.  Yamahas come from the factory jetted pretty well, and usually require only minor tweaks, assuming you are running the 30:1 (32:1 is close enough) ratio specified in the manual.  I usually start with the clip position, and depending on how that goes, decide if I need to change jets.
Also keep in mind that the plug itself can be changed to better suit conditions.  I have two YZ 125s that are used mostly for woods racing and trail riding.  I use "8" plugs in the woods on both of those bikes, but I will change it to a "9" for the track or deep sand conditions.
Lastly, no need to buy expensive plugs.  The bike will work just fine on NGK "ES" plugs.  Lots of guys use them in smokers.  I started using them back when I was doing a little two stroke jet ski racing.  They are $2.50 each at any auto parts store. I use a  BR8ES for the woods, and a BR9ES for the track.
 


Ok sounds good. I run 40:1 in all my 2 strokes. And I agree about es plugs. That's what I run in my Superjet and the 125. Thanks for the tips.

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12 hours ago, kylechandler1300 said:

So my 2001 yz 125 keeps fouling plugs. Fouled 2 plugs in the last week. My pilot jet was a 35 when I fouled those two plugs. So I thought I must be running real rich. Today I went to 30 pilot jet, 2 turns out on fuel screw, and now it runs bad in the pilot circuit. (lean bog, then it passes once revs pick up). Figure i'll turn fuel screw out a bit more and report back.

Now from what I've read people say that high idle on cold startup means lean pilot. Well I was fouling plugs with my 35 pilot jet and cold starts were revving to the moon. (intermittently pushed stop button until it warmed up a bit). 

So these two scenarios kind of contradict each other. Any ideas/thoughts?

You're backwards on your screw adjustment idea.  4 strokes have a fuel screw.  Screw it out for richer.  2 strokes have an air screw.  Left is lean.  Turn it in for richer.

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Adjust you float height, no matter how lean you jet it will run rich if the fuel level is too high,

a worn inlet needle and seat doesn't help as well.

 

Keep track of gearbox oil level, if it's constantly dropping and there are no apparent external leaks ,your need a right hand crank seal.

 

30$ worth of hardware store supplies makes you a homemade leakdown test fixture,

a must for troubleshooting 2-strokes (rules out air leaks and gearbox oil injestion thru bad seals)

 

While you're at it, inspect the reeds for damage, chips, not sealing around each petal etc.

 

No need to change the premix ratio,

pending you don't putt around the backyard all day, properly jetted and with

everything in a good state of tune (top end, reeds etc.) your YZ125 won't foul plugs at 32:1

 

Get yourself a service manual and read it over and over (free online downloads can be found)

AIR screw / jetting charts, float height etc, all the answer to 90% of your future questions are in there.

Edited by mlatour

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Your problem my be needle clip position.  30 on the pilot is extremely lean.  Even a 35 is probably too lean.

I would recommend taking the needle out of the slide and checking the clip position.  It should work best on either the middle groove or the second groove from the top.  See where that clip is and go from there.

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Your problem my be needle clip position.  30 on the pilot is extremely lean.  Even a 35 is probably too lean.
I would recommend taking the needle out of the slide and checking the clip position.  It should work best on either the middle groove or the second groove from the top.  See where that clip is and go from there.


Needle is set at middle clip. I thought 30 was stock?

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no matter what carb if the needle is on the engine side of the slide its a fuel mix and on the airbox side its an air. can make it easyer to identify what mixture screw you have (some carbs have both)

beyond that these blokes know what theyr on about and are leading you in the right direction I just wanted to add my 2 bobs worth. good luck

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I have a cr125 with a pro circuit exhaust, it's lacking power in the top end.  It just isn't crisp.  When the exhaust was installed the jets and clip position stayed the same.  I've already changed the main jet from a 380 to a 370 as was recommended by pro circuit.  Should i also change the clip position from 3rd to 2nd as recommend? thnx

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if the engine is 8 stroking sounding rich then give it a go after all the main jet and slide needle are responsible for the top end mixture also read your plug colour to see if its rich or lean.

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Ok. Went back to 35 pilot and 2 turns out on air screw. Lean bog is gone. Changed to a BR9ES plug, thought everything was going well then it fouled out again while riding. What exactly makes the plug stop working? Cause I took the plug out, cleaned it thoroughly and put it back in and it won't fire (so plug actually quits functioning/sparking properly). Then put brand new plug in and it starts first kick. Is the plug getting to hot and just "cooking" itself? How does this work...

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did you use some sand paper to clean and de glaze the electrode and earth in the gap and then re gap the plug clearance??? could be a dud plug.

generally carbon build up is the cause of a plug to stop working as the electricity tracks through the carbon to earth instead of jumping the gap to form the spark.

cracks in the porcelain insulator can do the same thing by leaking the spark to earth before the gap( hence its not a good idea to drop or throw a plug down)

plugs can be defective from new iv had brand new quality plugs be duds from the box. being a mas produced item your bound to find a dud or two along the way best you can do is spend the little extra on a better quality plug.

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beware of using a plug with too high of a heat range as it can burn a hole in your piston. stick to oem recommendations

the problem you have with fowling plugs may not be the plug and may not be the fuel mix (rich or lean) but may be the oil and or fuel quality you are using.

are you using premium fuel and full synthetic oil??? or low end pump fuel and cheap oil?

low grade pump fuel, ethanol blends and mineral oil can cause your engine to burn "dirty" causing more carbon build up and tarnish to adhere to internal engine parts.

like most things you get what you pay for.

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