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woods jetting suggestions for yz 125 (06)


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My 06 yz 125 was running great in the woods with stock main (410), pilot (40), and accessory needle (6BFY 43 74 needle with clip at position 2, which should be 1/2 step leaner than stock if I'm reading the manual right) during the summer months, but now that the weather has  cooled (50s), I'm fouling plugs left and right.  

 

The motor seems to run way to cool in this weather.  After a riding session with the above jetting, you can put your bare hand on the motor with no discomfort at all.

 

I should also note that there are no obvious rpm ranges where jetting is off with the above jetting.  It runs great and pulls hard at all rpm ranges and does not spooge out the pipe, but as soon as I back off race pace, it starts to load up and foul.  Should I just try a hotter plug?

 

When I use the stock needle (6BFY 42 74) one clip leaner than stock, the bike runs better and does not foul, and it gets up to an appropriate operating temperature (as judged scientifically by a bare hand motor touch!).  This is not the solution however, as the bike sounds "tinny" in the mid range, and will bog given a big handful o' throttle.  I can ride it like this, but it's not right. 

 

I was hoping someone could help me rapidly narrow down a solution with minimal testing.  I live in an urban area so i can't really test at home, and jetting at the riding area is a major PITA, plus I have a race in 2 weekends away I want to do.  

 

Some more info:

roughly 1000 ft elevation

32:1 on pump premium

motor is all stock except for weisco piston

BR9ES plug 

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This is kind of a worse case scenario but have you checked your transmission oil? You could possibly have a right crank seal going out. It happened to me on mine. I have the same bike basically with the same jetting and I can run it year round with adjustments of the air screw. I know that's not the correct way to do it but I have always gotten good plug readings and it runs good from pilot to main. So I would just see if your tranny oil is being sucked in there. With the air leak it could possibly make it pingy as well.

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This is kind of a worse case scenario but have you checked your transmission oil? You could possibly have a right crank seal going out. It happened to me on mine. I have the same bike basically with the same jetting and I can run it year round with adjustments of the air screw. I know that's not the correct way to do it but I have always gotten good plug readings and it runs good from pilot to main. So I would just see if your tranny oil is being sucked in there. With the air leak it could possibly make it pingy as well.

pretty sure it is not anything like that.  No excess smoke or spooge, and no oil loss as far as I can tell.  It runs great like it always did when riding hard, but it starts to load up as soon as you back off race pace riding.  Yes, the vast majority of the other bikes I've had needed little more than seasonal air screw adjustments.  

Edited by rpt50
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My 06 yz 125 was running great in the woods with stock main (410), pilot (40), and accessory needle (6BFY 43 74 needle with clip at position 2, which should be 1/2 step leaner than stock if I'm reading the manual right) during the summer months, but now that the weather has cooled (50s), I'm fouling plugs left and right.

The motor seems to run way to cool in this weather. After a riding session with the above jetting, you can put your bare hand on the motor with no discomfort at all.

I should also note that there are no obvious rpm ranges where jetting is off with the above jetting. It runs great and pulls hard at all rpm ranges and does not spooge out the pipe, but as soon as I back off race pace, it starts to load up and foul. Should I just try a hotter plug?

When I use the stock needle (6BFY 42 74) one clip leaner than stock, the bike runs better and does not foul, and it gets up to an appropriate operating temperature (as judged scientifically by a bare hand motor touch!). This is not the solution however, as the bike sounds "tinny" in the mid range, and will bog given a big handful o' throttle. I can ride it like this, but it's not right.

I was hoping someone could help me rapidly narrow down a solution with minimal testing. I live in an urban area so i can't really test at home, and jetting at the riding area is a major PITA, plus I have a race in 2 weekends away I want to do.

Some more info:

roughly 1000 ft elevation

32:1 on pump premium

motor is all stock except for weisco piston

BR9ES plug

Shouldn't it run leaner in the cold weather because the air is more dense? Weird that the coldness would be causing it to foul, could be something else causing it. Or I'm just bassackwards.
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Shouldn't it run leaner in the cold weather because the air is more dense? Weird that the coldness would be causing it to foul, could be something else causing it. Or I'm just bassackwards.

No, you are right.  Cold air is denser, so it should be leaner.  But the motor is just not heating up in this weather, even when I'm really hammering it.

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