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New 2012 YZ125 problems, need help please

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I bought this bike last week. Used it at the track on saturday no problems everything running fine & crisp. Now all of the sudden, using the same mixed gas used on Saturday when it was running fine, is fouling plugs like crazy & they're coming out black at about an hour of putting a new one in. Although I have to say after I fouled the one that came with it which was a BER9 something, I couln't find it & put in a BER8ES (I think). Can anyone give me direction?. Any help will be greatly appreciated.

Roger

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Yes I cleaned the filter. I'm puzzled to the drastic change on this bike's behavior that the more I think about it, I have no clue. after running so great, how can it become so rich that is fouling plugs an hour after they're put in?

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Stay with that br9es plug, the lower the number the hotter the plug on an ngk and the easier it will foul, cooler plugs are more resistant to fouling. Also, I would back out the air screw a quarter turn putting a little more air in the mix, then clean your airfilter and go ride. If that doesn't do the trick then raise your c-clip on the needle one position this will lean the fuel mix a touch and it should crispen up.

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Stay with that br9es plug, the lower the number the hotter the plug on an ngk and the easier it will foul, cooler plugs are more resistant to fouling. Also, I would back out the air screw a quarter turn putting a little more air in the mix, then clean your airfilter and go ride. If that doesn't do the trick then raise your c-clip on the needle one position this will lean the fuel mix a touch and it should crispen up.

I'm pretty sure colder plugs will foul more easily.

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Crankseal clutchside can provoke that.

Definitely can, but this is a brand new bike right so It's hard to imagine it would fail that fast. Is trans oil getting lower? Does Yamaha still give a 30 day warranty?

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Stay with that br9es plug, the lower the number the hotter the plug on an ngk and the easier it will foul, cooler plugs are more resistant to fouling. Also, I would back out the air screw a quarter turn putting a little more air in the mix, then clean your airfilter and go ride. If that doesn't do the trick then raise your c-clip on the needle one position this will lean the fuel mix a touch and it should crispen up.

Totally backwards. Rethink then repost

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Defiantly sounds like the jetting

Buy some jets and start tuning it in to your conditions

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Was the weather the same both days out or was possibly hot one day and then cold the next (if you were riding the same track) as this might throw the jetting off. Also id put the proper plug in if you can as it can have quite the impact and if the gap is different it can foul quite easy.

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The weather was the same on both days. You know the manual that came with it says a 30:1 mix, is that right?. That's 4.26 oz/gallon!, I live in the tropics where the temp is from 80o in the morning to 98o during the day & the humidity stays relatively high, does this mix makes sense for my area?. I'm going to have to do what buick_gn recommended. Buys some jets & tune it to my conditions. I'd like to thank everyone for your input.

Roger

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The mix has to make sense for your jetting and not for your area, for me your problems can be:

You clean the air filter and you put a lot of oil in it

Your bike jetting is not for that mix (more oil on premix makes your jetting leaner, less oil makes your jetting richer)

Crank seal (clutch side).....i don´t think so it´s a new bike but i don´t know

Just dirty carb

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Sorry, are you sure that manual says 30:1? because usually is 20:1 for break in and 30:1 after break in, how many turns do you have air screw? clockwise you rich the mixture C.clockwise you lean

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Does it still run fine or is it just fouling plugs?

What is your riding style?

Are you full throttle a lot or just lugging around the track?

It sounds like you might need to go a bit leaner with the needle and main jet. I am pretty sure they are tuned for sea level (which you are at) and for 60 degrees F.(which you are not) Also, the factory jetting is usually a bit rich just for safety sakes. Before you spend money on jets, try to move the clip on your needle up one slot at a time and see it it runns better. This will only affect up to about 3/4 throttle but it will give you an idea if it is too rich. Then after that go ahead and change you main jet. If your riding style does not invlove full throttle operation, then you do not need to worry much about the main jet. The problem I have with my advise or anyone elses advice is that you said it ran fine and then later on it started fouling plugs.

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Alright, right now I'm at work but will try that & report back. Hey, just wondering, how many turns does the air screw has to have to know wheather the pilot jet is to big or small?. Should I start there?.

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... The problem I have with my advise or anyone elses advice is that you said it ran fine and then later on it started fouling plugs.

Exactly. Why did it change... btw, I can lug my 125 around all day if I want and I've never fouled a plug. As long as you let it rip every once in a while even for a few seconds it seems to clean it out.

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Alright, right now I'm at work but will try that & report back. Hey, just wondering, how many turns does the air screw has to have to know wheather the pilot jet is to big or small?. Should I start there?.

The standard procedure is to find the idle airscrew position that gives the peak idle RPMs and compare that to the standard position listed in your manual as ideal. (For my 06 125 the service manual says 2 1/4 turns out). If you need more than this amount of turns your pilot jet may be too big, less and you jet may be too small. Check your manual under the Tuning section.

Here is what mine says...

EC716001

Pilot air screw adjustment

The richness of air-fuel mixture with full closed to

1/4 throttle can be set by turning the pilot air

screw 1.

Turning in the pilot air screw will enrich the mixture

at low speeds, and turning out it will lean out

the mixture.

EC71R010

Pilot jet adjustment

The richness of air - fuel mixture with the throttle

fully closed to 1/2 open can be set by turning the

pilot jet 1.

It is changed when adjustment cannot be made

by the pilot air screw alone.

Standard pilot air

2-1/4 turns out

screw position

Standard pilot jet

#40

*#45

*For EUROPE

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Take it back to the dealer. It's a week old.

Running fine and great and the very next day fouling plugs in one days turnaround? Does the bike smoke a lot too? If so, it is the crankcase seal (right side, tranny side)

Edited by j-bo

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