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yz250 CARBON FOULING

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Hello, I recently purchased a 2004 yz250. The bike has a full rebuild, including replated cylinder (have all receipts).

The compression is very good so my problem is not a worn top end.

 

So my problem is the bike will carbon foul the sparkplug every couple rides (about 5 hours riding). When I say carbon foul I mean the plug has carbon built up on it and it is black but pretty dry (not covered in oil like most fouled plugs).

 

The Carb has a 50 pilot, and 178 main. I am not sure the position of the needle clip.

I ride a couple hundred feet above sea level at around 20 degrees celcius.

I am running Motul 2t mix at 40:1.

Also, I am running a br8eg sparkplug (stock)

 

The bike smokes a fair bit and has a decent amount of spooge, so I am positive that its a little rich.

 

The bike runs great when I'm riding it and the throttle is nice and crisp. But after multiple hours it will begin to feel boggy and then die within about 10-15 minutes.

 

I only ride trails at an intermediate level, (varying from first gear trails to flat out on logging roads). I mostly ride in power band and the pipe is very hot after ive been riding.

 

I believe sparkplugs carbon foul because the motor is not running hot enough, but correct me if I'm wrong.

 

I am not sure what to do as I do not know why it is doing this!

 

Any help would be much appreciated!!!

 

Thanks!

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What type of fuel are you running ? Type of pipe ? I have never had good luck with motul 2 stroke oil. Is the motor all stock ? I have found that br8es are a much heartier plug then the eg series. Try running a better oil like klotz r-50. I run the needle on mine in 4 th position.

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Get good quality premix , like guy said klotz r-50,supertechniplate , amsoil etc . You dont have to buy the $20 per litre stuff . Run it at 50-1 , take jetting back to factory size , pull pipe and inspect for buildup and powervalve operation. If all looks good , button er back up and go from there . You need a baseline to work off of with jetting , the mix will be fine at 50-1 , stock main,maybe one size bigger on pilot , adjust airscrew ,itll tell you whether you need to go up or down on pilot. Good luck , yz's are awesome machines !!!

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Does have an fmf fatty pipe and powercore2 silencer.

Thanks for the advice, ill get the bike out in the next few days and get to work on it!

Just a couple questions for anyone who knows the answer.

would bad muffler packing, partially plugged up pipe, or leaking spooge where the pipe and cylinder meet, would any of these things contribute to fouling the plug? Just curious.

Also, if i have stock jets and a leaner mix than yamaha reccomends why would the plug foul?

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Use the klotz R-50 go up 3 main jets and switch to a br8es then let me know whst happens. I think you are getting too hot. Thosr plugs use super tiny eelectrodes and I feel your burning them up. A aftermarket pipe requires a larger main jet. I mix my R-50 at 40:1 which is 3 oz to a gallon.

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ok, yeah I use either 91 or 92, cant remember which.

 

If the electrode was burnt up would that plug be done for good?

Because the last plug that fouled I took it out and used sandpaper on the electrode/tip and the biked fired up again, then died soon after.

 

The reason I thought my bike was running cold was because I saw this on another site:

 

 

Carbon'd piston = lean, or near perfect
Clean, washed piston = rich

Carbon'd plug = cold
Black, oily plug = rich
Clean plug = lean, or near perfect

 

 

Is that true??

 

I just don't really want to buy$30 jets to "experiment" with and then realize I don't need them,

 

Thanks again for the input!
 

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Jets are $5. Buy one 3 sizes bigger then you have in there and a br8es plug so your out $8 dollars. A dry plug is never rich. You can not run stock jetting with a aftermarket exhaust it will lean the bike out.

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im kinda surprised noone asked you what elevation your at where you ride too?? that has a huge factor to jetting. so need to know that???

 

i believe i was running 48/175 jetting on mine, but was at near sealevel too.  but you do need to go down

 

how far do you have your idle/mixture screw at also????  i think you could go down pilot too maybe, but depends on altitude too

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pretty much sea level, about 200-300 ft above sea level.

 

Please excuse any dumb questions as I know very little about jetting,

 

airtime mentioned I need to go up a few main jet sizes but Denn your telling me go down??

 

Is a 175 main richer or leaner than a 178 main?

 

And is a 45 pilot richer or leaner than a 48 pilot?

 

 

I'm not sure if you can watch this video but if you can I am the blue bike on the far left, so you can see amount of unburnt oil coming out of my exaust.


https://www.facebook.com/photo.php?v=10201374748522228

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Smaller #'s on jets = leaner

 

Are you losing any tranny fluid?

 

Something else is wrong, your jetting is close enough that you shouldn't be fouling plugs, and you have running issues. Gas or pre mix ammount isn't going to fix anything either. Something is mechanically wrong and my guess is RH crank seal leak.

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It does smoke quite a bit (looks blueish though, not white)

 

Once the bikes warmed up it smokes less.

 

I'm starting to think crank seal, I will change the oil to see how much comes out.

 

How hard is it to change the RH seal? Any special tools mandatory?

 

Thanks again

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Blueish smoke, sounds like RH seal. Best way to tell is freshly fill oil with required amount measured exactly. Ride a couple hrs, drain and measure.

Never done a YZ but generally you can just remove RH case, clutch and primary gear, then carefully remove the seal and replace. 

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I don't have a pneumatic wrench, any tips for removing the main nut  on the clutch and the nut on the primary gear? without it just spinning?

 

Thanks

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I don't have a pneumatic wrench, any tips for removing the main nut  on the clutch and the nut on the primary gear? without it just spinning?

 

Thanks

 

Put a penny between the gears

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What type of fuel are you running ? Type of pipe ? I have never had good luck with motul 2 stroke oil. Is the motor all stock ? I have found that br8es are a much heartier plug then the eg series. Try running a better oil like klotz r-50. I run the needle on mine in 4 th position.

 

I'll agree that Motul is average, but I run Amsoil Dominator and have never had issues.  My old Gasser was bought used, the power valve had major carbon deposits 1/16" thick that could NOT be removed.  After doing the top end of my KTM 300 after 125 hours of riding/racing, there were no carbon deposits. 

 

This YZ is definitely running rich on the jetting, I would suggest mixing gas at 50:1 with Amsoil Dominator or some other high quality synthetic oil.  Try leaning out the needle one clip, see how the splooge and spark plugs change from that.

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Smaller #'s on jets = leaner

 

Are you losing any tranny fluid?

 

Something else is wrong, your jetting is close enough that you shouldn't be fouling plugs, and you have running issues. Gas or pre mix ammount isn't going to fix anything either. Something is mechanically wrong and my guess is RH crank seal leak.

I was going to suggest that too.  I think the  main seal on the clutch side is suspect.  For impacting that nut loose, I've got a 3in. long piece of copper tubing crushed flat on one end.  A little easier to grab on and pull it out of the teeth after you use it. 

BTW-OP I hate jetting too.  I am SOOO lucky my YZ was spot-on when I bought it.

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Ordered a new air filter and crank seal, should get here in the next couple days. I'll put in the new seal and see if that fixes the problem, if it doesn't I will start looking at jetting, but I  am pretty confident its a bad seal.

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can anyone tell me which way the seal should go in?  One side has a little bit of a pattern on the surface as well as the word "outside", I am assuming this side points away from the bike but the seal that is already in is the other way??

 

Did the previous owner install the seal backwards??

 

Also I have tried heating, and screwing in a sheet metal screw and pulling out, but the dang seal refuses to come out!!?

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