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2003 YZ250 ridden at Glamis in sand with paddle tire.

burrned up piston going up Oldsmobile hill. I run 40:1 mix. Pilot jet is 50, main jet is 182. Am I still too lean?

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IMG_3891.JPG

Edited by fftoddster
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Why not run stock 30:1 or 32:1 premix? Pilot will not effect your main while riding especially if you are climbing hills and spending all your time WOT. Remember that your needle and clip effect the first 1/4 to 3/4 of your throttle range. Main only kicks in past 1/2 throttle and up. Main looks good, but you could afford to go richer. Best way to test that is a plug chop. Also a leaky airboot will cause you to run lean even when your jetting is rich. Id check there as well.

thanks.

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Oil mixture has nothing to do with lean-rich.  The oil is simply there to lubricate the crank and conrod bearings.  Adding too much oil can displace fuel and cause a lean burn condition.

That piston shows evidence of either lean mixture, which will cause excessively high combustion temps and thus melt the aluminum piston, or it is evidence of detonation caused by too much timing advance, which can also scour the top of a piston.  I'd go with too lean on the main jet because the piston erosion is all at the exhaust port, which is where the temperatures are the highest.

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21 hours ago, CDNSXV said:

Oil mixture has nothing to do with lean-rich.  The oil is simply there to lubricate the crank and conrod bearings.  Adding too much oil can displace fuel and cause a lean burn condition.

That piston shows evidence of either lean mixture, which will cause excessively high combustion temps and thus melt the aluminum piston, or it is evidence of detonation caused by too much timing advance, which can also scour the top of a piston.  I'd go with too lean on the main jet because the piston erosion is all at the exhaust port, which is where the temperatures are the highest.

I guess you didn't reall read what I posted. But that's cool. 

I too suggested a larger main, but also made the suggestion of diagnosing any air leaks which can cause a lean condition and contribute to high combustion chamber temps. I think a check through of the carb and making sure all your jets are clean to ensure proper flow, and a decent plug chop test will help you out. If your sitting WOT in the sand you have to jet rich to make sure you aren't burning your bike up.

 

Sidenote, oil does effect mix, and I'm pretty sure that the recommenced ratio from the factory puts you in the right direction. 40:1 to 32:1 is not a large jump, but it will make he mix leaner from a gas perspective. Thanks though bro!

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I guess you didn't reall read what I posted. But that's cool.  

I too suggested a larger main, but also made the suggestion of diagnosing any air leaks which can cause a lean condition and contribute to high combustion chamber temps. I think a check through of the carb and making sure all your jets are clean to ensure proper flow, and a decent plug chop test will help you out. If your sitting WOT in the sand you have to jet rich to make sure you aren't burning your bike up.

 

Sidenote, oil does effect mix, and I'm pretty sure that the recommenced ratio from the factory puts you in the right direction. 40:1 to 32:1 is not a large jump, but it will make he mix leaner from a gas perspective. Thanks though bro!

I did read what you wrote.  However an air leak should be fairly obvious as it would make the engine run at elevated idle speed.  The OP went WFO up a long hill and seized near the top.  That screams lean mixture and the most likely source is main jet.

Hunting for air leaks isn't as simple as it sounds.  I had an obvious air leak at the intake on my RZ350 and none of the normal "methods" of diagnosis showed anything wrong.  In the end I just opened the thing up and resealed the reed cage and intake rubber and the leak stopped.  

You are correct, oil does affect fuel mixture but that bike didn't seize from too little oil.  The oil is only there to lubricate the main bearings, not to prevent the thing from melting the piston.  Adding more oil, as you suggested, would only make the lean condition worse and make it more likely to destroy the piston.

Cleaning out the carb is never a bad idea and considering the melted piston, probably a good idea.  If I were him, I'd still go a couple sizes bigger on the main jet though.  Would suck to destroy another piston because assumed the jetting was not the source of the first seizure.

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