THIRD TIME

[Woodsie]

I had been posting previously. Look at "Leakdown Test"
'02 YZ125. Wristpin sides of piston melted. Electrode on plug melted. 1st full tilt run after top end rebuild, same thing. Checked everything I could think of. Leakdown test, good, sprayed starting fluid all over motor, no rpm rise, changed crank seals anyway, had carb checked out and cleaned, checked timing, dial indicator and everything. checked reeds, good water flow, new gas mix, B8ES plug. Only thing I have left is power valve. Checked for free movement and linkage, everything looked good.
Anybody know if PV malfunction can cause an extremely lean condition. I don't think so but I'm at wits end.
Sorry for the long post, think I need counselling. This makes the third time it has melted!

[Aus_Rider_22]

There are a few different things to look at.

May I ask how you warm your bike up? And for how long?

Check here also: [Only registered and activated users can see links. ]

[Stangmata50L]

I'm going through nearly the same issue with one of my snowmobiles.

I have $600 into a motor that has moved 50 miles in 3 years.

I feel for you.

[Woodsie]

The bike is always at operating temp, B4 we go enough throttle to get into the pipe. At least 3 or 4 minutes of slow riding. The first time this happened, my kid was riding for at least 20 minutes, then full throttle on straight, melted. Second time was after top end rebuild. Broke it in, by the book. Ran fine. First wide open run, up to 3rd gear. Melted. Went through all the checks and measures, cleaned up the piston, (probably not recommended, buy didn't want to destroy a new piston,just in case). Put bike together, slow riding to check everything out. Screamed it a little. Melted. Bike is completely stock. Jets and all. We bought it used. We rode it 4 months B4 the first melt. Changed nothing.

[Aus_Rider_22]

That seems to point at cooling maybe. Have you checked all radiator hoses for kinks and make sure there are NO blockages.
Also make sure radiator isn't blocked up with grass or foreign debris as this stops air rushing through and cooling coolant and engine.

[Woodsie]

Water circulation, eveything looks good. warms up pretty fast though. This wouldn't have caused the plug to melt. Has to be an extremely lean condition at full throttle, to melt steel.
I'm baffled!

[drman]

It might not be the cause, but how is your left side crank seal?

[Woodsie]

I took the left sidecover off, sprayed starting fluid all around with the motor running. No rpm rise.

[cr2504life]

your main jet isn't clogged with something is it??

[Woodsie]

Had the carb cleaned and jets checked between 2nd and 3rd disaster. All stock jets, checked the numbers. Air screw 1 1/4 turns out. Didn't change jets or any settings, from when we got the bike. Rode it fast and hard for 3 months B4 anything happened. I was actually getting ready to rebuild the top end, when it blew the first time. Figured it was about ready. Guy rebuilt top right B4 we bought it.

[stickboy]

get a new bike

[Woodsie]

Just give me your motor!

[stickboy]

sorry owuld like to but cant i need it till the end of october

[02mncr250]

What kind of piston are you running?

is the cyliner stock?

no porting?

nicksil plating, or has it been sleeved?

WOT it burns,,,,,,it sounds like a mainjet issue,,,,,BUT you say everything is stock. How about the needle and needle jet?

What do you have for exhaust?

let me know

Jeff

[stickboy]

250f

[Woodsie]

Cylinder is sleeved. The original piston I think was ProX. I put a Wiseco in. Measured ring gap, it was good. By the book.
Other than that it is completely stock. Pipe and everything.
Needle is the same setting as it was when I got it.
Does the needle setting have anything to do with open throttle? I think the needle pulls almost completely out when the slider is up. No?

[stickboy]

ask Vet912 whats up he might now he just did alot of stuff ot his piston on his crf450

[YZGreg]

The needle controls 1/4 to 3/4 throttle and usually will be in the middle position on a stocker. If the clip is above the middle slot it will be leaner and the opposite if it's in one of the lower positions. So I would richen it up a bit (by moving the clip into a lower postition). Also how much compression are you running, maybe it was set up for racing and has very high compression which would want to drink race gas. I knew a guy that ran one tank of regular in his pinger and it melted a hole through the top of piston.

P.S. Stickboy, thumpers and pingers are a totaly different beast so I don't think it would be the same problem.

[Stangmata50L]

I knew a guy that ran one tank of regular in his pinger and it melted a hole through the top of piston.

We've never met before :

[Woodsie]

Do you mean regular as in....no premix oil, or regular octane?

I do use regular octane, But it did have right mixture premix oil.