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Hi everyone. I picked up a 2003 blaster with the 240 vito big bore kit has the +3 stroker and a full fmf exhaust  and I rode for maybe a half hr n she was done melt the exhaust side of the piston and not sure why. I have boysen carbon fiber reeds. No sure the carb size the needle is set half way and the main jet is at a 340. Running 32-1 yamalube too.  Not just if it's my jetting or if I was sucking air some where.  Thanks



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Ideally, after observing the damage thru the exhaust port and determined a possible air leak,

you should have performed a crank case leakdown test before tearing into the engine.


Nevertheless with no past maintenance history on it, new crank seals are in order.

Make sure to use gasoline resistant sealer (Yamabond) and not regular automotive RTV around the reed assy and wherever required

and, still perform a crank case leakdown test once the engine is re-assembled but before running it.


I notice a lot of dirt lying around the cases, vacuum all that up before you contaminate the bottom end.


Any up/down play in the connecting rod?  or any movement felt when wiggling the flywheel ?

If so, that also will need to be addressed.


Not familiar with the Blaster and main jet size but, weren't those engines initially equipped with oil injection?

Meaning the main jet size was originally determined to flow gasoline only, when premixing

the oil is now displacing some gas meaning a larger main jet is required to flow the same amount of gasoline as before.


In a 2-stroke, the oil lubricates but it's the gasoline that cools the engine's internals, preventing expansion of the piston and seizure.

You'll also want to confirm if fuel delivery was a problem, say a lower than spec float level which leaned out the mixture during sustained high rpms

and, that nowhere along the air inlet tract can debris get past the air filter element, allowing dirt to be ingested.


Troubleshooting this properly will be important if you don't want the same fate to happen to your newly rebuilt top end.


Post some pics of the spark plug's tip and, top of the piston as they might reveal other clues.

Edited by mlatour

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