Blew A Hole In Piston

[blacknred03]

The same thing happened to me. I have a FAST port stock carb/stroke setup. The first rebuild lasted about 10 tanks then either I grabbed the wrong gas can and put straight gas in it or I had a massive airleak, both pistons were melted/detonated smearing to the exhaust ports.

 

The second time around I bored to the next size and fattened my jetting alittle. I am at sea level 20cc domes 360 main 27.5 pilot 4th clip 40:1 Motoul. It ran great for about 20 tanks and last week it felt weak so I pulled the head and saw signs of detonation on the right cylinder. I thought the main had trash in it, so I took it apart and it was clean. Now I am debating on going to a 21 cc dome or taking out some timing. i had 150psi on a cold engine. It ran fine for so long that I do not want to get domes what should I do?

[THE GRIM RIPPER]

Any hole burned through a piston can almost always be attributed to detonation, or pre-ignition, or both. A strictly lean jetting condition will cause extremely high EGT (Exhaust Gas Temp.) and melt the piston as the exhaust exits the cylinder leving you a tril of melted aluminum down the side of the piston and cylinder on the exhaust side. In your case, I think you definitely had a problem with detonation, being that you were at 180. I am at 195 on a guage, stock porting, and I can squeak by with a 50-50 mix of 94 and VP C-12. Attributing to your problem was the fact that you did not re-jet for the colder temps. This heated things up inside the motor, and when it got hot enough, the detonation starting taking it's toll, and you blew a hole through the wickedly hot piston. Now, even though you didn't crack the skirt off, (uncommon on a Forged piston, common on a Cast) you still have the debris from the melted piston crown floating around in the case. And I don't know where TX was going with the changin oil theory, your tranny oil is just that, tranny oil. It doesn't contact the crankcase like a 4-stroker.... Pull the motor and clean it out good, take your time putting it back together, and if you have any more questions, someone here should be able to help.... Otherwise, I have a strong feeling that you will be needing a new crank...

 

 

What he said to a T... And as far as going from 80F to 10F?!?!?! You shoulda been rejeting at 20 degree incriments along the way... Heres a good jetting link, even though its pinned already I think... http://www.dfn.com/benkaren/jetfaq.html