All,

I have a challenge for you 2 stroke pros!

2001 Yamaha blaster, my son was riding in the dunes and over revved the motor, broke a piston skirt and locked the motor.

I tore it down, fished all the aluminum out of the case, honed the cylinder, put a new top end kit in and now is starts great, has great compression, idles great, good low end power, as soon as it hits the power band in (starts into the main jet) it falls on it's face with no power almost like there is a sock it the intake, a grain of sand in the main jet, a reed is broken.

Here is what I have checked so far. Bare in mind that it ran perfectly before the piston broke.

I have done a compression check that came out good. I have gone through the carb, not changed any jets, but cleaned and checked all passages, taken off the intake, cleaned the air cleaner, made sure there is no blockage, pulled the reed cage, ispected reeds, pulled the exaust checked for blockage, all this turned out great. I pulled the TPS off the carb and it is farily simple, it is a simple button that is either on or off with continuity. It works and seems to be adjusted properly.

I am absoutly stumped. I have riden and worked on 2 strokes for 20 years and am at the end of my rope.

Does anyone out there have any ideas of somethings I can check?

Here are my assumptions: If the motor starts, has good compression and runs great (on the low end) there is nothing wrong in the motor its self.

Because the motor ran great at this jetting before, I am sure the jetting isn't the problem.

What i suspect: Could there be a problem with the oil injection? would this actually make the motor run the horrible? How can I measure it?

Does anyone have any suggestions of what I could check?

thanks for your opinions in advance.

OK, tonight I bypassed the oil injection and mixed fuel at 32:1 and put it directly into the gas tank. I started it up, it ran much better, I rode it around the block and came back basking in my glory that I had tamed the beast. My son hopped on it and it bogged down and did the same thing... So another variable ruled out. Nothing to to with the oil injection.

Any ideas?

Sorry I didn't see earlier, The piston is in backwards. Hope this helps. It's classic.

What would the piston in backwards do? Was this the final fix?