Melted Same Pistion Again..HELP!

[blowit]

Can't believe my jetting was that far off. Hate learning the hard way but it's only time and money, neither which I have lots of. I'll rebuild and use bigger jets and try again. Thanks for all the help guys.

 

 

Can you post a pic of your piston? I also did not see your list of mods. If you are running stock porting, 300 certainly should not be melting pistons. twice on the left as well leads me to some other ideas. You need to do some careful testing when it runs again to ensure things are right. plug color, ignition, jetting etc. Also need to make very sure the brass washer is under your main jets. That will cause things to go lean. You many also have a severe carb balance issue that is leaning that cylinder out at WFO.

 

 

 

Mull Engineering

[Driggs]

Check out these pics and let me know if this is from being lean? If so, why does my other piston look so different?

 

 

 

 

 

[Kingsley78]

Check out these pics and let me know if this is from being lean? If so, why does my other piston look so different?

 

 

 

 

It looks like detonation has caused that, note the broken ring gland.

[Driggs]

What causes detonation?

[Kingsley78]

Too lean can do it and too high compression also. More so if you have both. Too much advance in ignition aswell.

[Driggs]

If everything is equal, I don't understand how the pistons can look so different. I'm sending my stuff off to FAST and let them sort it out.

[bansheesandrider]

If everything is equal, I don't understand how the pistons can look so different. I'm sending my stuff off to FAST and let them sort it out.

1 side is definitely leaner than the other one, so that means you probably have an air leak. I don't know what all you are sending to FAST, but if it is just the topend they may not find it. Once the motor is back together, you need to pressure check it. It needs to have everything on it except the carbs. Air leaks can happen at the crank seals,head and base gaskets, case halves, intake gaskets, etc. Also, if it blew like this the first time, did you get all the shrapnel out of the bottom end when you put it back together?

[Driggs]

1 side is definitely leaner than the other one, so that means you probably have an air leak. I don't know what all you are sending to FAST, but if it is just the topend they may not find it. Once the motor is back together, you need to pressure check it. It needs to have everything on it except the carbs. Air leaks can happen at the crank seals,head and base gaskets, case halves, intake gaskets, etc. Also, if it blew like this the first time, did you get all the shrapnel out of the bottom end when you put it back together?

 

 

I did a leak down test after the rebuild and also when I pulled the motor before posting these pics. No leaks.

[Driggs]

1 side is definitely leaner than the other one, so that means you probably have an air leak. I don't know what all you are sending to FAST, but if it is just the topend they may not find it. Once the motor is back together, you need to pressure check it. It needs to have everything on it except the carbs. Air leaks can happen at the crank seals,head and base gaskets, case halves, intake gaskets, etc. Also, if it blew like this the first time, did you get all the shrapnel out of the bottom end when you put it back together?

 

 

Oh and when it happend the first time, i replaced the crank so I know everything was clean. I'll pull the crank this week and make sure my new one is clean after this episode.

[cobrabc1993]

That sucks on your situation.... You had No air leaks.... did you make sure your carbs were synched right?? That can definately burn up a new engine.... It does sound like you may be jetted lean with 300s if you have port work..... Also, what kinda mixture were you running? 32:1? It says in the Yamaha manual to use an even thicker mixture during break in on the first tank of fuel...... Also, did you make sure your waterpump is working , and not stripped out?? make sure it is circulating coolant....?

And did you double check, for airleaks around your crossover tube where it goes into the carb boots...? And finally, are you sure those carbs are assembled correct?

You may wana try those new carbs you got from me the other day.... They were working perfectly, and just done up by Jeff..... Just double check the jet sizes in them

for your application........Goodluck man.....

[Driggs]

That sucks on your situation.... You had No air leaks.... did you make sure your carbs were synched right?? That can definately burn up a new engine.... It does sound like you may be jetted lean with 300s if you have port work..... Also, what kinda mixture were you running? 32:1? It says in the Yamaha manual to use an even thicker mixture during break in on the first tank of fuel...... Also, did you make sure your waterpump is working , and not stripped out?? make sure it is circulating coolant....?

And did you double check, for airleaks around your crossover tube where it goes into the carb boots...? And finally, are you sure those carbs are assembled correct?

You may wana try those new carbs you got from me the other day.... They were working perfectly, and just done up by Jeff..... Just double check the jet sizes in them

for your application........Goodluck man.....

 

 

I think it's the jets and something with that one carb even thou I've completly rebuilt it. I'm putting those on the shelf and putting the ones I got from you on next time. Thanks for the help. I'll check those things out too.