Motor down, Piston damage, detonation????

[outlaw700]

'99-2000 Polaris 700 motor, stock head, elev. 1200'. Engine has about 3-400 miles on total rebuild, bottom and top. Have rode it mostly in Cooke City with Hot Seat high compression heads. Converted it to stock head couple days ago. All looked really good when I had it apart. Jetting at 185's (keihins), Attac compensator reset for this elevation to ride around here a little. Rode a couple of miles and checked plug color, cylinders and wash. Looked good. Exhaust temps running 1000 at WOT, 1100 mid range. Got about 5 more miles and Mag side went down. Most of damage is on the exhaust side of piston.
Fresh gas, just picked up 2 days ago, 92 octane, but of coarse never know what your really gettin.
Lookin for a little advice on what you maybe think went wrong.

 

[BU1]

Looks surprisingly similar to my 700 Summit that went south 2 years ago. Never could get a good reason why mine took both sides out at the same time.

 

[CATSLEDMAN1]

judging by the wash on the piston you are running right on the edge of too lean or in other words too hot on the side that didn't burn, so most likely the other side was hotter, egt are exhaust gas temps that can just as easily be far colder than combustions temps or hotter, they are a good indicator of changes you make to jetting, but no fail safe. oK. that obvious.

So what to do................find jetting that will give you a 3/8" or better of wash on your pistons. If you error in you jetting you want to be on the rich side as its much safer, better for your engine, and lean jetting doesn't make power, often has good throttle response which can make you think you are making power.

The kiehin carbs proved to not be a real a good choice for Polaris back then, nice crisp carbs on two stroke dirt bikes, but too many hard to get rid of lean jetting in mid range and lots of 700 polaris's this vintage suffered the same problem as you have.

Going to keep this sled and motor? shop around on e bay or swapmeet on here and get a set of rack flat slides mikini's used on the later Polaris 700 and use their later jetting specs.

Really want to lean more? go to Cutler Performance web site and log into their advice /engine breakin section download, spend a couple hours reading this information, its right on the money and explains why egt, heads , gas etc
may or maynot help your engine.

 

[diggerdown]

I agree with it being too lean. The pto side looks real lean and the mag side usually needs 1 more jet side to even them out. I think you should have been 1 more size on the pto and 2 on the mag. The marks on the piston are from the debry banging around.

 

[devilrmk]

I agree with it being too lean. The pto side looks real lean and the mag side usually needs 1 more jet side to even them out. I think you should have been 1 more size on the pto and 2 on the mag. The marks on the piston are from the debry banging around.

My opinion also. Too lean

 

[outlaw700]

Revise my current jetting

I typed my original post up at 1 am this morning. Now reading your reply's.
The jetting I have in is, PTO 185 and MAG 195. I had noticed the mag side ran aabout 100 deg. hotter when I first installed the engine two years ago. Only had 195's in my parts box for richer jets. With Keihins that should be 4 sizes richer. That seemed to even out the exhaust temps though.

 

[theultrarider]

It does not matter what number jet is in your carbs when you look at your pistons and they are telling you that you are too lean, you go next size bigger mains. Listen your your wash and plugs first. Don't even look at egt's until you get them right. Then you can look at your egt's and use that temp as your baseline. Lastly, listen to what jets your bubby has in his sled or what his egt's run. They are all just enough different to burn one down if you get too close to the edge without actually checking your own wash.

 

[sidehill]

Lean

 

[rookie]

Lean

I run that motor with those carbs, and those jets for years at 3000-6000. If you went to 1200' you are way too lean. Did it idle faster? You probably should have changed out your slides to a 5 instead of a 6 (stock) the cut away in the 5 slide is smaller, allowing in less air and more fuel at idle. What needles were you running? Probably should have dropped that clip all the way down, until you could change to a richer needle. Were you WOT all the time? Or at least wicking the throttle? It looks like a mid-range lean to me. Just my thoughts.

~Rookie

 

[MARCUSO]

Good post

It does not matter what number jet is in your carbs when you look at your pistons and they are telling you that you are too lean, you go next size bigger mains. Listen your your wash and plugs first. Don't even look at egt's until you get them right. Then you can look at your egt's and use that temp as your baseline. Lastly, listen to what jets your bubby has in his sled or what his egt's run. They are all just enough different to burn one down if you get too close to the edge without actually checking your own wash.

This is the most intelligent post I have seen in a long time.

 

[outlaw700]

Problem found

After further investigation and reading all the reply's, I believe I discovered my stupidity.
So here in Minnesota we finally are going to get a dumpin of snow so I converted it the night before to stock head and stock jetting for this elevation. Big key to the whole burn down is the ATAAC system requires 20 below jetting. Last year when I rode it around here I disconnected the ATAAC and ran 185 and 195 jets. Of coarse put the same setup back in the other day as it worked well last year.
Obviously way to lean with Ataac hooked up.
I should't have gotten so excited we were finally going to get snow and thought the whole setup through when I converted it.
Oh well, burn down and learn.
Good news is I had all the parts on the shelf and its back together and runnin.
Ataac disconnected, 185 and 195, wash is about a half inch on both pistons and EGT's came down overall.
Thanks for all the reply's.

 

[NoSecondChances]

After further investigation and reading all the reply's, I believe I discovered my stupidity.
So here in Minnesota we finally are going to get a dumpin of snow so I converted it the night before to stock head and stock jetting for this elevation. Big key to the whole burn down is the ATAAC system requires 20 below jetting. Last year when I rode it around here I disconnected the ATAAC and ran 185 and 195 jets. Of coarse put the same setup back in the other day as it worked well last year.
Obviously way to lean with Ataac hooked up.
I should't have gotten so excited we were finally going to get snow and thought the whole setup through when I converted it.
Oh well, burn down and learn.
Good news is I had all the parts on the shelf and its back together and runnin.
Ataac disconnected, 185 and 195, wash is about a half inch on both pistons and EGT's came down overall.
Thanks for all the reply's.

That's why I threw my ATAAC in the garbage and just jet the thing. Don't keep it at the same throttle position until you get it jetted right using plugs and wash, glad to hear you figured it out though.

F-bomb has some really good info on his site for setting a baseline. www.F-bombracing.com

 

[winter brew]

At least you know your ATACC was working.

 

[rab]

hard way to find out