Hole in My Piston

[87TwoStrokeShee]

My banshee bogged on me at the Dunes over Memorial , brought it home and pulled the motor. I had no compression on the right side again. Motor only had 4 hours on a fresh rebuild. I pull the head off it and there was a hole in the center of the piston. I run 19cc domes, 110 octane with Caster 927. What would have caused this? the other side looks a little warm on top also. I dont have a air leak ither. Im lost......

[The Blaze]

what colour were the plugs?

[87TwoStrokeShee]

the left side was dark and the right side that had the hole in the piston was grey and dark.

[woodro77]

sounds like it leaned out , air leak , crank seal , same thing happen to me , just rebuilt it i hope it doesnt happen again

[Animalman294]

Did you do a plug chop on it after you rebuilt it, just to be on the safe side? Also when you went to the sand did you go to the same elevation that it was jetted to, or did you change elevations so you needed to rejet............?

 

It also sounds like the jetting between the sides was off if one got hotter and the plugs were not the same between the sides.................

Edited May 31, 2006 by Animalman294

[DIRTMAN350]

sounds like your jetting might have been off or your carbs were not in sync assuming you have dual carbs.

[87TwoStrokeShee]

I got a 2 into 1 intake and it ran just fine and jetted fine. I just dont know..

[boonman]

Your jetting likely wasn't off, BUT, it could have contributed to the problem, and made it a little worse. You had a detonation problem. A hole in the piston is 95% of the time a detonation problem. As mentioned, thejetting being a little lean could have contributed a little, and heated things up a bit quicker, but a hole in the piston is from detonation. Every one I have seen. And every one I have known about. It could have been from bad fuel, a timing issue, or a bad squish. On a fresh rebuild, there is a couple things to do before the motor is fired. One is a squish test, which can be done with solder. Another thing is a leakdown test. To make sure there is no air leaks. After this is checked, heat cycle the motor. I have had motors burn up during break in from not checking the squish. It's a must.

[woodro77]

do a leak down test , if it is good you know it is jetting , or maybe timing is off, [ajustable timing ?}

[canyncarvr]

The last holed piston (RH side) I took out of a Yamaha twin 2-stroke was caused by a leaking crossconnect (enrichening..call it choke) tube.

 

That shouldn't be an issue with a manifold.

 

The gray color you refer comes from the aluminum that used to be part of your piston.

 

While there is a difference between pre-ignition and detonation..the result can be the same.

 

Obviously you can be 'too lean' and have your jetting perfect...the lean part coming from something other than air through the carb.

 

Question: Is a burned hole in a piston due to a point source of heat?

 

Detonation involves huge pressures resulting from mixture 'blowing up' instead of 'burning'. Yep..that can eat engine parts, too.

 

Where is the hole? Top of the crown? Directly under the spark plug?

Edited June 1, 2006 by canyncarvr

[87TwoStrokeShee]

I pulled the head off before i went to the dunes and it looked ok. I ran it all of 10 minutes and it bogged. the hole is in the very center of the piston. You can see the connecting rod through the top of the piston. Jetting could be a problem but i dunno. it was the same temp at both locations. I didnt have a air leak did that test and a coolant test on it. What are the other test that you are talkin about? and how do u do them?

[KlotzBanshee]

If everything checks out so far (good jetting, no air leaks), it could be as simple as old/bad gas? I've heard of that happening with ppl running their snowmobiles for the first time after sitting all summer...??