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Author Topic: Place yah bets! Whadda yah think caused this!??  (Read 9706 times)

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Offline Nutz4sand

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Place yah bets! Whadda yah think caused this!??
« on: April 22, 2009, 09:27:48 PM »
My last rebuild on a Polaris Scrambler 400 2 smoke did a similair thing to the piston. It was jetted a tad rich. Had a pipe and reeds. And compression below 150 PSI.

I swapped a sled for a Polaris 400cc Two smoke Trailblazer and it ran out great. The compression was about 128 PSI. The machine was stone stock. I had it out in the Dunes at Silver Lake just getting around on it for fun. About a week ago I was out riding it and I thought I heard it detonate/detonating at high throttle. I stopped and looked it over but it seemed fine.   

I was riding it around more and it was running out well then I went at the big hill and halfway up it started to detonate for sure. I backed off and spun around and mildy rode it back to my parked truck. It had noticeably less power.

Got it home and finnaly pulled the jug today and found this.
Weird on a stock machine. The carb is clean inside and no plugged ports. Main jet is a 230 which is what the manual calls for with my altitude and temps I was riding it in.

I am starting to think the octane ratings on the pump are bogus as all hell. 87 Octane is OK to use (according to the  factory manual) but I had 92 in it.

So whatcha think?

The cylinder looks great as does the head. Nothing I can see that would have caused a hot spot to start detonation. But detonation sure got me.

Yah think the head looks like it was lean? The Plug is actually darker than the pic makes it look.

I pressure tested the crank in positive and negative pressure before I pulled it apart and it passed that with flying colors.
 
Never had these issues with my Banshees. But that was years ago.

The only commmon thing between these two Polarises I have (Besides being the same engine from the factory. On stock. One mildly modded) Is the gas. They each have different oil in them. Yet are scorching pistons the same way. Hmmm   ???

 
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Offline Boostinjdm

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #1 on: April 22, 2009, 09:43:37 PM »
It's obviously the driver's fault. ;D
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Offline BDKW1

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #2 on: April 22, 2009, 10:08:26 PM »
It was running the carb out of fuel under a load. That's a lean burn on a piston if I've ever seen one.
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Offline Nutz4sand

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #3 on: April 22, 2009, 10:11:28 PM »
Quote from: BDKW1 on April 22, 2009, 10:08:26 PM
It was running the carb out of fuel under a load. That's a lean burn on a piston if I've ever seen one.

The needle and seat are not plugged and when I drained the tank to remove it the fuel flowed quite fast. I will have to look at this more though. How it may flow fuel fine at times and not at others?
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Offline fabr

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #4 on: April 22, 2009, 10:20:57 PM »
it wasn't detonation.det causes breakage not melting.
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Offline Nutz4sand

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #5 on: April 22, 2009, 10:23:53 PM »
Quote from: Masterfabr on April 22, 2009, 10:20:57 PM
it wasn't detonation.det causes breakage not melting.

I have heard that mild detonation can make cylinder temps go up and melt pistons? Is that not true? (Serioulsy asking)

I heard it knocking and it backed off the noise as I backed off the gas.
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Offline fabr

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #6 on: April 22, 2009, 10:28:03 PM »
Nt that I'm aware of. PREIGNITION that many mistake for detonation will tho.
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but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

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Offline Nutz4sand

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #7 on: April 22, 2009, 10:30:41 PM »
Quote from: Masterfabr on April 22, 2009, 10:28:03 PM
Nt that I'm aware of. PREIGNITION that many mistake for detonation will tho.

I always thought they were related and nearly the same. If not the same.
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Offline fabr

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #8 on: April 22, 2009, 10:38:15 PM »
nope. Detonation usually is due to the flame front starting at the plug normally and then the process erupts into an explosion instead of a steady flame front propagation resultin g in very high mechanical stresses but not a lot more heat.. Similar to diesel operation. Preignition is due to a hot spot or too advanced timing or lean F/A and will localize the heat in an area such as you see burned. Detonation would have likely have broken the top ring and likely a ring land instead of melting.
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"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
 " You have all the right in the world to believe any damn thing you'd like, but you don't have the right to demand that I agree with your fantasy"

Ozpilot

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #9 on: April 23, 2009, 02:13:16 AM »
I took a piston exactly like that to the guy that runs the crank shop a while back and he said straight off "looks like detonation to me".  He then asked about compression ratio, jetting etc.

Looks to me like it may be bad fuel if the other things that usually cause lean condition etc can be ruled out.
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Islander

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #10 on: April 23, 2009, 05:41:10 AM »
I'm with Fabr on this one.
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Offline fabr

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #11 on: April 23, 2009, 05:51:07 AM »
Quote from: Ozpilot on April 23, 2009, 02:13:16 AM
I took a piston exactly like that to the guy that runs the crank shop a while back and he said straight off "looks like detonation to me".  He then asked about compression ratio, jetting etc.

Looks to me like it may be bad fuel if the other things that usually cause lean condition etc can be ruled out.
Looks like he doesn't know what detonation is. ;D ;D
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"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
 " You have all the right in the world to believe any damn thing you'd like, but you don't have the right to demand that I agree with your fantasy"

Offline fabr

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #12 on: April 23, 2009, 05:53:58 AM »
Detonation, BTW,  is much less likely with a hemi combustion chamber. Detonation usually is more prevalent in a wedge combustion chamber.
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"There can be no divided allegiance here.  Any man who says he is an American,
but something else also, isn't an American at all.  We have room for but one
flag, the American flag... We have room for but one language here, and that is
the English language... and we have room for but one sole loyalty and that is a
loyalty to the American people."
Theodore Roosevelt 1907

-----------------------------------------------------------
 " You have all the right in the world to believe any damn thing you'd like, but you don't have the right to demand that I agree with your fantasy"

SPEC

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Re: Place yah bets! Whadda yah think caused this!??
« Reply #13 on: April 23, 2009, 07:57:38 AM »
That is definately not a det. killed piston...
When they 1st started adding alcohol to all the gas We had alot of those, not burnt all the way thru but a bad spot on the exhaust side, running down to the ring...causing the drop in power...a couple of factors come into play here, the fuel is most likely the cause...when they add the alcohol, it takes 1.4X the fuel to keep the stroker happy, thus making it a tad lean on top...Not all 2 smoke oils are compatible with "oxygenated fuels"...In some cases I've seen this shit is up to 40% alcohol...It runs great at the lower rpms and throttle area's...But on top when things tend to get lean it leans way out and you get a melted edge,
Coulda just been a fluke... When was the last time the motor was gone thru ?
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Re: Place yah bets! Whadda yah think caused this!??
« Reply #14 on: April 23, 2009, 08:56:29 AM »
I believe i have yet to see a 2 stroke popo "4 wheeler" that hasnt burnt at least one piston in its life...
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