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Took the LT to dune fest for its first hopeful ride to come back still running. about 30 minutes in lost all compression. Pulled it apart last week and this is what I found.
about a month ago I had a local shop replace my stator, CDI, and coil. I'll be verifying the timing on the stator, then I'll get a new piston in it with new base gasket covered in threebond, new powervalve window gasket also with threebond, and new powervalve o-rings then I'll do a leak down test to make sure I don't have any air leaks. Cylinder wall and head look good no gouges or any damage. Anything else that I should look at? Maybe crank seal as well?
I checked my plug before we went and it was running a little rich. When I pulled the plug out after the poorly timed death it was running VERY lean.
Sorry for the terrible pictures.
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A hole in the piston like you have pictured is usually due to one or a combination of the following:
1. Too much compression for the octane rating of the fuel being used. (fuel looses octane rating points with age. )
2. Ignition timing too advanced.
4. Spark plug heat range too hot.
5. Exhaust system has too much back pressure (Spark arrestor plugged, and or Stinger size too small for the power the engine is developing )
6. Maintaining a constant speed at partial throttle when the engine RPM is high enough for the pipe to be in tune. On a 250, this RPM range is usually starts at about 6500 RPM and goes to the peak RPMs. High performance two strokes should not be left at the same partial throttle position for more than a few seconds at a time!!!!!!!!!!!! As the power level of the engine goes up due to improved porting and improved pipe technology the engines will tolerate less time at partial throttle with out experiencing run away detonation even with a well tuned carburetor, high octane fuel and correct ignition timing. -
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Dang! That thing nuked!
How's the cylinder look? -
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Thanks Jerry,
I didn't even consider the Stinger being the issue. Since this is still a std. bore, port and the stock carb it shouldn't be putting out too much power. The fuel was new 93 octane mixed at 32:1. The plug was a NGK BR8
When it happened I was climbing a hill, noticed a power loss but it was still running. Then tried the hill again, power loss and then it died, I immediately noticed the lack of compression and did not try to start it back up.
The cylinder still looks good but I should measure it out just to make sure. From the looks of it, the rings were fully compressed and no longer making contact to the cylinder, I literally had to pry them out of the piston head. I'll check my pipe to make sure its flowing correctly, as well as the timing since the local shop installed the new electrical components, I have not verified that they didn't advance anything or it slipped. -
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Run 100 octane when you get it back together.
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Will do from no on that's for sure.
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What jetting are you running?Stock jetting?
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What jetting are you running?Stock jetting?
No its not stock, I forget what I came to in the end. I ran it for about an hour before this trip and checked the plug it was just slightly rich with normal riding. I figure I could have leaned it out some but I didn't change anything as I would prefer to be a little rich on the sand.
What ever caused this, has to be a new condition. Like an air leak or exactly what everyone has said and I need to run a higher octane fuel. The local bike shop recommended me to run their JP-8 but I didn't for this last trip.
Odd thing is I did not notice anything different about the bike, it was running great, not running away like an air leak would. Just lack of power and then all of a sudden no compression. So I would believe either advanced timing causing too hot of spark. Or the low octane fuel.
I checked my Pipes Last night and the seem to be flowing fine with no blockage, so that can be marked off.
Edit: I guess it would also be possible that I got old gas from the station. I would find that unlikely but a possibility. -
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What jetting are you running?Stock jetting?
No its not stock, I forget what I came to in the end. I ran it for about an hour before this trip and checked the plug it was just slightly rich with normal riding. I figure I could have leaned it out some but I didn't change anything as I would prefer to be a little rich on the sand.
What ever caused this, has to be a new condition. Like an air leak or exactly what everyone has said and I need to run a higher octane fuel. The local bike shop recommended me to run their JP-8 but I didn't for this last trip.
Odd thing is I did not notice anything different about the bike, it was running great, not running away like an air leak would. Just lack of power and then all of a sudden no compression. So I would believe either advanced timing causing too hot of spark. Or the low octane fuel.
I checked my Pipes Last night and the seem to be flowing fine with no blockage, so that can be marked off.
Edit: I guess it would also be possible that I got old gas from the station. I would find that unlikely but a possibility.
Normally low octane you'll hear. -
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When it happened I was climbing a hill, noticed a power loss but it was still running. Then tried the hill again, power loss and then it died, I immediately noticed the lack of compression and did not try to start it back up.
The top of the piston shows where detonation has removed all traces of carbon and had death written all over the top of the piston before it burned the hole in the piston.
An engine that produced a piston that looks like the ones pictured was telling you that it was failing at least 5 seconds before the piston crown reached a high enough temperature for the top to blow through or to expand enough to start transferring alum to the cylinder or smearing over the rings with aluminum.
You did notice something wrong but unfortunately you ignored it. The sudden power loss is one of the many warning sign that something is wrong. The INSTANT you get one of these warning signs is when you grab the clutch and let off the throttle. Most of the time a rider that has had a lot of experience (blown up a lot of engines) riding two strokes would have heeded the warning sign "got of the throttle" and saved the piston from being damaged.
I am not trying to make fun of you or belittle your riding experience but this is the only way riders gain experience and learn some of these signs of impending engine death, is by killing many pistons and other related engine components. It took many pistons and crashes resulting from piston seizures when I raced motorcycles to get me to pay really close attention to every subtle change in engine tone, slight sag in power, vibration etc. After enough engine failures you begin to remember those familiar sounds, or subtle power changes or vibrations that occurred just before some of your previous seized or holed pistons.
High performance engines will not usually tolerate being very far out of tune without having failures if the rider does not heed the subtle warnings, stop and make the necessary changes to prevent the engine from failing.
If you had stopped the engine the instant you felt the power sag the 1st time, let it set for a minute and then slowly ridden it back to camp, removed the spark plug, you would have seen microscopic pepper like specks on the porcelain cone that surrounds the center electrode. The microscopic pepper like specks on the spark plug are from the outer edges of the piston and head and will only appear on the spark plug when the engine is experiencing detonation. Most of the time it requires a 2x to 5x magnifying glass to see these specks.
When you see these detonation specks on the spark plug the problem needs to be corrected before the engine is run hard again or you are going to kill another piston.
I do not believe in trying to build a recreational engine to run on pump premium. If the engine requires 91 to 93 octane fuel to keep it from detonating and you actually get 93 octane from the gas station today, it will not be 93 octane in a day or two if the fuel was left in the fuel tank on the bike or in a plastic gas jug. The fuel tank on the bike is vented and the plastic gas jugs are not usually absolutely air tight. Fuel will keep for a year or more in a steel air tight can or drum.
The fuel tanks at the gas stations are vented and the octane rating also diminishes as time elapses. Always buy fuel that has an octane rating that is at least a few points higher than what your race or recreational engine requires. Our cars and trucks have computers that are making thousands of tuning adjustment every minute that will prevent the types of engine failures we see in our high performance recreational engines. Our recreational and racing two stroke engines rely on the rider or his pit crew to make the necessary adjustments to keep the engine from failing. -
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Right here ^^^^ words to live by. Jerry is right, Since this is my first and only two stroke I don't have the keen ear for listening to the difference in how they run. After looking at my plug that I pulled, it does look pretty different then I've ever seen. just like you explained it looks "peppered" almost completely white. Checked my timing which was dead on. So I now repent my cheap ignorance and will swear my bikes to running race gas from the barrel. now I get to wait until my piston and gaskets arrive.
Thanks for the help with this. -
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i had no idea that deto would clean the carbon off the top of the piston. I couldnt figure out why your piston looked brand new - aside from the giant hole of course. lol
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Spark plugs from a properly tuned engine will be clean and white but without microscopic pepper specks.
Reading a spark plug for the optimum main jet size and power is another one of those interned topics where the want to be tuners are grossly misleading the new comers to the motor sports world.
A spark plug is not a sensor for air fuel ratio, a spark plug is not a dyno or performance measuring device. A spark plug is just a device that sparks and ignites the mixture.
The only accurate conclusion we can come to when looking at a spark plug is whether the various parts of a spark plug were operating in it's optimum temperature range
A spark plug must operate at a temperature that is hot enough that it keeps deposits burned off the porcelain cone that surrounds the center electrode, but not at a high enough temperature that the hot spark plug ignites the mixture before it sparks (this is what is called pre-ignition. This temperature range is only about 500 deg.F wide. When the plug temp. is too low, carbon deposits are not burned off and the plug fouls.
I can make spark look black and on the verge of fouling when it has jetting lean enough that power has dropped below max power and the exhaust temperature is high enough to burn the exhaust side of the piston. This can be done when the heat range of the spark plug is too cold and or the coolant temperature is too cold and or the time one spends at full throttle is very brief.
I can make a spark look white and on the verge of causing pre-ignition when it has jetting rich enough so that the engine is rich misfiring. This can be done when the heat range of the spark plug is too hot and or the coolant temperature is too hot and or the time one spends at full throttle is excessively long.
Tune the engine so that it has a slight rich misfire and lean the various circuits so that it just runs clean while monitoring the spark plug for traces for detonation. Engines tuned with the method will be reliable and will produce very close to optimum power. Once you have the engine tuned using this method of tuning, install the heat range that produces a clean porcelain cone and does not have any detonation specks.
Tune the bike under the same conditions as it will be ridden. Running the bike up and down the highway wide open and doing plug chops does not simulate riding the bike in the woods, dunes or race track. -
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X2 for MotorGeek, spark plug is not the best way to indicate a rich condition, rich bike produce lot of smoke and not rev at peak rpm. it stumble at idle end not idle very long.
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Quick update to this. I noticed that my pickup was running crappy. This was using the same gas that I used in the LT just not mixed with oil of course. So I emptied some of the gas from the LT into a glass mason jar and let it sit for like 15 minutes or so, to find that my mixed fuel separated from a bunch of water. I then checked the gas that was still in my 5 gallon Jug to make sure it was not just the gas in the bike, and found the same thing.
Needless to say, I won't be buying gas from that station for anything in the future. -
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Quick and short update for everyone:
Had my cylinder bored just a little over stock to true up the bore, reason for doing this was, because when I placed the stock piston into the cylinder without rings it was a little sloppy. got the cylinder back from the shop along with the piston kit. Put everything back together, with good usage of sealer to be sure I wont get any air leaks. and I started haveing a coolant leak on the front of the head gasket, this was using a paper type head gasket so I replaced it with a OEM type I had laying around. Leak then went away.
Now it has great compression, no air leaks, and starts with the first kick. I have not yet been able to take it out for a ride yet, but I am pretty confident that as long as I stay with the 110 octane it will be fine. -
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Quick and short update for everyone:
Had my cylinder bored just a little over stock to true up the bore, reason for doing this was, because when I placed the stock piston into the cylinder without rings it was a little sloppy. got the cylinder back from the shop along with the piston kit. Put everything back together, with good usage of sealer to be sure I wont get any air leaks. and I started haveing a coolant leak on the front of the head gasket, this was using a paper type head gasket so I replaced it with a OEM type I had laying around. Leak then went away.
Now it has great compression, no air leaks, and starts with the first kick. I have not yet been able to take it out for a ride yet, but I am pretty confident that as long as I stay with the 110 octane it will be fine.
Richen it up at least a few main jet sizes and a couple of clips positions on the needle at least for the 1st tank of fuel. Then lean it down if it runs too rich -
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Will do. Is this a good idea to do for all new pistons? or just in this scenario.
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