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Old 11-17-2010, 09:49 AM   #1
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Default Detonation, or fatigue?

Just cracked open a crankcase, check this **** out (yo)!

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Intake port (just inside the reed valve):
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Best angle I could get of the exhaust port (aluminum shrapnel is in focus)]
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Top of the rod:
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I also had a pic of the head but doesnt look like it uploaded to photobucket when I was on my way out the door to work this morning. It looks relatively normal with a patch of aluminum shrapnel towards the exhaust side.

This came off of a 1972 yamaha LT2, which is a 100cc 2-stroke dualsport/enduro. My friend's mother rode it while she was in college, it sat for years in her uncle's basement and then she got the motorcycle before she went to college herself. It never ran right, and it never ran for more than a week when it did run while she was in college. I got my hands on the bike to fix, and within about 2 hours, it was running like a top. After a couple weeks, she started having overheating problems with it. I played with the fuel mix thinking that was the issue. It only really overheated if you'd been doing 55-60mph for a couple minutes (which, on this bike, is WOT in high gear and aerodynamically limited). We were going up a hill towards her house to finish a ride one day (mind you, we're not 2-up on this little bike, I'm well behind her on my SV intent to keep traffic off of her rear bumper in case the engine should lock up the rear tire causing a lowside, god forbid) and it overheats, and locks up - which is no biggie, I've properly taught her to clutch at the first sign of any trouble, and she's done well with that. So we pull off to the side of the road for about 30 seconds, at which point she kicks the starter, and it fires right up, we continue up the hill for about 300 yards when it dies again on her, again, she instinctively clutches and pulls off, but this time there's a bit of smoke coming from somewhere, and there's no engagement of the engine, kick starter doesnt even feel like it's doing anything. Her house is 1/4 mile up the hill, so I ride my SV to her house, and walk back down to her to finish pushing the bike up the hill, she'd already made over halfway up at this point. I examine the bike and it hits me like a ton of bricks - I had never actually considered cleaning the heatsinks off of the combustion chamber or head of the engine - there was 38 years of crud built up on them, could be part of the overheating problem (that, and running it for several minutes at a time at WOT.....who woulda thunk?) Anyways, took the bike to my garage, and told her I wasn't doing any work by myself; if she wasn't there, I wasn't working on it. She had to study for a national medical exam which she took yesterday (will have her doctorate finished in summer) and afterwards she came over, we made large alcoholic drinks, and proceeded to tear down the bike. I was relieved when I saw that the only catastrophic failure was the hole in the top of the piston, and that the rod and crankshaft, as well as all seals and bearings, were in perfect condition. $50 later, I had ordered a NOS piston, rings, and full engine gasket set off of eBay. I would like to know what you all think about the condition of this piston. Was it likely detonation, or was it more likely simple metal fatigue from overheating and being nearly 40 years old? Upon removal, the spark plug was in normal functioning condition, and the electrode had not been crushed.
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Old 11-17-2010, 09:58 AM   #2
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the area on the edge of the piston below the stamped 52 kinda makes me think detonation.
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Old 11-17-2010, 10:12 AM   #3
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Agreed.
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Old 11-17-2010, 10:59 AM   #4
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I'll get some macro shots of that area and post 'em up when I get home
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Old 11-17-2010, 11:27 AM   #5
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Looks like detonation to me too... plus it sounds like as you came up the hill that you stuck the piston a couple times. Not an uncommon thing on a 2 stroke when you lean it out too much (ask any kart racer). You're lucky the piston didn't stay stuck, I've seen plenty of cylinders get tossed because the piston simply wouldn't come loose.
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Old 11-17-2010, 12:01 PM   #6
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I fully intend to figure out fueling on this thing and get it running right. Any idea what the chances are that this is timing related? From what I know about this motorcycle, the coil was replaced a few years back. I haven't investigated very deeply into the electrical system yet with regard to how the coil signal is set up, but it looks like it draws the signal wire from beneath the stator. The coil is a simple 3-wire pack: positive, ground, and signal; is there a way to set timing on the coil or should I be looking at the crank for an adjustable timing sensor?

I'll probably figure this all out myself when I get back to my garage tonight; we had only enough time yesterday to remove the engine, split the case, examine and remove necessary internals, pull the crankshaft, and order parts. By that time it was 11pm and we both have early wakeups.
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Old 11-17-2010, 06:24 PM   #7
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Here's the cylinder head, and a more in-focus view of the top of the piston.
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Old 11-17-2010, 06:38 PM   #8
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Lean, baby, lean.
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Old 11-17-2010, 07:25 PM   #9
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I wanna see a pic of the plug, too.
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Old 11-18-2010, 08:41 AM   #10
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Quite sure we removed the plug back when it died on us; I think it ended up in a trailer in her barn and has probably made it to a landfill since :( When we pulled it out to start working on it, I saw the plug was still gone, and I thought "ah, crap".

I had always suspected it was running lean, but I couldn't seem to tune it out of it. I'm not fully educated on carbeurators, but I feel like I might need a new jet for any sort of long WOT running...
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