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Discussion Starter · #1 ·
my yz80 top end keeps blowing... it blewa stock piston and a wiseco with in 1 day the wiseco blew and the stock lasted a weekend im runing a br9es spark if that makes a difference and its a 1993 yz80... andthe rings are fine just the top of the piston
 

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Discussion Starter · #5 ·
haha no its not but the camera sure did make it look like one... if you look at the cylinder you can see were the piston stops and it wouldent hit the head... i was probly losing performance like that tho... lol im about to put "another" top end in it and just sell it and not ride it and let someone else deal with it... but i would still like to no what the deal is with it :D
 

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fuck for the life of me it looks like a contact mark. On the correct side for it too.

I'd check the squish clearance when you get it back together.
 

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yea definitely it would. It'd explain that bizarre looking punch thru too. Especially if that piston had a partially collapsed skirt and was rocking.

I took one look at that egg and it looks like foreign material ingestion while she was hella warm. Something made physical contact there.
 

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Discussion Starter · #9 ·
ok so i just did a very small little experiment :D i put the spark plug in the head and rested the piston on top the the head... nothing is touching or going thru that hole but i did notice if i look like "in" the piston it looks like somthing came from the crank case poped up and hit the piston with how the hole is made idk how to ecxplain it and i might have my laws of physics completely wrong but if you look at the pics u might understand what im saying
 

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the air (oxygenated) was blowing up thru that hole when it punctured.

Where did the squish zones touch the piston when you set it on the head? around the hole?
 

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Discussion Starter · #11 ·
what hole the one on the piston? or the mark on the squish that you think was contacting the piston? if it is what you think its contacting the piton lines up on that mark but it has a little play in between it... but the hole on the piston isent contacting anything
 

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Discussion Starter · #13 ·
o well idk i dont think that line is made from the piston hitting, because on the other side of that line (side farther away from the spark) its flat... its not curved... but i did notice on both pistons the holes are closer to the exhaust so its happening in the same spot... if that makes a difference
 

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it'll always be closer to the exhaust. First off 'cause it's hotter there and in the event of contact it'll generally be on that side because the piston pin isn't in the center of the piston, it's offset to the exhaust side by about 2mm, so when there's pressure on the surface of the piston it pushes down and creates a bending moment so the piston will rock, lifting the exhaust side and pushing down the intake side.

Now from the look of those marks, it's tough to say without being able to run a fingernail over it but it looks to me from that picture like a light contact mark, figure ballpark 2 or 3 thou of interference, nothing terribly serious if it was a valve in a 4-stroke touching a piston, In that circumstance I wouldn't worry about it but when it's on the squish zone it's no bueno, that's instant pre-detonation right there. and with the piston burning thru just inside and radially inline with where the mark aughta be heaviest.. the evidence sure points that way.

Like I say tho, it's tough to diagnose without having it in my mitts and with my glasses on.

If it was here I'd get that egg under the microscope and look for
1) oxidization in the crystals, which there should be given how hot it was. Aluminum will oxidize like a motherfucker above about 600c, picks up hydrogen too, which forms bubbles when it comes back below the freeze point and the hydrogen can't say saturated anymore. This leaves visible bubbles in the crystals, whereas a shrink or contact scenario will break the crystals, sheer 'em off.

2) I'd look for those broken, sheered off crystals, and for crystals that are substantially different in length and structure, meaning metal transfer from somewhere else, not likely the head either, those marks aren't heavy enough to have transfered any metal.
 

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anyhow, when you get it back together, check the squish on all 4 corners, you've got a decent caliper? digi or vernier, doesn't matter.

I'll walk you thru the process if you need a hand, actually I think I've gotta do a bigbore next week so I can probably snap some pictures and get a photo-walkthru.
 

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Keep in mind that mark you are referencing on the cylinder is the Ring mark....the top of the piston is going past that. I know thats a pretty obvious statement, but sometimes for me its the obvious things that are easiest to overlook.

How do the spark plugs look ? From all the pitting on the top of that piston I would say you have a REALLY bad case of detonation.
 

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Discussion Starter · #19 ·
spark looks fine a little burnt but is still gapped... and ya u noticed it was the ring marks after a couple of minutes that i posted that and was hoping no one noticed :p
 

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You are obviously running too lean. Did the bike run okay before it blew? Meaning throttle response? What i mean is when you hit the throttle did the bike momentarily bog then come up in revs and continue to rev for a little bit even when you closed the throttle?
Look for vacuum leaks / cracks in the boot (between the carb and the head)
Also check the left side crank seal. A bad seal in the left side crank case would result in a extreme lean condition and surely pop a piston.
I don't think that it would be your right side crank seal because there is no burned crank case oil deposits on the piston. Also you would have noticed the oil around the crank when you assumingly inspected it. (keep in mind that there is always a small amount of unburnt 2 stroke oil at the bottom of the crank case.)

Next, clean the carb really good. Rip it apart clean every jet and check for any unusual wear. If you have a clogged jet the bike sometimes would seem to run better, meaning that, if your main jet was slightly plugged the bike would run lean (more power) and once again, pop a piston.

Lastly, make sure your fuel mixture is right.

Hope this helps.
 
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