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Discussion Starter #1
:cowboy: I just bought a '04 Sportsman 90cc from a guy off of craigslist. He claimed the bike had a dirty carb. When he turned it over, I was thinking low compression... but he quickly chimed in, "See you can hear the compression".

Anyway, bought it cheap enough anyway, I tore the bike down finding the cylinder shield melted. I pulled the head and cylinder off and find the piston is severely beaten on the bottom side, the cylinder has a metal chunk that's been welded to the wall of the cylinder.

I've never actually done repairs on a bike before... anyway.

1) What are my chances that I can hone the cylinder and be fine. Should I bore & go oversized?
2) If it got hot enough to destroy the piston/cylinder, should I expect valve damage? (The case internals look fine).
3) There is a little play (left and right) on the piston rod. (None pull & push). Does this point to damage/wear?

Anyway, thanks for reading.

Ryan
 

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Hi, welcome aboard!

1) I would let a machinist decided that. It takes a trained eye to really be able to know. You will probably have to go oversized though. Its tough to tell though.

2) Its a 2-stroke, so it doesnt have any valves. The rest of the top end is probably fine. (Cases, head wise)

3) Where is the play, top or bottom of the rod? I would always do a new pin, circlips, and upper bearing when doing a top end on a 2-stroke. If the play is on the bottom of the road on the crankshaft, you may need to replace that pin and bearing, but usually not.

The good news about doing a top end on a 2-stroke is that it is much simpler and easier to do then any 4 stroke!
 

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Discussion Starter #4
Honed

Well, these cylinders are available all day under $50 on eBay so I went ahead and attempted a hone. It actually came out very nicely. There are no scores or major damage on the cylinder wall . Anyway, I'm gonna slap a new Wiseco piston in it, rings, gaskets for $80 off ebay and run it. I'll let y'all know.
 

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Discussion Starter #5
I got it all back together today. In my journey to tear it all down I found that the oil injector cable was about to snap, so I ordered one on eBay and installed it today.

Anyway, I busted across the field and it seemed to have good power. On my second lap it started losing power. After 2 more laps it's got nothing and there's a little whistle down by the cylinder and smoke rolling off the cylinder area too. I'm thinking blown head gasket or cylinder gasket.

This is my first two stroke project ever so I need all the help I can get. Let me know where to start.

Thanks!
Ryan
 

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Assuming you torqued your head, you must then also allow the engine to run a complete heat cycle (get to running tempurature for perhaps 30 min..? And then allow to completely cool) and then retorque. This is due to it being aluminum. If it does not get retorqued, this could cause low compression, or a blown gasket
 

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Discussion Starter #7
Assuming you torqued your head, you must then also allow the engine to run a complete heat cycle (get to running tempurature for perhaps 30 min..? And then allow to completely cool) and then retorque. This is due to it being aluminum. If it does not get retorqued, this could cause low compression, or a blown gasket
. Good to know. I'll retorque the nuts tomorrow. It might be too late for the cylinder gasket tho. Although i wouldn't think there would be a lot of pressure there o blow that gasket. Anyway, if it is blown it will be cheap-unfortunately it will set me back another week of playing.



I'll check the head bolts torque again tomorrow. Think it's too late. :). I mean we're only talking a few bucks on eBay, but the wait is killin me. :)
 

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two strokes are easy to rebuild, but you should have done a leak down test after the rebuild. I would do one now if I was you.
 

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Discussion Starter #9
I retorqued the head nuts and the power is back with no smoke. It takes ether to start it up, but it pulls me around pretty good. I was hoping for more from the powerband, but maybe I'm dreaming with only 90cc. Why the hard startup though? Dirty carb? Choke not working (the choke doesn't seem to make a difference starting/driving)? Weak cylinder?
 

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Okay so, glad to hear she runs. The 90's are pretty pokey, however the only one I rode a was a predator so perhaps it's a little different. First thing I would do, change the plug. Makes a big difference sometimes. And yes, you must make sure your choke works, check your connections, and cable. If it still isn't running, clean out your carb. Let me know how it goes. Don't start it on ether if you can either, this is bad for your engine.
 
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