how can you tell when you need a top end rebuild

I have a 125 sx 05 had i dont know how many hours it has on it... but it starts on first or second kick... but now i have to hold the throttle A little to keep it from Dying

Their are hundreds of tall tale signs of needing a rebuild, yes what you explained could be a sign but also look for power loss, abnormal engine noise from top end piston area. Honestly just rebuild it right now since you don't know how many hours are on it, it could save your butt in the long run and it would freshen up the bike.

Best way to find out and be 100% sure is get a compression tester and test it. All you do is take out the spark plug, screw the compression tester into the spark plug hole. Than kick it until the gauge stops going up.

When i bought my 03 250sx, the guy said it had recently had a top end rebuild less than 10 hours on it. I opened her up anyway and replaced the piston and rings e.t.c. just to be safe. lucky i did because 1 of the circlips was not in properly and could have fallen out any minute. It now runs like a champion starts up first kick and has a F**k ton of power. :)

 

If i was you, like the post above says, id just give it a top end rebuild anyway then you know 100% that its good to go. Save you alot of hassle if it blows up. They are relatively cheap and can do it yourself easy if you have the service manual, if you dont have one, get one :)

Edited by stevensonnz

What is the idea behind the compression tester, what does low compression or high compression suggest? are you not pumping air into the gap between the rings and cylinder wall into the engine to the point where something breaks? lol Idk how it works

What is the idea behind the compression tester, what does low compression or high compression suggest? are you not pumping air into the gap between the rings and cylinder wall into the engine to the point where something breaks? lol Idk how it works

 

You use a compression tester to see how much play is in your powerband. If the powerband gets too soft it can break off inside your engine and cause all kinds of damage.

You use a compression tester to see how much play is in your powerband. If the powerband gets too soft it can break off inside your engine and cause all kinds of damage.

don't troll or post crap like this when serious people are looking for help, trolling on a serious forum like this could lead to someone blowing up their bike if they take you seriously...

What is the idea behind the compression tester, what does low compression or high compression suggest? are you not pumping air into the gap between the rings and cylinder wall into the engine to the point where something breaks? lol Idk how it works

No you cannot do any damage to you cylinder with a compression tester.  You are not pumping air into the engine.  The engine itself is basically an air pump.   The compression tester is reading how efficiently it pumps air.  

 

A low compression reading on a 2 stoke is *usually* a sign of worn rings or a worn cylinder.  

 

http://www.youtube.com/watch?v=urz12y5tkKk

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