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02 cr 125 wont start after top end rebuild

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hi guys a friend of mine has had his cr 125 topend rebuilt and the bike will not start it had new topend gaskets piston and ring got good spark and fuel had the stator and all electrics tested they was all good could a crank seal make this problem cheers guys 

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Was the piston installed facing the right direction ?

On "windowed piston" design engines (not sure about your CR but quite common)

the window (port) must face the intake side.

 

Pull the pipe off and have a peak thru the exhaust port,

if you see a 'hole' in the side of the piston, it's installed facing the wrong way.

 

Worn crank seals would likely not prevent it from starting but rather create driveability & tuning issues.

Edited by mlatour

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Like @mlatour said, make sure the piston is installed correctly or else it won't get proper air and fuel. The "window" in the piston is part of how the air fuel mix makes it to the combustion chamber. Following that, you need to make sure the four necessities for combustion are there: fuel, compression, air and spark. Something is missing or not up to snuff. 

I.E.:

Have you checked compression? Clean air filter? Good fresh gas? You said you have good spark but did you try a new plug? Is the plug coming out wet? Is the gas turned on? Did the bike sit and then the old fuel gum up the carb? 

Lots of things to consider, and those 4 items need to be present to have good combustion. 

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