Question about 2008 Yamaha yZ250F

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I have a big problem with my YZ250F 2008, I installed the aluminum radiator protectors and by mystake I forgot to refill the radiators with anticoolant... so Yeah, I went for my Sunday common ride and after 1:5 hours the byke just stop and I couldn't start again.. I check the head, valves, crankshafts, piston, cylinder and ring and they were just OK, so I only changed the head and cylinder gaskets.. After thet, I refill the radiators and the byke just run.. but the engine feels like some valve where leaking.. I just disassemble and assemble the byke like 8 times and I couldn't get it on.. so I just bought a new complete head, cylinder, piston and rings and right now it's assembled but I can't get the bike to start :-( I don't know what else to do, please help me Oscar

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Assuming it all back together correctly as you had so much practice now I sure all your timing set correct and your valves clearance are all spot on
There is a few things I would be attempting firstly being its virtually brand new motor I would be checking your stator with multimeter testing its resistance readings - let me know if you want info on this ?
if the stator all good then I would be suspicious of the CDI unit especially as you have cooked motor beneath it the heat could have killed it ?
if these two theory fail then your next option would have to be leak down test to confirm your new head is sealing valves in it are sealing and you not losing compression past the rings
From what you have written I would be mostly concerned with electrical side first Dont be fooled either just because spark plug has a spark does not mean stator and CDI are correct?
have you tested kicking over with spark plug laying on head bolt or something is it sparking at all?
regards Jamie

Posted on Sep 13, 2011


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