Problem getting bike started after valve rebuild! HELP!
  • Hienomiesh
    Posts: 1
    Joined: Wed May 24, 2017 12:04 pm

    Problem getting bike started after valve rebuild! HELP!

    by Hienomiesh » Thu Jun 01, 2017 1:53 pm

    Ok so i took the head of my CRF250R to dealer because my valves were .,they replaced them and i got my bike running..after few miles one of the valve springs blew off and the valve got bent..i took the head back to dealer and they replaced the bent valve and did a inspection of all valves and cylinder and piston. Everything was ok so i put everything back together,and now the bike wont start,all it does is backfire? i guess the timing should be ok,tips of the camshaft are facing backwards and marks on it and the crank.. wonder what is wrong? timing is okay,i have a new plug with good spark and still the bike only backfires? valve clearances are okay,they were re-shimmed after head rebuild. ](*,)
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    JimDirt
    Posts: 2617
    Joined: Sat Nov 25, 2006 6:31 pm

    Re: Problem getting bike started after valve rebuild! HELP!

    by JimDirt » Thu Jun 01, 2017 5:40 pm

    Welcome to the site !! \:D/

    I am a bit concerned that the valve hit the piston and they found no damage to the crank bearing , if it hit hard enough to bend , chances are it made a flat spot in the connecting rod big end bearing....maybe you got lucky , but i would be concerned and hope they will stand behind it it if goes , since its basically their fault the valve hit the piston , i would think they would have just replaced everything..... hopefully you did not sign anything saying if something else happens they are not responsible , i personally would have made them replace EVERYTHING including all bearings , crank/rod assembly , piston/rings all with new matching components of what was there when it blew apart ...hope i am wrong ....but i have seen a valve hit a piston before , show minimal damage only to have the rod bearing fail shortly after .....just be prepared and get something in writing before continuing much further , otherwise it "could" be a major expense out of your pocket because of their doing


    With that said:

    The backfiring issue can only be a couple things , it is either the timing IS off and your not catching it , or its a fuel issue or a massive air leak (gasket sealing or valves not seating as described and leaking past allowing a lean condition) , since the bike presumably ran fine before the rebuild and directly afterwards until the valve cut loose , i will stick with a timing issue first

    Cam lobes facing 2 o'clock rearwards , 2 slash marks on cam sprocket level with head like so - - , large round plug on clutch side has arrow and dot lined up facing each other , and ignition cover small round plug with mark , also make sure there is NO SLACK in the timing chain , if there is , all 3 marks will not line up and your timing will be off as the engine rotates ... also make sure tensioner is functioning correctly

    Start with double/triple checking the timing is in fact 100% perfect , something has happened between first tear down and re-assembly this time , if you are the one doing the work then just take each step and confirm each item is as should be , it has to be something you did or did not do and are just missing/overlooking it , it happens a lot , just confirm each step is correct and move to next one ,,,,,,,,,,report back with results...
    02 CRF450R
    06 CRF450X

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