Changed piston out and now it won't FIRE

6 replies to this topic
  • craigroyse

Posted November 25, 2007 - 07:16 PM


Hello. I have a 2007 yz450f. I changed the piston, rings, and timing chain out today. Now my bike won't fire. Everything on the bike ran good before I tore it apart. I don't know why my bike won't fire. I checked to see if the plug was sparking, and it was. I tore the cams out and changed them around like 5 times because It is SO HARD TO FIND T.D.C.. My flywheel has like a " |-| | " on it. Looks like an " H " sideways and then a mark up and down. What I also did was took the plug out and stook my 1/4" extension on top of the piston and when the extension was at the highest point (piston at the highest point) The sideways H was turned clockwise almost out of view. Please help me find T.D.C.. Also, why wouldn't it be firing? AGAIN, MY BIKE RAN FINE BEFORE I TORE IT APART, JUST PUTTING A NEW PISTON IN FOR NEXT YEAR.

  • Kent Rathgeber

Posted November 25, 2007 - 07:21 PM


I know this is gonna sound stupid, but have you turned the gas on??

  • craigroyse

Posted November 25, 2007 - 07:23 PM


Yes, it had gas. I even took the jets out and made sure they weren't clogged up.

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  • rexbond007

Posted November 25, 2007 - 07:38 PM


did you check to see if you had spark, the spark plug coil wire might not be plug in all the way.

  • grayracer513

Posted November 25, 2007 - 07:55 PM


The marks look like this:

|-| |

<------ They move that way as the engine turns. The 3rd vertical mark "|" is TDC, not either edge of the "|-|". If you have had the flywheel off, verify that the "|" will line up at TDC. Of it will not, you may have the flywheel key out of place.

  • craigroyse

Posted November 26, 2007 - 05:19 AM


I do have spark. I will check the flywheel. There is a slot on the flywheel when you put it on that the woodruff key goes in so I don't understand how the flywheel can be off. But that is something that I will check.

  • craigroyse

Posted November 26, 2007 - 06:39 PM


I fixed the problem. I took the flywheel back off and put it back on. I didn't see any noticable difference that it was off. But it fixed the problem.

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