07 starting problem?

7 replies to this topic
  • Family Man

Posted November 24, 2009 - 03:59 PM


I just replaced the piston on my 07 yz450f, 13 to 1 proX, and I cant get it to start. I have the bike timed to the last mark on the fly wheel when rotated counter clockwise the singular "I" mark after the two I-I mark. thats where TDC is also. The cams are aligned with the head as per manual or as close as they get with the head. 13 pins from 12 O'Clock to 12 O'Clock. I torqued the cam caps with a snap on torque wrench. However, the bike wont start just occasional backfire. I know I'm getting spark and gas at least. Bike is kicking over but with allot more compression than with the old piston and rings. Any ideas as to whats wrong?

  • Family Man

Posted November 24, 2009 - 08:29 PM


The backfire occurs with just cycling the piston I don't even have to give it a good start-able kick.

  • grayracer513

Posted November 24, 2009 - 09:54 PM


Clean or better, replace the plug, turn the fuel screw in 1/3 turn, and try cracking the throttle slightly when starting (may also try the hot start along with the choke).

  • Padgett

Posted November 25, 2009 - 06:40 AM


Man I got the same deal going on. I traded for a 06 450 about a month ago and the dude had replaced the piston and rings (ran fine before he changed the parts) and it has never busted of with kick starter again. You can pull it and it will start and run like a grape ape, but when you kill it your done, wont start. So I have gone over every little detail, timing is dead on, valves good, got spark, got fuel, got killer comp out of the top end and thing still wont start. I walk away from it for a while. Sorry I couldnt help much, but I'll get back on the bike next week and if I figure it out I'll let you know...better yet fix yours and let me know what it is. Have a good day.

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  • Family Man

Posted November 25, 2009 - 11:09 PM


I did a search and read that sometimes the valve bucket shim not being centered can cause the bike not to start? I think thats probably my next step along with a new plug. What are the other symtoms of a misalighned valve shim? Oh and it is cold here too. Probley in the 30s now. The bike does have way more compression when trying to kick start, so much that I cant find the sweet spot that I normally kick it at and get it to start. The Plug does get wet with fuel after a few attempts at starting.

  • mxracer206

Posted November 26, 2009 - 06:56 PM


Check the auto de-compression pin in the exhaust cam and make sure it moves in and out smoothly. Also I always look at the marks on the cams BEFORE diss-assembly, this helps let you know how badly the cam chain is stretched. The chain should actually be replaced on most top end services anyway, they are cheap and just as responsible for bad performance as a worn piston. If all this checks ok and it is really cold there then you may need to twist the throttle a few more times to get extra fuel in the cylinder.

  • Family Man

Posted November 27, 2009 - 01:46 AM


I checked the valve shims in the buckets and found one of the exhaust shims wasn't centered. It is now. This was probably the problem. Find out soon. I gotta get a flywheel puller, than I'm swapping out the cam chain. Got a new cam chain in the bag for my bike, but I didn't think to order a puller.

  • Family Man

Posted November 29, 2009 - 10:25 PM


I replaced the timing chain and the bike fires right up, first easy kick. I don't even have to find the sweet spot. Starts easier than it ever has. Sounds Awsome too. I saw a thread from Gray in a previous thread that said if the timing chain is stretched too much it can cause the bike to be out of time. Thanks Grey. I didn't know how stretched the old chain was, until I installed the new one. With the old chain I could just spin the cams to time them by raising the chain above them. Just wouldnt time right. PITA. No way with the new chain, way less slack. But the new chain isn't as tight between the cams. The old one acted as if it had no slack between cams with the cam chain tensioner installed. No matter which way it was timed it was hard to kick over and wouldn't start. It was a pain to start before I began working on it. It runs great now.

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