Top End Rebuild, Won't Start!!!!!

22 replies to this topic
  • SincYZ450f

Posted February 15, 2009 - 03:24 AM


Recently did a top end rebuild on my 07 yz450f. Put it back together and now it wont start only gives the odd backfire. Has good spark.
Any ideas wat my problem could be

  • William1

Posted February 15, 2009 - 04:24 AM


Re-check the cam timing

  • SincYZ450f

Posted February 15, 2009 - 05:34 AM


Ye did that i think everything seems to be fine. I didn't mark the timing chain when i took it apart but did the timing by the manual and it seems to be fine. Seems ok to kick (not hittin any valves, good compression)

  • SincYZ450f

Posted February 15, 2009 - 05:45 AM


Which mark on the crank should be used 4 the cam timing. The marks on the crank are' H I ' I used the 'I' mark??? Is that rite

  • husqy360

Posted February 15, 2009 - 07:20 AM


Which mark on the crank should be used 4 the cam timing. The marks on the crank are' H I ' I used the 'I' mark??? Is that rite

yes mark on case aligned with the I on flywheel.

the best way by gray racer:
Remove the spark plug and use a screwdriver to find TDC, or eyeball it by lining up the flywheel key with the bore centerline and time and assemble the chain and cams with the whole run of chain visible. Foolproof, but be sure to double check the timing by pushing the flywheel and cover on by hand and using the real marks just before torquing everything all the way up. You don't want to have to take any more apart than necessary if you screw up.

and yes it works !

  • grayracer513

Posted February 15, 2009 - 08:21 AM


How long was the bike apart for the top end?

  • SincYZ450f

Posted February 15, 2009 - 08:30 AM


The bike was apart a while. A good few weeks. Would this make a difference??

  • grayracer513

Posted February 15, 2009 - 08:50 AM


It could if the fuel turned on you. Two weeks isn't much but you might try some fresh gas and think about cleaning out the carb. I ha done case where the fuel turned into something non-flammable in a week's time sitting at the shop, but that is unusual.

Fresh plug?

  • SincYZ450f

Posted February 15, 2009 - 08:53 AM


fresh gas and plug was used, carb was drained as well. you think i should go 4 a push start

  • grayracer513

Posted February 15, 2009 - 08:56 AM


If you're sure of the cam timing. It could be nothing more than the weather. How cold is it?

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • SincYZ450f

Posted February 15, 2009 - 02:26 PM


very cold!! snow on the ground. Could be that eh? A bump start the way to go then

  • barry450

Posted February 15, 2009 - 10:31 PM


Do you have compression? Check one of your shims isn't holding a valve open.....

  • SincYZ450f

Posted February 16, 2009 - 10:10 AM


Yes plenty of comp.

  • BC3

Posted February 16, 2009 - 11:14 AM


[COLOR="Blue"]Did you have the carb apart at the same time you did the top end?[/COLOR]

  • BC3

Posted February 16, 2009 - 11:21 AM


[COLOR="Blue"]When I trouble shoot someones bike for "No Start" I take the plug out and put the plug wire back on the plug and ground it to the head so I can see the spark happen when I kick it over......I want it to spark on the compression stroke.....With the plug hole so far down inside the head I don't know if you could use this method (maybe with some help) By holding a finger over the plug hole and have someone kick it through.....The compression will blow your finger off the hole right when the plug sparks.....At least you will know your timing is close.....If your happy with the spark being in the right place the carb would be my next place to go (Fuel)

Good Luck [/COLOR]

  • MX1967

Posted February 16, 2009 - 12:35 PM


My friend had this same problem after re-building his 03 YZ450. It turned out to be a shim that wasn't centered on top of the valve stem & that wouldn't let the valve close completly. We know the local Yamaha mechanic really good & he came over after work & spotted the problem in about 5 minutes. I guess the shims sit on top of the valve stems in a little dished out seat. When my friend put everything back together that shim was off to one side & was sitting at an angle instead of flat in its seat. Hope this helps.

  • yam 3

Posted February 16, 2009 - 05:50 PM


2nd on the shim! Check your clearances, mine was off on one exhaust valve, and i had the same problem. Reseated and fired right up.

  • carson red

Posted February 16, 2009 - 07:19 PM


I just did a quick valve adjust, and the same thing happened. Kicks over fine, just backfires after about ten kicks.

  • mitchicle

Posted February 16, 2009 - 08:55 PM


I just had this problem with mine and it was my timing. Hope you figure it out.

  • gdbarr1976

Posted February 17, 2009 - 01:45 PM


Over the years I have had a learning opportunites by screwing up a valve adjustment in every possible way. Got the timing off, bent the valve, Timing chain was loose and skipped, bent the valves. Over tourqued the cam caps, seized the cams. Good, expensive lessons.

Double check the valve timing. If you are sure, and I mean SURE it is right, then pull the main jet and clean it. It's easy. Should start.

One more thing, did you use assembly lube?

Related Content


Should I buy this bike? help....... by Numsa Hul

Dirt Bike   General Dirt Bike Forums   What bike should I buy?
  • * * * * *
  • 10 replies

05 yz450f not fast enough by RedeyeYZ

Dirt Bike   Make / Model Specific   Yamaha   YZ 400/426/450
  • Hot  38 replies

Is it worth it? (Cam and High compression piston) by macgi77

Dirt Bike   Make / Model Specific   Honda   CRF 150/230 F/L
  • Hot  36 replies

Yamaha YZ450F 2017 by Chris.GVS

Yamaha YZ450F 2017
  • * * * * *
  • 1 review
  • Last review by Tommyk_55
  • On January 15, 2017

Wilson the Blue Privateer by jetfuel

Dirt Bike   Special Interest Forums   Pro Racing
  • Hot  50 replies

Join Our Community!

Even if you don't want to post, registered members get access to tools that make finding & following the good stuff easier.

If you enjoyed reading about "" here in the ThumperTalk archive, you'll LOVE our community. Come join ThumperTalk today!

The views and opinions expressed on this page are strictly those of the author, and have not been reviewed or approved by ThumperTalk.