Cannot get my YZ450 started, ideas?

6 replies to this topic
  • devshee14

Posted July 28, 2012 - 11:58 AM


Hey guys im new to this forum, iv got a 2004 yz450f. recently i was driving it and noticed it started losing power i almost made it back to my truck befor it completly died. engine siezed. brought it home and ripped it apart, found the lower rod bearing has siezed to the crank throw but only in certain spots. it moved freely then it would sieze up. regardless i had a used crank out of a spare bottom end i received with the bike. i brought the used crank into work and cleaned it up, feels fine, brought it to a local shop and had them look at it, looked fine. cleaned up the cylinder also and had them look at it. looked fine. orderd new crank bearings and a new wisco piston. rebuilt engine with new gaskets and put it back in bike. i have kicked and kicked and kicked with no luck of it starting. i pulled the valve cover back off and checked the timing, roll it once to the dash and timing is dead on, roll it again to the dash and its 180 off, (just like a four stroke should be). so my timing it right on the money. pulled the spark plug out, looked fine, grounded it against the head and kicked, it sparks. replaced it anyways. put new plug in and checked the gap first. checked the spark on the new plug and it sparks also. removed the carb and even though it wasnt dirty i cleaned it anyways. all passaged clear. removed air filter and cleaned it, clean as a whistle. kicked and kicked and kicked, no luck. brought it to work and towed it behind a forklift, sounds as if you were trying to tow it around with the kill switch on. makes all kinds of compression, kinda hard to hold onto the rope from the forklift. i have heard a pop out of the exhaust twice so far but thats been over a period of an hour or two of trying to start the bike.

i am very mechanicly inclinde. i build diesel engines and repair them every day as my full time job, i dont know if gasoline is not my forte or what but im getting very tired of trying to get this old girl going. dealer down the road charges about 85$ an hour so id like to hear some opinions here befor i let them throw some very marked up parts at it to try and fix it.

any more information you need just ask.
thanks for your help

  • DC_Excitement

Posted July 28, 2012 - 12:54 PM


i wonder if the cam gear spun on the cam shaft? causing it to be out of time. your marks may be lined up but the cam really isnt.

  • devshee14

Posted July 28, 2012 - 01:12 PM


i looked at the lobes and none were touching the camshaft and they all looked to be about where they should, not only that with a tight tolerence bike like that it should hit the valve if its out of time. plus i did check the clearence on the exhaust valves and they wer right on.

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  • devshee14

Posted July 28, 2012 - 01:14 PM


none were touching the buckets sorry not camshaft

  • DC_Excitement

Posted July 28, 2012 - 02:24 PM


i know you say it has compression, but did you do a compression test? and is there any chance that it could be timed 180 off? did you see the piston at the top when timing her up?

it sounds like you know what your doing im just trying to think of some simple stuff..

  • grayracer513

Posted July 28, 2012 - 05:22 PM


Start by verifying TDC is where the marks say it is. Pull the plug out and locate TDC with a screwdriver, etc., while watching the timing marks. If they don't agree, check the key under the flywheel.

If it's been a month or so since it ran last, dump the gas and try fresh stuff. If that doesn't do it, open the carb up and check for blocked or partly blocked jets, particularly the pilot and starter jets.

Try a fresh spark plug, too. Sometimes dry looking plug insulators can be conductive enough to short the spark under compression.

  • devshee14

Posted July 28, 2012 - 05:49 PM


hey guys, as per the checking of compression, i did not do a test as i think i remember trying it before and it would not read accuratly because of the auto decompression on the exhaust camshaft. i also recall reading afterwords that you could not check it with the camshafts in. as for timing i went over the bike again today and like i said i checked the timing as per yamaha's repair manual, it checked out fine and i also did pull the plug out and checked it with a seal pick to assure the piston was at TDC when the marks were aligned.
the spark plug in the bike is brand new as i said above and i gapped it as per spec. also i think i mentioned above that i did clean the carb and it wasnt dirty in the first place, all passages are clear. the fuel in the bike is about three weeks old. its deffenitly not dirty fuel as it does not smell dirty or feel like varnish and the bike wont fire on ether either.

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