04 450 wont start after new topend

30 replies to this topic
  • jakester1967

Posted May 14, 2008 - 05:01 PM


Well I am by no means an expert mechanic on these, but if the valves were off would they cause the bike not to start?

  • milomilo

Posted May 14, 2008 - 05:24 PM


take a pic of the marks and post....

also, is this true that either top dead center can be used?? and is it like this on like the crf's kxf's and rmz????

  • FriscoKLR

Posted May 14, 2008 - 07:39 PM


How far turned out are the timing marks? It sounds like the cam chain may be stretched a little too much.

  • 2000yzf426

Posted May 14, 2008 - 07:49 PM


i got the timing all figured out, the exhaust was off 1 tooth, timing chain is brand new motor hasnt started with the new chain on yet. ive got tons of spark, good compression im still thinkin its a carb issue even though ive had it off and cleaned it a few times.

  • grayracer513

Posted May 14, 2008 - 08:31 PM


... im still thinkin its a carb issue even though ive had it off and cleaned it a few times.

Pilot jets are very difficult to clean out by simply spraying carb cleaner in them. Run a piece of fine stiff wire, like a single bristle from a wire brush through the metering hole and any cross holes. Some will even use small "number" drills, not to enlarge the holes, but to be sure the holes are totally cleaned of dried varnish deposits.

Blow air through all of the air and fuel passages, including the air jets at the face of the intake bell. Run carb cleaner spray through each until you see where it come out from.

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  • spriggan

Posted May 16, 2008 - 11:35 PM


i had the same problem recently after removing both cams to replace valve shims...... took me about half an hour of kicking to get the odd backfire. when i put them back i aligned the timing marks perfectly in relation to the head. long story short took it apart again and the timing marks @TDC were right but the cam lobes were iffy. turns out both cams were advanced one tooth. check that the exhaust and intake cams are a mirror of each other at TDC.

  • wardy27

Posted May 25, 2008 - 10:38 PM


I just did the top end on my 426, it was doing same thing,, kick and kick nothing but an occasional back fire..drove me nuts for a couple of hrs...Come to find out it was the shim on the center intake lobbe was not seated in the center of the valve spring,,it held the valve open just enough that it would not fire...Just a thought

  • matt4x4

Posted May 26, 2008 - 08:35 AM


I always find when taking a top end apart, it's easiest to mark both the cams and chain with a grease pencil for reference since the chain can't slip off at the bottom due to the guides, if replacing the chain, also mark it at the crank gear, then lay the new chain next to the old chain and transfer your marks to the new one - once you install it, there's no guesswork anymore.

  • brent426

Posted May 26, 2008 - 03:47 PM


the cams would only line up every 2nd time as the cam gears have twice as many teeth as the crank thus giving it half the rpm as the crank. It doenst matter what stroke your on, as long as your at tdc, then change the cams to proper timming. As others have said you have to make sure the front side of the chain, closest the front of the bike) is tight with the tensioner on the other sided pushing out taking up the chain slack on the back side. Roatate the engine a few times and check the timming marks again as the few roatations will let the tensioner take up any unwanted slack. Hope this helps and goodluck

  • matt4x4

Posted May 27, 2008 - 04:54 AM


I think there are enough links off this page that you can get an accurate picture of how it SHOULD look on the YZF - 250 and 450 look the same TDC on the cam marks. Just remember, the article talks about changing the WRF TO YZF timing since the WRF is timed different.

  • 2000yzf426

Posted May 27, 2008 - 03:03 PM


well ive got the timing marks all lined up, took the carb off once again and cleaned everything, checked valve clearences and still gettin the same thing, the odd backfire every now and then. im about to give up and take it to the shop

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