Yz450f wrong timing symptoms

12 replies to this topic
  • dbikie

Posted December 14, 2014 - 05:48 PM


I need help with my 2007 yz450f. The problem im having is after rebuilding the top end my bike doesnt want to start. It ran fine before i worked on it. The things i tried was removing the cams and turning the engine over by pushing the kickstarter with my hand, with all the valves closed it has good compression. So the piston and rings i installed are good... when i put the cams back in and time it correctly it turns over really easy and i cant seem to find tdc because all the air is escaping out of the cylinder head vent hose, and it doesnt start. I lined the punch marks to the edge of the head where the gasket goes and the mark on the magneto with the mark on the case. so i know its timed correctly. It seems like the valves are open during the compression stroke. i dont know what else would cause it to loose compression after i install the cams. I thought maybe i installed the cylinder head gasket or base gasket upsidedown but they're uni gaskets so couldnt be that and their new gaskets. It has spark and i went through the carb completely and its working fine. does anybody have any ideas on what i could do or have had this happen to them before, i really dont want to take it in to the shop because it ran fine before i tore in to it and because i only rode it once after i bought it. i didnt check the anything on the cylinder head(valve clearences, springs, ect), but it should be fine because i wasnt having problems like this before. 

  • drtrcr400

Posted December 14, 2014 - 06:09 PM


Have you compared your timing marks to pictures posted here or elsewhere? Or, you can try posting a few pictures yourself. A lot of us have set up dozens or even hundreds of these and can tell quickly if it's timed correctly.


Posted December 14, 2014 - 06:17 PM


could you possibly have mixed up the valve shims when you installed them?

  • Nautical Miles

Posted December 14, 2014 - 08:38 PM


check the decompression flywheel weight on the exhaust cam. sometimes they get a little sticky. when its on the inside circumference of the exhaust cam, it "decompressing". when its on the outside, its "closed". also, like frantik said... make sure its shimmed correctly. if your marks are lined up correctly and it looses compression when the cams are hooked up, a valve is letting compression out when its not supposed to.

  • dbikie

Posted December 14, 2014 - 11:28 PM


At first I thought it was a gasket not sealing so I went ahead an purchased another set, only this one came with the shims, tomorrow I will dig into it and and let y'all know what I find.

  • racer8

Posted December 14, 2014 - 11:32 PM


Where ya at in Texas?

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  • dbikie

Posted December 15, 2014 - 12:34 AM


Dallas ft worth area

  • grayracer513

Posted December 15, 2014 - 08:00 AM


Air is supposed to come out the vent hose.


Check your valve shims.  It's a common mistake to have one tilted in its pocket so that there is no clearance (checking clearances is a good way to catch this). 


Also, if the bike sat for several weeks while you worked on it, it could well be that the pilot jet is clogged with dried gasoline.

  • dbikie

Posted December 15, 2014 - 10:56 AM


But it doesn't seem to get hard in one spot when i kick it over, and the air coming out of the vent hose is really strong when i put my finger over it feels like when you have the spark plug out and put your finger over the hole to check for compression is strong as that.

  • grayracer513

Posted December 15, 2014 - 11:00 AM


The piston moves up and down in the crankcase, you know, and when it comes down the bore, 450cc of air has to go someplace, thus the vent tube.  Ignore that, and start by checking your clearance.

  • dbikie

Posted December 16, 2014 - 12:12 PM


Here are the timing marks I'm checking the shims right now, kinda hard to do without a feeler gauge hopefully I'll find one that's sideways

Attached Thumbnails

  • image.jpg
  • image.jpg

  • dbikie

Posted December 16, 2014 - 12:45 PM


You were right grayracer513 one of the exhaust shims was off to the side, problem solved, I forgot the importance of torquing the cam caps.

Attached Thumbnails

  • image.jpg

  • grayracer513

Posted December 16, 2014 - 01:21 PM


You were right grayracer513



Imagine that

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