2003 YZ450F Won't Start

9 replies to this topic
  • edwin.smith2

Posted June 03, 2012 - 10:08 AM


The issue is the last time I rode my bike, I ran it fairly hard for almost an hour, then stopped to talk to a friend, and while idling, it just quit. It didn't rev up or make any weird noises or backfires or anything...just quit, like I'd hit the kill switch. I pulled the carb, cleaned it thoroughly, moved needle clip back to stock position #3, moved from a 42 Pilot jet up to a 45, 165 Main Jet, 77 Starter Jet, cleaned the filter, drained the fuel and put fresh back in, and put a new spark plug in it. It won't bump start or backfire. I pulled the plug wire/coil off and held it while I kicked it over, and not "shockingly,"...I got shocked. My next step is to check the resistances on the primary, secondary and source coils per the service manual as well as take a look at the CDI module to look for any obvious failures. I'll also check valve clearances just to make sure everything's in spec. The compression, at least while kicking-it over, feels fine, i.e. no grinding or locking up, so a catastrophic top-end failure doesn't seem likely, but I don't have means to do a leak-down test right now. I wouldn't think it would jump time to the point of not even a backfire w/o some kind of rough-running conditions prior to quitting. My feeling is the problem is electrical since I can't even get a backfire, even if bumpstarting. I've searched the forum as best as I can to find someone with a similar problem but I haven't come up with anything. I'm not hugely experienced with working on it but I know how to follow instructions, hopefully after looking for electrical gremlins I'll find an easy-fix but any help is MUCH appreciated.

  • Krannie McKranface

Posted June 03, 2012 - 10:33 AM


Curious as to why, after you had a good running bike, you felt compelled to change all the jetting?

Sounds like you are chasing your tail.

  • edwin.smith2

Posted June 03, 2012 - 11:42 AM


Sorry I was a little vague on the jetting, I kept the main jet and starter jet the same as before, the only one I went up on was the Pilot jet. I've consistently had issues with difficult cold-starts, so while the carb was off I decided on the bigger Pilot jet. Chasing my tail is an accurate description of how I felt after I put the carb back on!

  • Krannie McKranface

Posted June 03, 2012 - 08:42 PM


Pilot jet is for idle only.

Cold starts are tuned with the choke, which is tuned by the power jet size.

Most people don't properly adjust the fuel screw, and never learn the proper starting procedure for a modern four stroke with a pumper carb.

Do you know about either?

  • edwin.smith2

Posted June 03, 2012 - 09:45 PM


I'm pretty familiar with it, I've had the bike since 2007. For cold start: choke on, 3 or 4 twists to prime it, kick through til I get top dead center, bring the kickstarter back to the top and kick fully through. The fuel screw is two turns out from lightly seated.

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  • grayracer513

Posted June 04, 2012 - 07:06 AM


Start by checking the cam timing and valve clearances. Then test for spark. Even if you appear to have spark, test the stator continuity too.

  • edwin.smith2

Posted June 12, 2012 - 03:59 PM


Update: pulled the valve cover off today, piston at TDC, the exhaust cam is in time, but the intake side cam is way out of time. I zip-tied the cam chain to the frame, pulled the cam cap off and in a horrendous turn of events, I accidentally dropped the camshaft circlip down into the head! :ride: :devil: I'm at a loss as to what I'm gonna do to get it out. Good news is that since it was out of time, I may have figured out why it won't run. Any advice on getting this clip out? I can see it down inside the head so maybe an antenna-magnet?

  • grayracer513

Posted June 12, 2012 - 05:33 PM


If it went down between the valve springs in the head, it should be pretty easy to fish out. If it went down the chain tower, it might be a little more fun, and you could be ahead by pulling the flywheel cover.

  • marv02

Posted June 12, 2012 - 06:40 PM


[color="#000000"]Interesting[/color] I never seen a circlip type cam chain before I have built many motors I just seen the rivet type of cam chains before.

Back on the subject you can try to pull the Stator and flywheel and see if you can fish out the Circlip from there.

And about the circlip type of cam chain I would make sure the head of the clip pointed in the [color="#000000"]correct direction so it wont get knocked off when the motor is running.[/color]

[color="#000000"]I would ordered my self a riveted cam chain.[/color]

[color="#000000"]I just be scared if the circlip fell off inside the motor when it was running thing could get very ugly if that happens.[/color]

Good Luck

  • edwin.smith2

Posted July 08, 2012 - 02:50 PM


Well Grayracer was on the money as usual. I put a new cam chain in and re-shimmed the valves and now I'm up and running again. I had a long delay and some major headaches because of inadequate tools, but once I got access to the right tools half of my battle was won. It runs better now than when I first got it, best part is I done the work myself, no service costs. Thanks for all of the input, this is the biggest maintenance I've ever done and without this site I wouldn't have attempted it.

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