07 YZ450F starting issue. Tearing into it tonight and would like gameplan help

Hey guys


Purchased a 2007 450F last summer and put about 10 hours on it. On its last ride, it started to act like it was out of fuel. I pulled over, checked gas tank and it had plenty.  tried to kickstart it a bunch and it wouldnt start.  walked it back to the car and let it cool down.(it was a hot hot summer day).  Once it was cooled down, i tried to kick it over again and it started just fine. I quickly loaded it on the trailer and took it home.  Next weekend it wouldnt start and hasnt started since.


I will also say that it is most likely in need of a valve clearance adjustment.  I am unsure if I should try to readjust valve clearance(wait for new shims to arrive) or try to get the bike to start and running properly before i do the adjustment.  opinions?


so i was thinking this

1.) take airfilter off, spray some starting fluid and see if it will run and how long. If it runs, then I have spark.

2.) if doesnt run, then ill check spark at the plug.

3.) If runs, but eventually dies, then ill check to make sure fuel valve still works.

4.) If fuel valve works, then it has to be something in the carb. tear carb and clean everything.

5.) If running fine now, then let it cool down and work on the valve adjustment.  (can someone validate its .10-.15mm intake, and .20-.25mm exhaust)



anything im missing or should look at?

Edited by squid4life

I would check the valves before doing anything else.

alright. sounds like i have a gameplan

When trouble shooting I like to get all the basics out of the way first. Is it getting fuel, air, and spark? And don't forget to check the fuel filter inside the petcock assembly. I have seen places that have dirty gas clog those little screens up over time and it starves it of fuel. From there I would go to the harder issues like valve clearance, dirty or clogged carb, etc. You never know, it might be something really simple? Good luck.

Compression, ignition, carburation. With those three near enough to optimal an engine will start. I'd say checked out in that order but 4T singles with automatic decompressors make it less obvious if they are low on compression in my experience. You were never in any doubt in the days of a Matchless G80 with a valve lifter lever. So I usually start with a check for a spark. Starting again after cooling down could be valves, but it could also be a stator winding problem.


You have to have the tank off anyway to check for a spark. If there is a good one then is it wet with fuel, if so carry on and check for tight valves. If there is no spark there is no point doing anything else unless you can find out why.

hey guys, updated.

checked valves: perfect in the middle of the spec

spark: spark seemed decent but plug was alittle black(ill replace)

fuel: pitcock was working so fuel was getting down in there


Took a few kicks then i hit it with starting fluid and it started up just fine and ran till warm. So i noticed my choke was stuck and was open.  There is a ggooood chance it was stuck and i didnt know back when i last rode it as I remember a buddy fiddling with the carb adjustment because during the morning when it was running it had some popping. 


well i let it warm up and noticed the exhaust pipe was molten red.


im guessing I have it too rich which potentially is making it flood, foul plug and why it eventually flooded out till it evaporated out. 


sound right?   just need to make it alittle leaner till the pipes stop turning red right?

No.  The pipe normally glows.  A normal, completely healthy YZ450 will turn the pipe a fairly bright red after no more than a minute of idling/running at a standstill when viewed in reduced light (even a reasonably well lit shop at night).  They glow worse if the idle speed is high and they are running LEAN, or if there's an ignition timing problem.


Try to adjust the idle mixture with the engine at normal temperatures and see where it comes out:  http://www.thumpertalk.com/topic/271735-how-to-confirm-your-pilot-circuit-setting/


A sooty, black looking spark plug is a sign of weak ignition.  You haven't tested the coil yet, from what you've written.  I recommend you do so, both cold and once the bike has started to act up.

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