07 450 No Power

Rebuilt the top end 15 hrs ago and recently checked valve clearances(piston and rings,valve seals,cam chain,gaskets,valve adjustment)Runs great been changing oil/filter every 3 rides. I just adjusted the clutch cable and replaced the air filter,changed oil(did not ride it yet since doing this)

Here is what i belive caused my issue,I started it first kick starts right up wtih the choke on and then pretty much hit the rev limter and cut it right back off.I regret doing that.i know it was not wise but who hasnt done something stupid while drinking.

So i go to ride it the next day and it has absolutely no power,no wierd noises,bike sounds the same when you give it gas but doesnt really effect my speed

I pulled the engine out and "inspected" engine timing,valve clearance,spark plug,top of the piston but not the rings,valve springs,valve seals,valves dont look bent,engine oil was good and filter clean(everything is looking okay).

Im looking for some advise at what to look at next?Any help is greatly appriciated thanks

Edited by Mxdude91

Update,compression check and I only have 90 psi.

Bear in mind that compression testing on any high compression single cylinder is fairly inaccurate anyway, since many of them can barely even be kicked over at full compression, but it's an almost completely pointless exercise when you have auto decompression.  "Somewhere around" 90 psi is about what I'd expect.


Double check your cam timing one more time.  I can't think of anything else of the top of my head that would have related directly to revving it hard and chopping it.  If the cam timing really is OK, then ignore the fact that the "stupidity" ever happened and troubleshoot it as if you've never seen the bike before.

Ive checked timing.its spot on,How would I check the auto decompression to see if its working as it should and could a carb cause a issue like this?because I had just changed the air filter possible for particles to fall into intake...thanks a lot for your opinion on the compression test,I'm use to doing compression test on car/trucks.and I do agree that I may of feared the worst and jumped to conclusions on the engine...

Even with 4 or more cylinders and an electric starter, conventional compression testing is more useful in finding significant variations between cylinders than on absolute numbers, unless of course the readings are way off the normal range.


The auto decomp mechanism is virtually too simple to fail, and is normally only affected in any way that really shows up by the cam timing being off.  You can operate it by hand, and watch it work as you turn the engine.  Read: http://www.thumpertalk.com/topic/593945-06-yz450f-compression-issue/#entry5732518

I do not believe my engine is to blame after I was able to get a hold of a cylinder leak down tester.


It just hit me the Clutch cable is to tight is my guess IMA put the bike back together.

are you still drinking?   :naughty:

Haha,yeah Im feeling a lil stupid...but when my bike stopped running right after a cold hard rev I instantly jumped to conclusions...and had forgotten about how I had adjusted the clutch cable after riding last time..so after checking everything I could imagine it came to me.the engine could be running fine but if the power of the engine was sorta getting to the transmission it would cause this issue.thanks gray racer for your help it really saved the day because I thought 90 psi was low for a compression test.I rode yesterday evening and man was it good to be back on the trails

I find it interesting how the problem of a slipping clutch was expressed as a lack of engine power.  That threw the entire diagnosis completely off the mark.

The bike would move it just didn't have the torque it had before..I would say thats a lack of power

If you had a bald tire on wet concrete, you'd have exactly the same condition, but it would be inaccurate to say that it was a lack of power.  Problems need to be accurately understood and described in order to be correctly diagnosed.

Thanks for your help grey,sorry I felt like the bike had no power,I described it to the best of my knowledge at the time I made the post and tried to keep the post up to date with where I was at with diagnosis.

Thanks for your help grey,sorry I felt like the bike had no power,I described it to the best of my knowledge at the time I made the post and tried to keep the post up to date with where I was at with diagnosis.

You learn as you go. With a slipping clutch, you will hear the engine "sounding" good, but it will rev up without there being a corresponding increase in speed. That should have told you there was a problem with the stuff connecting the wheel to the engine.

If it was truly down on power, it wouldn't sound quite right, and you would not hear it revving up without the expected acceleration to go with it.

See the difference?

I follow ya

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