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Bike won't start after re-shim

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Hey everyone, I decided to check and re-shim my valve clearances over the weekend for the first time. My bike is a 2007 YZ250F and it started first or second kick before I took it apart but it wouldn't start after I ran it for a while and it got hot (I would have to wait about 15 min for it to cool off). This is why I decided to check my clearances and put new shims in because a few of you suggested that tight valve clearances would be the most likely reason it wouldn't start hot.

 

Now the bike won't start at all. It sounds fine when I'm kicking it over but it won't fire up. It will give me a little backfire once in a while but that's about it. It also seems harder to kick over than before I re-shimmed it. Is this normal? I put a new spark plug in so I think its getting a spark.

 

What would you guys suggest I do? I already to it apart again and re-checked my clearances and it seemed good. Thanks in advance for the advice.

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Sounds like your cam timing is off. Or did you make any drastic changes in shim thickness .

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I'm willing to bet that your cam timing is off by a tooth. Did you check the timing after the cam chain tensioner was installed? This is the number 1 mistake people make when setting the timing.

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Sounds like your cam timing is off. Or did you make any drastic changes in shim thickness .

 

No they only needed adjusted by about +/-0.05mm.

 

I'm willing to bet that your cam timing is off by a tooth. Did you check the timing after the cam chain tensioner was installed? This is the number 1 mistake people make when setting the timing.

 

My cam timing should be good. When I took it apart a second time I made sure the cams were in the correct position. Would one tooth make that much of a difference? If the flywheel was off by a tooth would that also make a difference because now that I think about it I didn't check the flywheel position the second time.

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Thats what I would say timing or valves not shimmed right. The crank needs to be at tdc when you set the cams. I usually check the valves after I shim them to double check there right.

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Since it kicks over harder than before, that rules out any possibility of a valve staying open due to a mis-positioned shim. You probably are off on your cam timing. Verify the timing by using the timing marks on the flywheel and the cams. That can happen if you didn't have the timing chain tight on the front of the engine while setting the cams. In the future, after doing any cam work, always check the timing after you release the tensioner. And then, spin the motor thru very carefully by hand two full turns and verify timing one more time before buttoning everything up.

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Would a stretched cam/timing chain cause the bike not to start?

 

if the tensioner is actually working correctly, how would you know if the cam chain was stretched?

that is the sole job of the tensioner!

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Since it kicks over harder than before, that rules out any possibility of a valve staying open due to a mis-positioned shim. ..........

 

Aaaaahhhh,........... not exactly.

 

What if it was so far out of time that the piston hit a valve and bent the stem?

The piston would hit every revolution. 

 

I had a bike jump time from worn small sprocket, and it quit running.  Valve bent so far the head of the valve was off the seat and hit the piston every revolution.... which is not much bending with this 250F

 

Only way to tell if valve is off the seat would be a leak down test, you'd hear the air rush straight out.

 

Now with that said, I have had another failure that caused harder kick issue.

The spring on the auto de-comp flywheel broke off and allowed the de-comp to stay out all the time... no de-comp.

 

A worn cam chain is easy to check.  Just remove it and drop it on the table or floor.  The pick it up by 1 link and see how many links kink up.  Kinked links means it time to replace the chain.  And you better check the small sprocket cause the chain will take out the small sprocket.... and the only way to fix that is replace the crank !!!

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Thanks for all the replies guys. Sorry for taking so long to get back to you all but I have had shingles for the past couple weeks so I've been pretty useless.

 

I went back to my bike last night and checked to make sure the timing was right and I believe my flywheel was off by a tooth but the cams were timed correctly (12 links between the marks). With the chain tensioner installed and the cams TDC the flywheel has the horizontal I showing (not the vertical line). Is this the correct position?

 

I also re-installed the cam towers and I believe I had the bolts over torqued which may have caused the harder kicking (hope I didn't damage anything).

 

So I tried starting it after doing all this and after a few kicks it started but only ran for a second and died right away. It didn't sound that good either. After about 10 more kicks it did the same thing. So I kept kicking and now it won't start at all but it will backfire once in a while. Would this maybe be because the fuel has been sitting for almost a month?

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Torque spec on cam tower bolts is 7 in lbs. which isn't much so yeah, that could be stiff kicking.  I'd check underside of cam tower for scoring before starting again.

A check for TDC is remove the sparkplug, turn crank to TDC, then drop a long dowl or other suitable non-destructive "stick" down the hole (personally, I use a long screwdriver and yes I'm careful with it), and then rotate the flywheel slightly to feel for TDC.  Then, I compare marks with what I feel.  Usually the marks with match what you feel unless you sheered the Woodruff key.

 

Most likey the sparkplug is either starting to foul out, or the carb pilot jet is getting clogged up, or your starting technique is slightly off.  You want to wick the throttle 2-3 times to squirt fuel into the intake for cold starting.  You have to learn your bike for that.  And keep your hand off the throttle while kicking.  And yes, fresh gas might help.

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