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Ever See a Cam Gear Spin?

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The gears on YZ250f cams are pressed on / heat shrunk.  Has anyone ever seen one move?  My motor is running like crap, and sounds like the cam timing is off a bit.  It is popping and has a really harsh sound...like when an alcohol burning engine is out of tune.  The dot on the intake cam gear used to line up the cam to set timing will no longer lines up.  It is either too high, or too low.  Something wierd is going on...

Edited by Blutarsky

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yes they can spin, ususally if the cam seizes.

 

Cams are old but pristine.  Both lobes and journals are in spec. Valve train has 20 hours on it since a full Millenium rebuild (new grind, new springs, new stock Ti valves, new valve seals).  Buckets are pretty worn on top.  They look bad, but they feel smooth to the touch.

 

Is there a way to tell how the cam is oriented relative to the gear?  I see no specs on valve timing that I could use to degree in the cams....

 

Really scratching my head on this one.  Pulled the motor disassembled down to pulling the cylinder off.  I see nothing suspicious....other than the dot on the intake cam no longer lines up where it used to.  I used a long travel dial to find TDC exactly...just like I always do.

Edited by Blutarsky

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Thanks SU....  I am going to check a few more things...but I keep coming up blank....  Everything points to the gear having spun. 

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When you say new grind,are you refering to having your stock cams hardwelded and then a diff profile ground on them?did you ask millenium if they actually removed the cam gear and bearing?the only bike that i have seen the camgear actually slip was on a 450 kawi that had the gear pressed off of it in an attempt to re-degre it for better performance.What year is your bike?if you think that you need a new set of cams,let me know,i have a set or yammie race cams that i would be willing to part with.they are low hr gytr units.p.m. if your interested.and you might want to make sure that your timing chain is not worn out,like old n bold says,that will def make em not align up like when the chain is new.did you check the valve lash before you tore down the motor?maybe it just jumped a tooth on the sprocket,i would def check the valves to make sure that they are o.k.let us know what you find out.i had a 09 that started running bad,and it ended up being a air leak on the carb boot.

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When you say new grind,are you refering to having your stock cams hardwelded and then a diff profile ground on them?did you ask millenium if they actually removed the cam gear and bearing?the only bike that i have seen the camgear actually slip was on a 450 kawi that had the gear pressed off of it in an attempt to re-degre it for better performance.What year is your bike?if you think that you need a new set of cams,let me know,i have a set or yammie race cams that i would be willing to part with.they are low hr gytr units.p.m. if your interested.and you might want to make sure that your timing chain is not worn out,like old n bold says,that will def make em not align up like when the chain is new.did you check the valve lash before you tore down the motor?maybe it just jumped a tooth on the sprocket,i would def check the valves to make sure that they are o.k.let us know what you find out.i had a 09 that started running bad,and it ended up being a air leak on the carb boot.

 

My reference to grinding was the fact that I had a complete head rebuild, with new seats...in truth, they are not ground...they are cut.  The cams are stock. 

 

I am sending the cams to Crower, and he is going to re-time them.  I measured everything on the cams...and they are in good shape..

 

Intake:

•    Journals ODs are in spec.  Spec is .8645-.8650".  Measures .8648" at both journals, in two directions.

•    I measured journal bearing clearances (plastiguage) twice.  Spec is .0011"-.0024"  Measures .0015" - .002".

•    "A" dimension (major):  Spec is 1.1923"-1.1947".  Measures #1: 1.1921"  #2:  1.1933"  #3:  1.1930".   (Note that the first lobe has measured .0002" under spec since it was new...the low # is not due to wear)

•    "B" dimension (minor):  Spec is .8839"-.8878"".  Measures #1:  .8866"  #2:  .8863"  #3:  .8860".   (Note that the first lobe has measured .0002" under spec since it was new)

•    They spin free when fully torqued down (10 NM)

•    I did not measure runout, but I have no reason to assume they are bent.

•    The bearing feels like new

Exhaust:

•    Journals ODs are in spec.  Spec is .8645-.8650".  Measures .8648-.8649" at journal, in two directions.

•    I measured journal bearing clearance (plastiguage) twice.  Spec is .0011"-.0024"  Measures .0015" - .002" just like the intake

•    "A" dimension (major):  Spec is 1.1968"-1.2007".  Measures #1:  1.1986"  #2:  1.1985 "

•    "B" dimension (minor):  Spec is .8839-.8878".  Measures #1:  .8862"  #2:  .8861".   (Note that the first lobe has measured .0002" under spec since it was new)

•    They spin free when fully torqued down (10 NM)

•    I did not measure runout, but I have no reason to assume they are bent.

•    The bearing feels like new

 

The cam chain only has 10 hours on it...but I guess I could get a new one.

 

I am also going to replace the buckets.  I think that is the source of the problem.  They are starting to get scoured and scraped on top..and I think that is where all the drag is coming from.

Edited by Blutarsky

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Either way I always mark the cam and gear with a paint marker so this issue is easy to diagnose in the future. 

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Either way I always mark the cam and gear with a paint marker so this issue is easy to diagnose in the future. 

 

Good idea...

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Ive been having a problem with my kx250f i got dirt cheap.. Wouldnt run right... Felt like a carb/jetting issue... Wound up being INTAKE cam gear spun!!! Fixed it and the bike runs perfect... The holes on the cam should line up.. Mine were about 3 teeth off... Bike ran but very lean... Cleaned carb several times.. New jets etc. another carb... No luck... All timing marks were perfect.. I was a hard find but i stumbled onto it.... Bike is killa quick now and runs beautiful...just passing on some good infoImageUploadedByThumper Talk1426356922.189747.jpg

Spun cam gear

ImageUploadedByThumper Talk1426356957.041673.jpg

Fixed

ImageUploadedByThumper Talk1426356986.359007.jpg

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You guys think this will be good to go!!! Or should i change out... Its currently in bike running great now!!!! What a kick ass this gave me finding this defect,,,

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You guys think this will be good to go!!! Or should i change out... Its currently in bike running great now!!!! What a kick ass this gave me finding this defect,,,

I think you should buy a new one. Who knows how long that thing would hold up and it could break and cost you more money.

Edited by FactoryJake

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Posted a picture of a 2014 YZ250F cam gear that moved or spun on the end of the camshaft. I had seen this on a KX250F before so I knew what to look for. Not sure what cam is the right one it's been a while and I just posted this picture for anyone needing proof this happens when the cam cap or cam tower gets too hot and grabs the cam to tight, causing the gear to move slightly, but slightly is enough to throw timing way off.

The only way to easily confirm when this happens is to buy the new cam and  put the lobes straight up, and as in the picture with the lobes up, one gear does not match the other. Another note, when this happens it usually causes a hot spot or bluing on the cam tower showing where it locked the cam for a split second. This 2014 YZ250F had the owner put the oil filter in backwards, first the clutch went, then the cam gear spun and it quit running. I figured it out and we installed new cams and a clutch and everything was okay for now...2014_YZ250F_cam_spin.jpg

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