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Hey y'all, I crashed my 2005 rmz250 while going up a hill, and after that, once I tried to kick it again I could hear piston slap. So I guessed my timing jumped so I pulled the motor and it did jump a couple teeth. Valves looked ok and piston was fine, valves had right amount of clearance, so I reset the timing with a new tentioner, and chain seemed tight. Installed the motor again. Tried starting and it feels like it has no compression when I kick. I'm a two stroke guy and don't know much about 4 strokes. Had lots of compression before I crashed it. Any help is appreciated.

20200517_232458.jpg

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Hold the cylinder head upright and spray parts/brake cleaner into the intake and exhaust ports,

lift it above your head and observe for leakage around a valve(s).

 

Agreed with 650rrrr above, if the cam chain jumped time, you likely bent a valve.

Another possibility is a spun cam gear, putting the lobes out-of-time with the gear/marks.

 

Edited by mlatour
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2 hours ago, mlatour said:

Hold the cylinder head upright and spray parts/brake cleaner into the intake and exhaust ports,

lift it above your head and observe for leakage around a valve(s).

 

Agreed with 650rrrr above, if the cam chain jumped time, you likely bent a valve.

Another possibility is a spun cam gear, putting the lobes out-of-time with the gear/marks.

Alright, I'll pull the motor again tomorrow. I'll check to see if I bent valves or if they leak, but how do I know if my lobes are out of time with the marks on the cogg?

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That I am unsure,

to my knowledge unless you have the cam specs, a degree wheel and dial indicator setup, there is no sure way to know.

 

It's actually not uncommon for cam/gears to 'spin' for a sudden stop in rotation following piston to valve contact or, when the chain jumps time.

Start by checking for leakage.

Edited by mlatour
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I don't know about "possibility is a spun cam gear".  I don't know that bike, but my kawa the cam shafts are bolted to the cam gears, no way they are going to spin unless the bolts shear.  The cam gear probably did jump on the chain.  Time the cams as directed in the manual, make sure you have the engine @ TDC, on the firing stroke.  I think on some bikes you don't have to worry about that due to it fires each time the piston goes over TDC on each rotation.  Yours might or might not, you have pts or electronic?

I would suspect your problem is bent valve or something up in there.

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2 hours ago, durielk said:

I don't know about "possibility is a spun cam gear".  I don't know that bike, but my kawa the cam shafts are bolted to the cam gears, no way they are going to spin unless the bolts shear.  The cam gear probably did jump on the chain.  Time the cams as directed in the manual, make sure you have the engine @ TDC, on the firing stroke.  I think on some bikes you don't have to worry about that due to it fires each time the piston goes over TDC on each rotation.  Yours might or might not, you have pts or electronic?

I would suspect your problem is bent valve or something up in there.

Yeah the timing is in spec with the owners manual and crank was tdc when I set timing. At this point I'm guessing it's a valve is shot too. Still have yet to find time to crack it open again, hopefully in the next couple days I can though.

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Bump, I took the head off again and pulled the valves, they all look good and are all perfectly straight. What should I check next? Timing was tight and it was correct. I don't see why it could be the top end losing me compression because it was fine before timing slipped originally and looks fine now. A couple marks where valves hit the piston. Any help is appreciated.

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Rather than looking if the valves are 'straight' 

as suggested earlier on, with the cylinder head fully assembled and held upright,

spray brake/parts cleaner into the intake and exhaust posts and

observe if any leaks out from around a valve.

 

A 'spun' cam gear is pretty much impossible to tell by eye.

Many CRF owners (likely similar no matter the brand) who have had impossible to start rebuilt engines following a failure and were re-using the cam(s)

have replaced them as a 'last ditch' effort and the engine woke up once again.

Edited by mlatour

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46 minutes ago, mlatour said:

Rather than looking if the valves are 'straight' 

as suggested earlier on, with the cylinder head fully assembled and held upright,

spray brake/parts cleaner into the intake and exhaust posts and

observe if any leaks out from around a valve.

 

A 'spun' cam gear is pretty much impossible to tell by eye.

Many CRF owners (likely similar no matter the brand) who have had impossible to start rebuilt engines following a failure and were re-using the cam(s)

have replaced them as a 'last ditch' effort and the engine woke up once again.

Alright, I can order some new ones, should I go OEM or order a hotcam? And if the cam spun relative to the cogg, wouldn't there be piston slap, I couldn't hear any when kicking it over.

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Start by checking the valves/seats for leakage.

 

Edited by mlatour

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You need a lot of help, do you have anyone that can read the reply's and explain them to U what they mean, then have them do that while you watch?

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