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Found the culprit, but how did it happen?

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I recently rebuilt my 2003 YZ250F motor (new top end and timing chain). Everything else is bone stock. And even more recently blew it up (or so I thought).

My motor break in procedure was 4 cold to hot starts, and a gentle ride. About 30 minutes into the ride going up a steeper hill, the motor quit on me. I experienced a serious loss of compression (I've felt this before on my very first 125cc when a piston circlip popped off and scored the entire side of the cylinder & piston). So I pulled the clutch in and rolled to a stop. I could shift fine, but it was very difficult to kick over, so I didn't. I found neutral and rolled/pushed the bike to a fire road. I got my riding partner to head back to the truck and pick me up.

A few weeks later (yesterday) I finally found the time to start tearing down the motor. I found the camshaft circlip on the exhaust missing...the exhaust camshaft bolts were still as tight as the intake bolts. There were very few metal shavings, but quite a bit of not magnetic black grit, I'm assuming from the timing chain guides. After I pulled the head off, I could see the mangled cam circlip down at the bottom of the motor. It was sitting between the timing chain and crank sprocket. With a magnet I was able to easily fish it out. I also found a little piece of the clip sitting next to the tappet. I haven't had time to measure it all, but I believe almost 100% of the clip is accounted for. I applied tension to the timing chain and was easily able to turn over the motor!

What would you do from here? Attached are some pictures of what it looks like. A little scoring on some parts, but I believe for a weekend warrior like myself I can probably polish them out. Also how could this have happened? It must have been my error in installing it properly I'm assuming.

This is what I had in mind:

Polishing the parts down than need it. Making sure 100% of the clip is found (hopefully with pulling the stator off and using the magic magnetic wand)! Flushing the oil a few more times. Install a new oil filter, and timing chain, two new camshaft circlips, checking the valve clearance, and then go for a ride.

Here are some pictures, you can see where it worked itself. It looks like it must have been holding open the exhaust valve just enough to not allow detonation.

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A little scoring on the lobe

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Intake clip vs exhaust clip

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Not sure how it happened but that would be my #1 priority is to find out how and fix that. I would check for any other damage like you said and IF that is the only damage then IMO and if it was my bike and my poor broke ass I would polish it out and make sure there is no burrs or deep scratches, replace the clip, put it together and see how it goes. If you had money I'm sure other guys would do something else but again I'm broke and would salvage all that I could

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Damage does not look too severe. You dodged that bullet. If you can get your hands on an accurate scale, compare the peices to a whole.

Any diea how come the clip was not in, think it just fell down the tunnel and was not noticed? (Another reason to always stuff the tunnel with a rag until the cam caps are tight.)

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watch of all these videos from first to last in order!just incase you dont know what your doing :thumbsup: you might wanna watch the other vids this guy has in his channel.i watched all of them they were so interesting and i did learn quite a bit.the guy buys an 03 yz250f and he completly rebuilds the entire bike and for everything he did,he made a "how to video".the bike turns out sweeettt!go check it out!and good luck with the motor,pay more attention next timee!someone got luckyy!

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Damage does not look too severe. You dodged that bullet. If you can get your hands on an accurate scale, compare the peices to a whole.

Any diea how come the clip was not in, think it just fell down the tunnel and was not noticed? (Another reason to always stuff the tunnel with a rag until the cam caps are tight.)

I really like the idea of the scale! Thank you!

I'm almost 100% sure I put the cam exhaust clip in properly, because I remember examining the intake clip to see which direction the clips bevel faced. I do remember when initially tearing down the motor, the intake clip sitting snugly in the cam bearing, where the exhaust clip easily felled out. During installation I used some thicker gear oil to help keep the clip in place on the cam bearing groove during installation. But the fact that neither the cam bearing nor the camshaft hold down bracket (not sure of its proper name) have a single scratch on them leads me to believe that I must not have put it in correctly. I just want to make sure it doesn't happen again.

A little clarification:

I didn't drop the clip down the tunnel. The clip was definitely installed on the top end of the motor for the first 30 minutes of riding (I'm guessing it must not have been installed correctly if it worked its way out). Then I'm assuming it wiggled out of its home (or probably the the place that it installed it) and worked its way to the cam lobes, the cam lobe gobbled it up causing the lack of compression (exhaust valve being stuck open), then fell down the tunnel, which is where I found it. I'm glad this is a non-interference engine (or maybe I lucked out) because the piston looks as good as new, and the valves appear to still be seated nicely. Luckly I didn't try to keep kicking it, and mangle up the bottom end.

Edited by joeprunc

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watch of all these videos from first to last in order!just incase you dont know what your doing :thumbsup: you might wanna watch the other vids this guy has in his channel.i watched all of them they were so interesting and i did learn quite a bit.the guy buys an 03 yz250f and he completly rebuilds the entire bike and for everything he did,he made a "how to video".the bike turns out sweeettt!go check it out!and good luck with the motor,pay more attention next timee!someone got luckyy!

I had my laptop out in the garage watching all of these videos during the initial tear down and rebuilt. They were EXCELLENT!!!! Then used the manual for proper torque values.

Edited by joeprunc

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When I install the cam, the cap and clip are already together with the cam, so I can't drop the clip in the timing chain tunnel. When I remove the cam I remove the cap with the cam, again I can't loose the clip.

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i would debur the cam just on the shinney surfaces. non-shinney dont matter.

maybe pull the flywheel (it is a very strong magnet) and clean it out as well.

maybe pour a half gal of diesel from the top down and flush it out the oil drain hole.

double check your oil filter for any parts.

put a new cam circlip in. fill with oil and start it.

please post any more info as you find out.

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That is right, the flywheel is magnetic, that is probably why the clip stayed relatively close once if fell to the bottom end. I'm going to change the timing chain again since I'm sure a little extra stress was put on the chain (and I know that is one of the weaker links in the motor). So while I'm in there I'm going to fish around and make sure it all looks OK.

I never thought of dumping diesel down to help clean it out. Will that have any adverse effect on any gaskets or bearings in the bottom end? Also usually when you change your oil not 100% of the oil is flushed, how can I help flush all the diesel out? I guess just do an oil change after a good warm up.

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i would lean it over at 45 degree angle to the kick start side to help drain it all out.

you could measure how much you pour in and catch how much comes out. if almost the same then no worries.

i would use some low cost oil (gosh if somebody that has been testing oil in these bikes for 10 years would just chime in) like say Shell Rotella T 15w40 and only run it a short time and then change it again. diesel is NOT gonna hurt anything. you are just using it to flush stuff out. not run the motor with it in.

a lot would depend on how much metal i found on the inside of the flywheel or just sitting in the bottom of the case. if none i would maybe flush 1 cup of diesel and see if it gets anything out. if not, then maybe 1 more cup. if nothing then. stop.

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Diesel is not going to do much harm to the engine. The worst it will do is thin your oil so yeah run diesel then get cheap oil and run it a few minutes then drain it

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I have used Kerosine to flush the motor with no adverse affects. If you can get a magnet on a wand to the oil pump sump it is possible some metal is there but I doubt it, everything should be on the magneto.

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Yeah, diesel is basically like really thin oil so it won't do any harm to any seals or anything, but it is pretty good at cleaning. Pouring some diesel in the top end and draining it out the bottom isn't a bad idea, especially if you have a magnetic drain plug. It definitely looks like you dodged a bullet as the damage looks very minimal. You should be able to de-bur the cam lobe and check the lifter bores for any scoring in case a piece got on top of the lifters at any point. If that all looks good I would change the cam chain b/c you don't know if it got stressed at any point during that ordeal and it is cheap insurance.

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Thanks for all the advice,

I carefully deburred the cam. One of my friends who knows motors like the back of his hand mentioned that many cams on high performance engines have a very slight slope (from side to side) on the contact surfaces which helps rotate the lifter to minimize lifter wear. Does anyone know if this is true with these cams? The reason I'm asking this is to see if this burr on the lobe may contact the tappet more than the opposing side.

Yesterday I flushed the oil, pulled the flywheel and mag (I didn't find any further debris on the mag or fly wheel). I then flushed it with some diesel, it looked fairly clean coming out, but some additional non magnetic particles showed up on the oil filter (I'm assuming from the clutch). I will flush it again just because I still have a quart of diesel that isn't going in any of my vehicles. Fish around under the crank with my magnetic wand to see if anything else is hanging around that wasn't flushed out.

The UPS man just dropped off all the new parts (new Yami TC is in there too) and some other items I really don't need :thumbsup:...new foot pegs, carb drain lines, air/fuel screw, fork bleeder valves, & an air filter. I initially thought the repair was going to be close to $800 before I tore down the motor, so after realizing how lucky I got I felt like I could spend a bit more. Hope I don't have to work too late tonight.

Thanks again everyone...and I learned if your bike craps out on you, best thing to do is walk it home and diagnose it. If I would have kept kicking, I'm sure it would have eventually started, but who knows what else I would have torn up.

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I'm almost 100% sure I put the cam exhaust clip in properly,

Take a close look at the cam cap, cam bearing, and head. There's no way that clip can just come out if it was installed correctly. As others have said you definitely dodged a bullet. Be careful since your luck probably won't be as good next time.

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yes the cams are slightly inclined to spin the buckets. but they will sit flat on the bucket as well.

honestly, i dont think the clip was IN the cam cap. it had probably fallen out and was sitting there ready to fall down at some point.

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honestly, i dont think the clip was IN the cam cap. it had probably fallen out and was sitting there ready to fall down at some point.

That's what I think as well. I don't believe there is any way for the clip to fall out of the cam cap once the cam cap is on, the clip sits in a groove that is completely encapsulated by the cam cap, so unless it somehow went through the cam cap there is no way it could come out.

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After throughly looking at the setup, you all are definitely correct. There is no way for the clip to come out once installed properly. There are no grooves or scratches showing it forcing itself out. However you can tell the cam bearing was spinning in the exhaust cam (which lacked the clip) where the intake looks like the bearing had never spun a day in its life.

The clip sure does want to pop off when installing the cam cap, I noticed this when I simulated the installment using the good intake clip. Sure enough the first time, it popped off while aligning the cam cap guides with their holes and the cam cap groove with the clip. I will be taking my sweet time rebuilding this motor unlike the fly by night operation I did previously.

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I find it is easiest to put the cam in place, then place the clip in the groove in the cam bearing, then place the cam cap over it, rather than trying to get the clip to stay in the cam cap while putting it together.

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