Messed up cam mod???

Posting here (as well as the 250 forum) as I am a ex-426er and know the brains on this forum.. Also, would be the same situation on a 426 as my 250F...


I may have really screwed myself somehow..

Doing the 03 cam mod on my 02 YZ 250F. Included pulling the flywheel and replacing the timing chain.. Tonight I had the chain on the new cam and was trying to turn the motor over to test my timing and noticed a grinding noise (at the crank) when I turned it over and it felt really rough and tight at sections, then would cycle through easily. At first I thought it was the force of the valve springs on the cams, but I realized that it sounded and felt worse than that. Took the chain off the cams and it still did it. Seemed to get worse, but could be my imagination/panic.. There is no spark plug.

I pulled the flywheel and the chain off again to see if there is anything obvious and there is some strange metal shavings in the case.. Sounds really bad and now I am not sure what to check next.. Again, have the plug out, no timing chain, no flywheel or stator plate and I can *not* move the kickstarter - and am afraid to really put pressure on it. Thinking the crank should easily spin over. Is this correct?

I was tired the first night and turned the motor over clockwise trying to find my timing mark. Manual says that can cause "damage" - but doesn't say what kind of damage...

Anyone have any ideas?? I am stumped and really nervous that I've got a major problem on my hands.

Thanks in advance.

Wow...sounds like a question for Dr. Grey. Hope nothing serious is going on in there!

You say you have the chain off. Is that all the way off, or just off the cams? It can get jammed behind the flywheel, or get a loop in it if it's not laying all the way down on the crank sprocket.

If the flywheel is still on, I'd pull it and the stator and take a look. The key may not be seated, and the flywheel grinding on the stator. Completely remove the chain and see what you get. If it still binds/grinds, open the other side up; it may be totally unrelated to the cam/chain.

In assembling the cam chain, the surest way to get it right is to do it with the stator plate off so you can see it all gets seated right. The flywheel key way is aligned with the crank pin, so when that's pointed straight up the cylinder, it's at TDC. Set up the cams, put the ignition side together, and double check the timing. You might have to correct it, but that's OK.

The thing to avoid is turning the engine backwards quickly or beyond the point where the intake cam starts to open the valves while the chain tensioner is off. That can cause the chain to slip out of time. For that matter, going forward can to, so be careful with that.

Thanks Gray. Right now I have the chain completely off (flywheel and stator plate off) and the spark plug out.. Very hard to turn (have to force it) and grinds somewhere..

Have no idea what could've happened.. Nothing obvious in the case other than a few shavings that I know don't belong. Anyway to mess up the crank bearings with this procedure? It ran OK when I took it apart..

I did the cam mod on my 426 without incident, and the only thing I could've done wrong is when I turned the crank the wrong way.. Huge mystery to me :cheers:


Any way to mess up the crank bearings with this procedure?
Not unless you got a frag of some kind in the bearing.

The only problem with rotating the engine backward is the one I mentioned earlier. If you have the chain off, and the whole left side disassembled, your problem is clearly not on that side unless it's chips in the left bearing. That leaves the top end (not likely) and the primary gear case. Looks like time to open up the right side. It's weird, I know, but stuff happens, even as a total coincidence.

Sure. Wondering if there is some kind of pawl that can break when turning the motor clockwise, or if a water or oil pump on the right side of the motor breaks parts going that direction.

Anyway, looks like I'll need to take the head off and check the crank bearings fo possibiliites. Hope I'm not splitting the case. That's when I call in the professionals and get out the checkbook.

Give you all an update when I find something.


Update for all you cam modders..

As I was turning over the motor finding, or refinding my timing mark while putting the cam in.. I bunched up the chain against the front side tensioner -the metal one. That distorted it and pushed it into the counter balance weight in front of the crank. The weight piece rubbing against the tensioner was my grinding, shavings, and resistance..

Wonder if I am the only one that this happened to.. Anyway, problem solved and lesson learned w/o breaking the bank. Those are the best kind of lessons.


That's why the engine should not be turned without some tension on the cam chain. If you need to move the crank while the chain tensioner is backed off, or the chain off of one or both cams, do so slowly, and only if you lift the chain with a finger or press in on the tensioner shoe (either one carefully). The arrangement of the tensioners and guides is intended to prevent the chain from dropping off of the crank sprocket by itself, but as you have discovered, the issue can be forced, and the results unpleasant.

Btwick, If I understand you it was the front guide out of position?! I give you credit for turning it over by hand prior to starting, many have skipped this part and paid a heavy price.

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