Intake cam cant make a full revolution

Ok, as some of you may know I'm having a few problems, but I've finally narrowewd it down. New valves, cams are timed right, exhaust cam opens and closes the exhaust valves with no problem. The intake cam starts to open the intake valves, and opens them all the way. Then it will not go any further. the cam lobes are basicly sticking straight down, and will no go any further. Anyone have any suggestions whats wrong? Being the little impatient kid I am, I tried to force it, and it wouldnt budge. It is so stuck in there that if you push hard enough the cam chain will jump a tooth from the pressure you are putting on the crank. The valves are definetly not hitting the piston.

School ends tomorrow, I should be riding right now. There are no visable problems.

Then you better check out the "work" that was done when the new valves were installed. It sounds like a coil bind problem, maybee there is a double shim under a valvespring causing a "lock up" condition, or maybee the wrong valves were used?

What started this problem? Did it quit while you were riding, or did you perform a rebuild and it's not going back to together correctly, etc..?

Don't force it, the valve train is full of relatively delicate parts and you're going create a big repair bill for yourself.

We put the valves in ourselfs, and ony did one at a time, so we woulnt mix anything up. That leads me to belive that we messed it up. All the valves are in the right spaces, by the : and - symbols.

Sirthump, the problem was the intake cam got way way way out of time somehow and the intake valves hit the piston. It happened right when I hit the kill switch so all we ended up with was some mildly bent valves.

I'm going to get to the point where I'm going to start breaking stuff, I mean I'm probally gonna end up stripping out some threads becuase ive taken the cam caps off around 10 times now.

Sure sounds like the cam timing isn't correct and the intake valves are contacting the piston. I'd start over, making certain the cam timing is correct. Put the tensioner in and release it so the chain is tensioned before you try to turn the motor.

Do you have a cam chain problem that started this whole thing? Cam chain worn out? Tensioner not holding? Hate to see you fix it and have the problem again!

The intake valves are definetly not hitting the piston, and the bike is definetly timed correctly. It is timed to the I on the flywheel.

YZDAD, how might I check / fix what you have suggested? I think thats the only thing that could be wrong.

Maybe one of the intake valve springs isnt seated correctly? Later on we will see if we can push the intake springs into their right positions.

I'm going to let this rest for now, I need to ride my minibike around to blow off some steam. Thanks for all the help with my annoying head rebuild. :thumbsup:

YOu are hitting the piston or have a bent valve.

Either way, take the head off.

Matt, I just replaced the valves and the valves are definetly not hitting the piston, seeing as when I stick a screwdriver down the spark plug hole the piston is pretty far down on the stroke.

make sure you didnt mix up the exhaust caps with the intake caps, they are different thickness. im not talking about the buckets, there the same. im talking about the caps that sit atop of ur valve stem tip

Ok here is an idea for you. This is a long shot and dosn't happen very often. But it can happen.

Maybe the intake cam sprocket has spun on the cam. If you just had a valve hit the piston it is possible that the sprocket spun. There is a lot of force when you hit a valve. Something has to give. The sprockets are only pressed on.

Here is some photo's that may help.

This is what the cam lobes should look like when everything is timed corectly


This is what my cam lobes looked like when the intake cam sprocket spun. The timing marks were lined up in this shot


You should be able to place a straight edge across the top of the cam lobes when everything is timed correct like in the 1st photo.

The cam lobes are 105deg fron max lift. Meaning that the lobes are roughly 15 degrees up from horizontal. They should form a shallow V so to speak. The intake pointing up and to the rear 15 deg and the exhaust point towards the front and up 15 deg.

Hope this helps. Just another possible piece to the puzzle.

Nope that looks right. I am at a loss. Do the cams spin smoothly in the caps with the caps torqued down? thought is if the journals are buggered up the cams might be binding in them.

Otherwise i would look at the assembly of the valves,springs, keepers, and bucket/shims. Something is amiss.

Pull the cams out, hold the cam chain up and slowly roll the motor over. Does it spin a full revolution with the valvetrain outta the equation?

Is the chain not sitting on the crank sprocket right causing interfearance problems with the cam chain sliders.

I dont know I am just throwing ideas out there.

Good luck.

Here's another longshot. It is possible to warp the hollow cams if you didn't have the engine at TDC when you removed the caps. The valve spring pressure can tweak the cam as you loosen the caps. It does sound more like the valve is stopping the cam though since it happpens at max lift. :thumbsup:

I would pull the head back off and disassemble it and double check everything.

Is the piston in the correct way?

Did something fall into the head under a bucket?

It will be very obvious when you find it.

remove the cam chain and rotate the engine to BDC. Attempt to rotate the camshafts manually. If it still gets stuck it's a spring/valve problem.


Is it possible that one of the shims is not seated in the bucket correctly? Not sure if the valve-to-piston clearance is that close, but an unseated shim might cause the valve to be extended and touch the piston. Just thinking out loud here...

Im beginning to belive we assembled the valves wrong. Last night we tried various differnt possitions of the piston when rotating the intake cam, and it still hung up at the last second. It has to be something installed upsidedown / backwards in the valves. Oh well, off comes the head for the 3rd time. Does anyone know if a head gasket will hold up if it hasnt been heated?

Yes the gasket should be alright. But it would suck if not!

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