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Jon_Escombe

Inlet camshaft not turning smoothly?

8 posts in this topic

I've just noticed something odd with the inlet cam on my YZF this evening - and wondering if it's normal?

I had some oversize valves installed a couple of weeks ago, and after checking the valve clearances tonight noticed that the inlet cam wasn't turning as smoothly as the exhaust.

With the camchain off and the cam cap torqued down, there's definately more resistance and a slightly uneven feel compared to the exhaust cam (which turns smoothly & freely until the lobes touch the buckets).

It seemed to be running fine at the weekend, so it's possible that it's always been this way & I've just not noticed before, but I'm a bit worried about just slapping it back together.

Wondering if something may have warped slightly so that it's binding? Don't know if it's significant but he valve clearances were very tight from one side of the head.

JonE.

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You are spot-on to be concerned. There are a couple of things to check and you've touched on both. First of all, was the "c" clip reinstalled? This clip properly aligns the cam radially. If it was omitted, the cam can slide a little. It's unlikely, but worth checking. Next, were the cam bearing caps properly torqued? Some owners have experienced failures when these caps were not torqued properly. Lastly, and this is what I feel is suspect from your description, is that the valve lash was a little too tight after the head work and that one or more valves have driven deeper into the seats creating too little lash. Like you mention, the cam never has a spot where it rotates easily. If the lash is too tight, there is constant pressure from the cam and you won't get that part of the rotation where it spins freely for a couple of degrees. This is an area where I checked my cams for free spin at TDC. At this point, with a cold engine, the cams should move easily just a bit. You should be able to grasp the cam sprockets and kinda rock the cams back and forth just a little. Perhaps all you need to do is do a valve adjustment since you've had extensive head modification done. Hopefully, you had a good shop do the work. To troubleshoot your problem, I would bring the crank to a few degrees past (or before)TDC and then remove both cams and then reinstall without the cam chain. Now, since the cams are not tied directly to the crankshaft, you can easily check that they spin freely. By doing this, you won't have to fight compression and you feel with your hands how the cams spin independently of each other, and for comparison. By the way, when I say to bring the crank a few degrees past, or before, TDC, I only use this position to make it easier to find true TDC when you've finished and then you can reinstall the cams without searching for the crank marks. Also, since the 426 has a "waste ignition cycle", you can find the mark and time the cams from there without having to worry about whether the ignition is also timed for this particular stroke. All this means is that the ignition fires just like a 2-stroke....everytime the piston comes up. Sorry about the rambling.

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Hi Boit,

Thanks for the suggestions. I'm moving the cams around tdc with the chain off & the lobes aren't touching the buckets. Could describe the feeling something like stiction on forks, there's just a partial resisitance to movement that's not there on the exhaust cam.

I've tried holding & turning both cams in the caps & the head seperately, the only one that feels like it doesn't want to turn as easily is the inlet in the head.

I'm tempted to put another hour on it and then see if it's the same/better/worse when I put the new shims in...

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I made a mistake in my description. I meant so say that if you remove the cylinder head, you can set the cams in the cradles and check for smooth spinning. You've done that. Does it seem to be a bearing problem?

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I REBUILT A YZ400 ENGINE A COUPLE MONTHS AGO AND IT HAD THE SAME PROBLEM BUT THE BIKES RAN IF EVERSINCE. I DON'T KNOW IF IT'S SOMETHING TO WORRY ABOUT THOUGH

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After deciding to rebuild the motor and put another couple of hours on it, I dropped a C-clip down the cam chain tunnel instead! :) Spent an hour of trying to get it out with a magnet before pulling the head and barrel back off. Must confess I had a bit of a sense-of-humour failure, my garage was not a happy place to be last night!! :D

So anyway the head is back on the bench. The cam feels stiffest when the cam-cap is seated but not tightened down. Once the bolts are torqued to spec it becomes much easier (but still not really free moving). I think it's the cradles rather than the bearing as this spins perfectly. I'd have to guess that something is slightly out of alignment, but it's bedded itself in whilst torqued down & running?

A friend is stripping his WR400 motor next week, so I'll take a look at his for comparision. I think all I can do for now is put it back together & keep a close eye on it...

Thanks again for the input,

JonE.

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Jon,

I read an interesting article in drn about relieving the camshafts to solve this particular problem. This involved lapping the camshafts to the cam towers. Procedure goes like this; Use fine grinding paste on the plain journals, apply finger pressure to the caps initially and start to turn the cam wheels in a back and forth motion moving the wheels about 1/4 turn every few moments. Gradually the bolts can be torqued down while still lapping to about 4ftlbs. Don't continue until the shafts spin free, stop when they will spin about 1 turn with 1 finger, because they free off slightly when the head is torqued down to final spec. Be sure to remove every trace of grinding paste from head or it can destroy a motor pretty quick. It was stated that that the end results on a 426 were amazing, crisper throttle response and much freer revving.

I'll will check this out and try it if needed on my next valve adjustment......Chris

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