78 XS400 Right Cylinder Excessive Tapping Noise

Peter Ronney

XS400 Enthusiast
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Hi everyone! I took my 1978 XS400 out today. All of a sudden it started making a terrible loud tapping noise and it sounds like it’s coming from the right exhaust valve (sorry for no video, I didn’t want to risk harming the engine). Of course my first thought is the nut worked itself loose and undid my valve adjustment job. Took the access port off, the nut was tight. I’ll go over all my valves tomorrow when the bike has cooled down fully anyways. I pulled the plug to see if there was any damage if it bent a valve or something and thankfully not. Are these interference engines? I want to make sure it’s OK to start the engine as is when I get into it again. It seems like a this is a common problem on the forum, has anyone had this problem and if so what was the solution if not valve adjustments? Thanks!
 
Update, I think this is the problem
 

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The nut is just to the left of the valve, got it all back together and it’s running as it should
 

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It could all be tricks of light and oil distortion but I would swear that one of the valve locks is not properly fully down in place and the end of the valve is not in good shape either. I could be wrong about that but both pics seem to show it.
 
It could all be tricks of light and oil distortion but I would swear that one of the valve locks is not properly fully down in place and the end of the valve is not in good shape either. I could be wrong about that but both pics seem to show it.

I wouldn’t be surprised if the rocker was knocking it like it was. The clearances are in spec so I won’t worry about it for now. I could probably use some fine grit sandpaper between the screw tip and the valve (with a towel to soak up any metal fragments) if that were the case but it’s a story for another time.
 
Did you yamabond that cover after you fixed it? It looks awful dry.
 
If that valve lock is truly sticking up it is out of place or possibly pulling the groove loose, either one of which can easily drop a valve. Your clearance means nothing there as it will remain correct until the valve drops. The retainer clearance is what changes and there is no spec for that.

Take that second pic and copy to computer and use something to blow it way up, you have good clarity there. That lock-to-retainer attachment does NOT look right at all.

Of course yours and do as you will with it.
 
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