First off let me say that I believe in preventive maintenance including valve adjustments. There has to be a reason that every manufacturer lists valve adjustments in there maintenance schedules.
In my experiences, most of the time the valves are too tight from wear. How many times have we read on the forums that a valve adjustment has fixed a running issue? All kinds of nasty thing can happen if the valves are too tight. If the valve lash is too tight your compression will be lower, due to the valve not seating correctly, however, slightly off valve lash may not show up in a compression test. Continued running of a motor with the valve lash to tight will lead to burning of the valve. This is because the valve seat dissipates the heat of the valve when seated properly, if the valve doesn't seat properly the valve will get hotter and hotter causing it to burn, this will cause compression to go down and eventually the motor will not start. In extreme cases, I have seen where a small portion of the valve has burned off and put a hole in the piston.
Too wide of clearance is a different animal. This will be noticeable by a clicking sound from under the valve cover. This noise comes from the rocker actually smacking the top of the valve stem. Too much clearance can cause damage to the top of the valve, damage the rocker and cam and in extreme cases something will break. If the intake valve lash is too loose there will be a lost of compression, now this may or may not be noticeable on a compression test unless you have a really good gauge and a benchmark of what you had to start with. In extreme cases there will not be any compression. Here is why, when the intake valve opens, the piston is going down to draw air and fuel into the cylinder, as the piston comes up, the intake valve closes causing compression of the air/fuel mix. If the intake is not opening enough, the amount of air entering the cylinder is reduced, thus not as much air to compress, equals lower compression. If the intake valve does not open at all, you will get zero compression. Also to much valve lash on the intake will lower manifold vacuum, and we all know there isn't much vacuum to start with on these motors.
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