What happens if a valve train clearance is to small on an engine?

What happens if a valve train clearance is to small on an engine?
When your valves do not have clearance, a number of things may happen. Your valves will not be closing all the way, you can lose horsepower/compression, poor fuel economy, backfires, hesitation, stumble when idle or take off, it will make your engine run like crap! When you lose clearance on the valves, the engine almost acts like it is out of timing. When the valves have excessive clearance, they will be noisy.