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My experience is basically the same with most "old" cars I have driven. If I very lightly tap the throttle to just increase the engine speed slightly the engine feels less shaky than it is when it is at idle. Why does that happen?

Is it because:

  • Because of the higher engine speed, cylinders fire closer to each other so each cylinder damps the vibrations made by the one before it?
  • The vibrations are absorbed by the mounts with higher engine speed better than lower?
  • The throttle plate is closed at idle so that makes AFR more exact making a harder bang?
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First two options are not true. By increasing the engine speed, you change (shorten) its vibration period. Car body is a receiver of vibration, produced by the engine, but has its fixed period. The more similar periods are, the more vibrations is transmitted from the engine on the entire car. Car period is rather lower, comparing to the engine at higher rpm, so the best resonance is when engine is at idle.

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