Is it firing on all cylinders? You can try to remove the spark plug wires one at a time, while the engine is running (replace them before removing the next one). Each time you remove a wire, there should be a noticeable change in the sound of the engine, because that cylinder will not be firing. If there is no change when a wire is removed, that cylinder is probably not firing, and may cause vibration or shaking.
Disclaimer: If the Afourians were an army, I'd be a Private at best.
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