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The very first 30ish seconds of engine operation, it runs rough because it's in low vacuum operation, it's running with the throttle plate open and de-timed as fuk to stop oil consumption on startup. Try this: before you start it, pump the brakes to deplete the vacuum in the booster, once it starts notice the lack of vacuum as the brake pedal remains stiff as wood. After 30ish seconds, you'll notice the sound change and the brake pedal will sink as the booster gets vacuum again.
This doesn't work for an automatic car though, as it uses a mechanical vacuum pump.
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