Reply to "Idle Rumble"

Carburettors rely on airflow to initiate fuel flow and in a engine fuel flow effects engine operation which effects air flow. This is a feedback loop.

A bigger cam obviously effects air flow characteristics at idle in such a way as to make the airflow more intermittent and manifold vacuum more intermittent. That intermittent air flow adversely effects carburettor function making fuel delivery more intermittent and thus increasing the rough idle which then increases the intermittent nature of the air flow and manifold vacuum. This then amplifies the problem.

Fuel injection, analog or digital can be programed to ignore the wildly varying air flow and manifold vacuum caused by the big cam and inject fuel to a predetermined calculation.

This breaks the feedback loop that causes carburettors to amplify the idle roughness.

I have replaced carburettors on 351C's (that had very rough idles due to big cams) with fuel injection systems that did not sense airflow to calculate fuel at idle and the change in idle quality was astounding.

Once that feedback loop caused by the way all carburettors function had been eliminated the idle became almost as smooth as stock.