Have you ever noticed how the engine is always throttled up real slow? Do you know why it's throttled up real slow? Because if the engine is equipped with a mechanical secondary carburetor a condition known as "fuel standoff" will occur and then a BIG backfire can happen. Fuel standoff happens because the velocity thru the venturi slows down so much under a sudden full throttle the fuel gets blown out of the carburetor in a "fog" that can ignite if the timing is a bit too early. And it is this fuel standoff condition that prevents mechanical secondary carburetors from being useful on street engines that have to operate under a very wide rpm range.