well use my example for instance, this motor pulls 8 pounds of vacume at idle, and it has a 6.5 power valve, which means when the vacume drops below 6.5 pounds the power valve opens increasing the fuel flow. so at idle the vacume may be too low to competely close the power valve letting extra fuel into the motor causing it to be too rich
and about the timing, it still cranks just like it did and doesnt seem to knock