Originally Posted by 1957willyman
good idea about drilling primary butterflies to get better airflow at idle done that today got rid of bad rich fuel smell also cracked open secondarys 468 bbc 550 hp 1941 willys coupe
Just for the record, drilling the primary blades is often not needed.
The idea of using idle bypass air (holes in the throttle blades, in other words) is to allow the primary throttle blades to be returned to a normal position in relation to the transfer slots.
Often this can be done by raising the initial timing. This increases the idle speed, allowing the throttle blades to be closed down, restoring the transfer slot vs. butterfly relationship. In some cases opening the secondaries a bit helps with this as well. Increasing the initial timing also has the added benefit of helping to get the lean idle air/fuel mixture (often encountered w/a performance cam) started burning sooner. This will eliminate the eye-watering exhaust odor that's often noticed.
The mechanical advance has to be limited to keep the total advance (initial plus mechanical) from being too high when initial advance is added. The extra work involved can pay big dividends in overall better performance and economy.
Click on image for details on limiting the mechanical advance and on timing, etc.: