by Art Gardner
Jon at the Carb Shop in Missouri suggests a slightly different approach to adjusting the idle mixture screws on an old Cadillac carb. According to Jon, the Shop Manual’s approach of first setting the idle speed and then tweaking the mixture is backwards. He says you should set the mixture first and then adjust the idle speed to achieve the desired/specified idle speed. Jon says that on a Carter 722, the mixture screws should be somewhere between 3/4 of a turn from all the way in to 1-1/2 turns. He suggests that one should break that up into thirds, so you would have more or less discrete positions of 3/4 turn, 1 turn, 1-1/4 turns, and 1-1/2 turns. He says for a brand new engine, 3/4 turn is typically the right position. As the engine wears, 1 or 1-1/4 turn become the norm. Moreover, he says that there is almost never a need to turn a mixture screw more than about 1-1/2 or 2 turns out from closed. He says that with these relatively steep mixture needles, once you turn the screw out about that much, the mixture passageway is fully open and further turning of the screw doesn’t make it any more open (open is open, after all). I followed Jon’s advice in fine tuning the performance of the Carter 722 on my last 1949 (sadly now enjoyed by someone else) and found it to be very sound. Give it a try!