I just got a rebuilt Q-jet 7042240 and after installing it's running super lean. So lean in fact that it won't run unless I hold the choke almost completely closed. The one I'm replacing was just the opposite and was running way too rich. The jets are 73, the primary rods are 44B and the secondaries are CT. I'm putting it on a 350 with a mild cam and I know it's originally for a 455 but it sure shouldn't be running lean. Any ideas would be appreciated.
Are you sure the old carb that was "running rich" was adjusted properly? Was the idle mixture set? Any time you run a 455 carb on a 350 it is likely to run lean at idle and low rpms. The air passages are much different on the 455 carbs than the 350 ones. GM spent tons of $ to get the best Q jet on each engine, I like to start with the correct carb and make small changes from there. I have used 455 carbs on 350s and it took tons of hours to get the idle air modified to work properly.
