Hotrod Lincoln is right, a qj is a very tunable carb, if you know how to do it. And again he is correct they adapted them to many engines. I've had my hands on many and pay attention to many things that are different. But, for down and dirty tuning, the rear metering rods are about as fast to change as taking the air cleaner off, and you don't have a puddle of gas all over your engine like a holley. The primary rods can be changed as well as the jets, but you have to take the top off for those. I always checked the linkage, some wouldn't open the secondaries all the way. Keep it original.
