Carb problems

Since you've also jetted your carb leaner that can cause it to pop through the exhaust when you let off the throttle from running too lean. The air gap don't atomize fuel as well as a regular intake because it is not heated from exhaust heat so it will require richer jetting than a normal intake. You may need to jet it way richer than you have it now. Believe it or not if you jet it too lean it can actually cause you to get worse fuel mileage because your producing less power so you have to open the throttle farther to get the car to go. Get it jetted correct and it'll get better mileage. Running a cold thermostat such as a 160 degree will help torque but also hurts fuel mileage because it hurts fuel atomization.

Do this.

Make sure your exhaust is fixed and not leaking. Make sure and run at least a 180 degree thermostat. Timing is at 34 degrees total timing in by 2400 rpm. And then play with jetting.