Air, Fuel & Spark just like any vehicle. Start with the basics. Clogged filters, check your plugs, vacuum leaks, bad gas, check your Dizzy & wires.
If I remember correctly the stock pressure on a 12A would be 4.5 psi
The return line carries fuel back to the tank. Some cars use a return type fuel system & some don't.
Take a look here.
http://www.racingbeat.com/mazda/perf...agnostics.html