Fundamentals
Before starting to tune your car, the fundamentals have to be right. The car must be able to start every time. A car that can’t start when hot, can’t be tuned properly. And it must not overheat after a few trips down the track or when idling for a long time. And a very worn engine with leaking valves, worn throttle axles and burning oil can’t be properly tuned either. It can probably be improved, but you’ll find that a worn engine will not act as expected on the testing needed, so results become inconclusive, and instead of being able to deduct for example whether it should have a richer or leaner idle, it’ll be guesswork.