Spark issues are also not always easy to diagnose. This dyno graph shows the example of a Late Model with a bad distributor. The spark wasn’t consistent and the car never ran clean. The team changes carbs thinking it was a fuel issue, and swapped ignition boxes thinking that was the problem, but it never went away. After chasing the problem for the better half a season, a day on the chassis dyno helped them figure out the distributor was bad, and that weekend they ran up front.