Diagnosing is an art. Sometimes it is learned by doing all the wrong things first, sometimes it is by making an educated guess.
Once I had a 73 Challenger that also idled fine, but ran out of breath at midrange. No Tach, so I can't guess the engine speed. The engine was a reringed 440 that went a tire smoking 14.01 at 100 mph in a Charger, so I knew the motor was fine. The ballast resistor felt hot, so I swapped in another known good one. It ran perfectly afterwards. Either the car gods took mercy and lifted the curse on the car or the ballast was AFU.