Gentlemen,
The specific use of EGTs is a very shaky business as a singular tune-up tool. UNLESS one knows where the A/F ratio peaks the temps (typically at stoichiometric or chemically correct for the fuel used), any target numbers are pretty spooky.
As an example, if the temperature where you are reading this comment is 70-75
0F - Is that rich or lean? It is an indication of temperature.
When isolating EGTs and evaluating the bell shaped curve of A/F ratios vs Temperature, one can be referencing the same temperature at two (or more) different exhaust gas temps.
Also as the thermocouples are fairly slow to react, you are looking at history more often than not.
Lots more could be said here, but sufficient to state that it is spooky business without the correct supporting inputs of other sensors.
Yours for less detonation and more fun,
Regards to All,
HB2