EGT Monitoring (exhaust gas temperature)

Finding the optimal EGT signature for your engine is a trial and error (hopefully more trial than error!) procedure, and other factors such as power output, plug readings and air fuel ratio equipment should be used to corroborate the data.  Once you have ascertained this ideal EGT, it should be repeatable regardless of climatic conditions:  simply tune for the same previously determined optimal EGT, and your engine should perform at full available output under any ambient conditions.

Easy error for new users of EGT: high temps indicate lean condition.  Not always true!  Excessively rich conditions will result in "after burn", where the fuel, which was unable to completely combust due to insufficient oxygen in the cylinder, lights off in the exhaust system, causing an unusually high temp reading.  If all other indicators still suggest a rich mixture, try leaning in small steps, and you will likely see the EGT go down.  Just be sure that the power does not also go down from the changes.  If you are on the right track, power should go up noticeably as you lean towards optimal mixture while EGT drops.  As you approach and then pass the optimal mixture point, the EGT will begin to climb again.  STOP! Richen by one step and you are there!  Now, when climatic conditions worsen (i.e.: hotter temperature, more humidity, less air density), lean until you get that optimum EGT again.  If conditions improve (colder weather, lower altitude, less humidity), richen for optimal EGT.  Bear in mind: if EGT suddenly changes for no apparent reason, you may have an aggravating factor (ignition problem, fuel pressure wrong, clogged air inlet, etc.) which is unrelated to tuning.  Be observant, and the indicators should guide you to the right tuning decision. 

Another caveat:  Air fuel ratios, which are not optimal throughout the entire available RPM and manifold condition range, will mislead you.  In other words, an optimal EGT signature at high RPM may not show an incorrect condition at lower RPM or different manifold pressure.  Although perplexing, this problem is truly the difference between a happy, powerful and long-lived engine and one that is trying to destroy itself slowly but surely.  It is one reason why the precision of fuel injection is usually superior to carburetion in both power production and engine life.  Just watch ALL the indicators, and remember: lean is mean, and fuel is power.  Instead of continuously trying to lean it for maximum HP, try to find ways to get more air to the engine, and thusly support the combustion of more fuel.  There's only so many BTU's in a gallon of fuel, no matter how you burn it.  Just try to burn more fuel!