Spark plug gap is critical. A parallel gapping tool assures even firing under severe driving conditions. Generally use the proper heat range plug; if your engine is carbureted, a hotter plug can compensate a bit for higher altitude. Spark timing requirements are related to atmospheric conditions, and at higher altitudes, horsepower loss can be recovered some by advancing the spark timing. As a rule, one-degree of extra advance per thousand feet of elevation works - do not exceed 7 to 8 degrees of added base timing. Return timing to its normal setting as altitude decreases, otherwise detonation (ping) will occur.