There are basically 3 things you need to consider for timing - engine RPM, air / fuel ratio, and volume of air fuel mixture in the cylinder. (mechanics.stackexchange.com)
Conventional cars use an air - fuel mixture in which all the oxygen and fuel is burned during combustion. (newscientist.com)
This is not really a case of lean fuel mixture but you might get the basic idea. (mechanics.stackexchange.com)