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)
A failed Air Fuel Ratio sensor can also cause these symptoms and fuel trim readings. (mechanics.stackexchange.com)
I'm learning about vacuum leaks on engine intakes and I'm hearing this terminology of lean air fuel mixture. (mechanics.stackexchange.com)