Nitro fuel is a combination of part radiator, part gas, and part oil. The engine needs the fuel to run and the fuel also lubericates the moving parts and keeps the parts cool. Too lean means that you are depriving the engine the necessary lubricants for the moving parts and allowing the engine to run hotter than what is considered normal for the engine...too rich the performance of the engine suffers which causes very hard starts, bogging and the engine dying a lot. I would check all fastners to ensure that they are tight.