Fitting different exhausts or air filters will also upset the carburation and can cause an engine to run 'lean'. The free-flowing nature of the new exhaust or air filter means there may not be enough fuel reaching the engine. This can result in a loss of power and response, jerky running and damage to components such as exhaust valves.

A 'lean' running engine runs hotter than normal. Think of the old Bunsen Burner at school – as you introduce more air it burns hotter and more efficiently (more power). Too much air and it will splutter (misfiring) – it's the same in an engine.