In all internal combustion engines, IC engines, the air/fuel ratio is of utmost importance for the engine function.  Usually the air/fuel ratio is referred to as the A/F-ratio, A and F signifying respectively air and fuel.  In order to achieve a satisfactory combination of low fuel consumption, low exhaust emissions, good runability and high efficiency the A/F-ratio must be maintained within comparatively narrow limits. 
The requirements that exhaust emissions from the IC engine to be kept low are becoming increasingly stricter.  In the case of car engines these requirements have led to the use of exhaust catalysers and to the use of sensors and probes positioned in the car exhaust system in order to control the A/F-ratio. 
However, for consumer products, such as power saws, lawn mowers, and similar products, this technology is difficult to use for mounting reasons and also for cost-efficiency and operational-safety reasons.  For instance, in a power saw, a system with sensors and probes would result in increased size and weight as well as a drastic rise in costs and possibly also cause operational safety problems.  Further the sensor or the probe often requires a reference having completely pure oxygen, which is a situation that it is practically impossible to achieve in some engines, for instance the motors of power saws. 
Expected future legislation with respect to CO-emissions from small IC engines may make it difficult to use manually adjusted carburetors.  Given the manufacturing tolerances that could be achieved in the case of carburetors it is impossible, with the use of fixed nozzles in the carburetor, to meet these legal requirements and at the same time guarantee the user good runability in all combinations of air-pressures and temperatures, different fuel qualities and so on. 
EP 0 715 686 B1 describes a method of controlling the engine A/F-ratio without the use of an oxygen sensor (lambda probe).  Initially, the A/F-ratio is changed briefly.  This could be effected for instance by briefly throttling or stopping the fuel supply.  In connection with the change, a number of engine revolution times are measured.  The revolution times relate to engine rotational speeds chosen in such a manner that at least one revolution of the engine is unaffected by the change, preferably an engine rotational speed that is sufficiently early for the A/F-ratio change not having had time to affect the engine rotational speed.  Further at least one forthcoming revolution of the engine is chosen in such a manner that it is affected by the brief A/F-ratio change.  In this manner it becomes possible to compute a revolution-time difference caused by an A/F-ratio change.  On the basis of this revolution-time difference a change, if needed, of the mixture ratio in the desired direction towards a leaner or richer mixture is made.  Thus using this method an optimal mixture can be achieved by testing how the engine reacts to a leaner or richer mixture.  However, the engine control method of EP 0 715 686 B1 is somewhat slow and also requires the product to be ran under a load when fine-tuning the A/F ratio.  Some machines, such as bush cutters, are not usually operated under constant load and are could thus be difficult or take even longer time to fine-tune with the method of EP 0 715 686 B1. 
US 20100011597 disclose a method for quickly finding an A/F ratio when free running the engine.  The A/F ratio is adjusted until a desired speed interval has been reached.  The algorithm finds an A/F ratio on the rich side of the A/F curve, i.e. it seeks a decent A/F ratio which later can be optimized using e.g. under load as described in the method of EP 0 715 686 B1.  However, it is sometimes desirable to find an optimal A/F ratio also when not being able to adjust under a load.