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One of the main drawback of the two stroke engine is the unstable running especially at light load which involves high emission of unburned hydrocarbons. In this case, the scavenging process has a low efficiency and the high quantity of residual gas in cylinder leads to incomplete combustion in certain cycles. The use of controlled auto-ignition combustion or ATAC, which is initiated by auto-ignition instead of a spark plug, provides a consistent increase of the thermal efficiency and a lower ignition of the fresh charge. Its main characteristic is a great cyclic stability. The experimental work described in this paper is focused on the analyses of the auto-ignition phenomenon. A FORCE outboard engine 1230 cc three cylinders is used in this research work with extra special throttles in the tranfer duct. Large windows are mounted in the cylinder head and allow us a complete chamber visualization in firing conditions at many crankangle position.