The dual fuel combustion mode, the use of bi-fueled natural gas and diesel fuel, is an attractive alternative to standard spark ignition or compression ignition combustion modes due to potential benefits of lower CO2 emissions, lower fuel costs and the use of a fuel with an alternative supply chain. Besides the potential benefits, the dual fuel combustion mode also has its challenges. There is limited data available in the literature that illustrates how the performance of a dual fuel engine changes over the entire engine map at various operating conditions; this paper presents a comprehensive set of experimental results obtained with the conventional dual fuel operation (diesel/natural gas) at ambient intake conditions to help fill this knowledge gap. In the current experiments, methane from gas cylinders is substituted for natural gas and is injected into the intake port, while standard diesel is directly injected and used as an ignition source on a single cylinder experimental engine. The presented results are obtained at optimized operating points in terms of start of injection timing, where the criterion in the optimization was to obtain maximum indicated efficiency, while satisfying limits on combustion stability and maximum pressure rise rate. The results showed that high load can be effectively achieved in dual fuel mode with high fractions of natural gas, up to 90% natural gas mass fraction, but as the load decreases the fraction of natural gas for effective dual fuel operation decreases. Below a specific load (total excess air ratio of 1.65) the switch to normal diesel operation should be made to obtain higher indicated efficiency. The results also show that if the effectiveness of the dual fuel operation is analyzed by a cost effectiveness or CO2 emissions rather than efficiency then higher fractions of natural gas can be tolerated at lower load.