This paper reports on joint experimental-numerical investigations of flame dynamics in a microchannel of diameter 1.85 mm. Experiments show that decreasing the diameter of the channel helps the arising of the FREI instability. Yet leaning the mixture is found to be a mean to make the flame more stable. The numerical model used is based on two- dimensional thermo-diffusive equations with one-step chemistry, and it is tested against the experimental results without any calibration. The model turns out to reproduce in a satisfactory manner all the features from a qualitative point of view, and most of them from a quantitative point of view, showing robustness not only with respect to geometry but also to equivalence ratio changes. Nonetheless, while the analysis of frequencies of the FREI instability shows a qualitative agreement, a quantitative discrepancy is unfold. The results suggest that instabilities in microcombustion of CH4/air are fundamentally triggered by hydrodynamics.