Incipient unstart and choking are investigated experimentally in a direct-connect, constant-area supersonic combustor with circular cross-section (28 mm in diameter). The experiments were performed in the arc-heated hypersonic wind tunnel ACT-II of the University of Illinois at Urbana-Champaign using ethylene as a fuel. The long test time (up to 1 s) of the facility and the full visualization capabilities allowed by the circular geometry are important novelties compared to previous studies. Both intrusive and non-intrusive diagnostic techniques were used. Heat-flux measurements at the combustor exit revealed a negligible contribution of combustion heating to the stagnation enthalpy of the supersonic core, excluding thermal choking as possible cause of unstart initiation. This conclusion is supported by OH-PLIF images, showing that most of the OH is confined in the boundary layer and not in the supersonic core. Pressure distribution at the wall revealed a combustion induced pressure gradient in the combustor, but not sufficiently strong to explain a possible boundary layer separation. Pitot pressure measurements were used to evaluate stagnation pressure and Mach number at the combustor exit. The two quantities were found to be well correlated by Fanno relation, indicating a predominant effect of irreversibilities on the Mach number reduction leading to choking. Finally, OH-PLIF and chemiluminescence images of a pseudo-shock provided information on the flame structure during unstart, showing that combustion takes place mainly at the interface between the shock-train and the diffusion region.