We investigated the multi-mode operation of long-cavity terahertz quantum-cascade lasers (l ≥ 7.5 mm). For QCLs based on an active region design with longitudinal optical (LO) phonon transitions, emission with 30–40 strong modes in a range of more than 270 GHz (9 cm<sup>-1</sup>) is observed. For certain operating conditions, we found evidence for stable frequency comb operation, which has been further proven by a self-mixing technique. In general, the multimode dynamics is characterized by a complex alternation of broad- and narrow-beat note regimes for these devices. In contrast, only a single narrow-beat note regime was observed for a different long-cavity device based on a bound-to-continuum active region, for which the emission comb spans only 33 GHz (1.1 cm<sup>-1</sup>). We further report a technique based on a tunable bandpass filter to confirm the presence of weak emission modes in the periphery of THz combs, which allowed for the unambiguous detection of modes within a dynamic range of 35 dB. We found that the 35-dB width of the comb can exceed the 20-dB width by a factor of two.