We demonstrate broad bandwidth and intense sub-100 eV high-order harmonics from diatomic carbon molecules driven by long-wavelength laser pulses. Up to now, one limitation of the intense carbon harmonic source driven by a 0.8 μm wavelength Ti:sapphire laser has been the low cutoff around ∼32 eV. In this paper, we show that this harmonic cutoff is extended to ∼70 eV by increasing the driving laser wavelength to 1.71 μm. Surprisingly, the carbon harmonic intensity is found to be high despite the long wavelength driving laser. Experiments show only ∼30% decrease in the harmonic intensity when changing the driving laser wavelength from 0.8 μm to 1.71 μm. Such intense sub-100 eV coherent X-rays would have important applications in various domains of science and technology.