Time-resolved spectroscopy is performed in bulk samples of single wall carbon nanotubes. The carrier dynamics is monitored by probing the transient change of transmission of the sample in resonance with the interband transitions of the semiconducting tubes. Samples containing either isolated nanotubes or bundles of nanotubes are investigated. A strong increase of the relaxation time is observed for isolated nanotubes. We interpret this effect with a model based on a tunnel coupling between semiconducting and metallic nanotubes within a bundle. This model also accounts for the quenching of the luminescence in such samples.