Wave modulation due to Bengamin Feir Instability (BFI) in the presence of long wave (swell) is investigated in the elaborate experiments. The mechanically generated waves are composed of three discrete waves; while the newly generated harmonics due to BFI still combining into discrete spectra with the same frequency difference step. These stable discrete waves allow us to further examine the basic properties of nonlinear wave-wave interactions, such as the variation of phase speeds and wave numbers along the wave tank together with speed of crests, especial for the large transient waves. The measured values have a reasonable correspondence to the available nonlinear model estimations. The technique proposed in this study is verified to accurately measure the phase speed and wave number of each harmonic in the wave group. Phase speeds of short waves have a significant amplification in the vicinity of big transient waves, while the speeds of longer waves are close to linear values. The relative long wave (storm wave) travelling with short waves (wind waves) dramatically change the local kurtosis and skewness and may have an important role for the generation of large transient wave and provide an opportunity for triggering the freak waves.