A new flame pyrolysis reactor for carbon black synthesis from liquid hydrocarbon fuels is developed. The reactor system consists of a surface mixed oxygen/methane diffusion burner, downstream of which the heavy hydrocarbon precursor is injected. The hot mixture flows directly into a high temperature isothermal plug-flow reactor, from which samples are directly taken for on-line measurement of aerosol size and yield. The reactor is designed to produce carbon black (CB) at a rate of 100 g/h at overall equivalence ratio varying from 2.5 to 4.0. For these studies Methyl-naphthalene is used as the hydrocarbon source for the CB which is injected downstream of the flame fuel in vapor form. Particles nucleate and subsequently grow by coagulation and surface growth in the isothermal reactor region. Nitrogen injection is used to quench the chemistry and aerosol dynamics and minimize transport losses during sampling. Particle size distributions are obtained on-line, using a differential mobility analyzer, and total carbon black yield is obtained by on-line oxidation of the CB and measurement of the resulting CO/CO2 concentrations. Our preliminary yield measurements with varying overall equivalence ratios indicate that the soot yield increases with the overall equivalence ratio at a given burner equivalence ratio. Also, the temperature independence of soot yield measured at furnace temperature between 1300C-1500C indicates that the soot formation process is not kinetic controlled over the testing temperature range.