Higher alcohol to oil molar ratios rather than stoichiometric value can increase contact area between alcohol and oil and so the reversible transesterification reaction shifts to forward and subsequently the yield of biodiesel increases.
Two factors can reduce biodiesel yield in higher molar ratios.
One of them is the reverse reaction between FAMEs and glycerol and decomposition of FAMEs, and another one is the reduction of critical temperature of reactant/product mixture by increasing the molar ratio of alcohol to oil.