The C=C bond is harder to break (higher bond energy) than a C-C bond and so there is more likelihood of elemental carbon being produced as soot by alkenes. The hydrogen from both compounds always is burned.
You really should have the same coefficient in front of the alkane and alkene, as it really should either be 3.5:4 or 7:8 comparing the O2 needed for the alkene and alkane.
Otherwise, charco is bang on the money (as always). Alkynes are even worse!