2 x 74LS181 4 bit ALU's cascaded to form 8 bit ALU, Cin problem?
When using Active High operands, Cn (carry in) and Cn+4 (carry out) are active Low (indicated by the circles on pins 7 and 16 in the logic symbol below). 

The least significant ALU's carry input should be set to No Carry, ie. Cn = High.