Is the Carnot cycle always 100% efficient?
No, the efficiency of Carnot cycle is a function of source and sink temperatures T_1 and T_2 and Carnot efficiency is less than or equal to 100%.
The efficiency of Carnot cycle
ηCarnot  = 1-(T_2/T_1) ≤ 100%
The efficiency of Carnot cycle becomes 100% only for infinite source temperature T_1=∞ or zero sink temperature T_2=0 case.