Check where the inner cable exits the sheath at both the control and engine ends. Look for damaged, frayed or dry spots - this causes it to bind at those points as the inner slides through the outer.
Lubricate the cables - if they've been neglected the inner can chew into the outer somewhere along their length and you'll need new cables. Liikewise if they've stretched or had a too severe bend in them (they wear out).
If the cables look good then check the control cams and rollers - the cable inner will connect to a bell crank, which often has a cam or roller at the other end.. A broken roller can make it bind up, requiring a good pull to get past the crack or broken bit.
If you don't have skills leave it to an expert - you don't want it stuck at full throttle unexpectedly.