TOPIC: Technical question on setting the timing 68 125

Ok so I have been having an issue getting full rpms on my 125. You merc gurus know that on the 1968 merc 125 the timing advances for quite a ways then it retards itself. Tim c thinks that mabey it is retarding to much.if this is the case and its not allowing me full throttle is it an easy job and could a moron like me do the job? I will shot a video of the motor running at the top of the rpm range before it looses power.its really weird because I can trim it out too far and it does not rev out. fyi 880lb boat with a 19 pitch aluminum prop.