Merc 25 spark advance query

Hi , I have a 96 merc 25 EL sn og377175. recently purchased with low hours fitted to a newly refurbished 14ft boston whaler . 11 pitch prop. the engine idles great and runs up fine to half / two thirds throttle, just about gets on the plane but then has nothing left for the last wot. whilst checking the linkages etc for wot I notice that the trigger is locked imovable via a small adjustable link arm attached to the block, and has no visible connection to the throttle. Is this correct , as I can see no mechanism to advance the spark and I am assuming that this is the cause of my missing top end power . I went throught the carb and it was all clean and clear The engine had been reviously fitted to a small rib , I was wondering it it had been altered to de tune it , any ideas
thanks pajic