Get a real Merc manual off ebay . . . they pop up now and again for $20.
also this is a pretty good write up. I wouldn't go as high as 25 degrees, b/c I'm a little more conservative.
http://www.hydrostream.org/ArticleArchives/Timing.htm
That's a great motor. I run the same powerhead (80's mid section and 90's lower unit) on my seacraft.
Disconnect the spark advance modules and throw them in the garbage . . . Also check the wiring trigger and stator while your at it. I'm almost 100% the spark adavance modules/idle stabilers module is your problem. They sit above the carbs on the airbox on that vintage engine.