03 25 mercury weak spark on top cylinder

[pvannattan]

boat was running rough at idle, and then when i try to take off it would bog and then suddenly have a burst of power and take off. Looking further into it, i ran it at the house on the water hose. I could pull the top plug wire off and it would continue to idle (roughly). Then when i pulled the bottom it would die immediately. i could hit the starter and advance the timing arm, and it would then fire and run on the top as long as it was advanced. i pulled the top plug and checked it for spark. it had a nice blue spark on it, same as the bottom plug. i have almost an entire spare motor, so i began switching parts. i switched coils, and plug wires. other parts like a cdi box or a switch box i do not have. nothing changed. i noticed while running it with the advanced lever pushed, i was getting a spark coming out of the bottom plug wire, jumping about an inch to the motor. upon running it on the bottom plug (having the top plug wire off) there was no spark jumping out of the plug wire, even stuck my finger over the end...nothing. so...due to lack of a volt meter, i checked it the old fashioned way. i clinched my teeth and hung on as i hit the starter. and let me tell you, there was a significant difference in the two. so that leaves me with a weak spark, and no volt meter or any other electrical tools to test anything else. i then bought a used cdi box and put it on. It seemed to have helped the problem significantly. but not completely. it still mainly wants to idle on the bottom cylinder, and only cough here and there and hit on the top. but once you get it plained out, it runs like a top. I'm at a loss! does anyone know what my problem is???