I purchased a 1977 Mercury 850 Thunderbolt 85 HP. I also purchased a 1979 Mercury 800 Thunderbold 80 HP. to use as a parts motor. the 800 came with the original T-Bolt tach and i'm trying to use it on the 850, however when the engine is at idle the tach gauge is pegged and bouncing. There is no movement of the needle until after the engine is running. Any thoughts?
