Mercruiser 470 Tach is fluctuating

[davidt71]

I have twin Mercruiser 470. The tach on my starboard motor was fluctuating when the engine was running over 2500 RPMs but the engine itself was not. I changed my tach and still am having the same problem. I am also not getting the performance out of my boat like I have been (I do not think the starboard motor is running 100% although I cannot hear a difference between the 2 motors). I am running about 5 mph slower that I have been and the fuel economy has decreased. My port motor is also burning more gas than the starboard motor. I traveled about 90 miles last weekend and I put about 48 gallons of gas in my port fuel tank as opposed to about 38 gallons in my starboard tank. Any suggestions????

 

[msi84]

Re: Mercruiser 470 Tach is fluctuating

I had these engines and this problem among others several years ago and solved them all by replacing both engines with 5.7Ls. This isn't much help so if I recall, I had replaced the points with electronic ignition and rebuilt the carbs to solve the performance issues. As for the Tachs, this turned out to be a wiring, noise problem. Try running a new wire from the coil to the tach. The problem I had not only appeared at certain speeds but also when I keyed up my VHF. Sound like electrical interference to you? Replacing the Tach wire solved my problem.