Mercruiser tach issues

[duped]

Have a boat with twin 1985 260 Alphas. Previous owner stated the port engine's tach sometimes needs a whack before it would work. Tried that, it seems like it has completely died.

No worries, bought a tach, made sure it was set to "8" and made sure all the connections are good. This one goes to zero when the key is switched on, like it should. But when the engine is started it reads at least 1500-2000 RPM higher than actual and seems to jump all over the place when the actual engine speed hasn't changed.

I've also been having a slightly rough idle on the this engine. Runs fine otherwise. I have ordered cap rotor plugs and wires just as a general maintenance deal, but I'm not sure if it will fix either of the issues, or if they are even related.

Grey wire Connection at coil is clean. Any other thoughts? Bad coil?

 

[alldodge]

Have a boat with twin 1985 260 Alphas. Previous owner stated the port engine's tach sometimes needs a whack before it would work. Tried that, it seems like it has completely died.

No worries, bought a tach, made sure it was set to "8" and made sure all the connections are good. This one goes to zero when the key is switched on, like it should. But when the engine is started it reads at least 1500-2000 RPM higher than actual and seems to jump all over the place when the actual engine speed hasn't changed.

I've also been having a slightly rough idle on the this engine. Runs fine otherwise. I have ordered cap rotor plugs and wires just as a general maintenance deal, but I'm not sure if it will fix either of the issues, or if they are even related.

Grey wire Connection at coil is clean. Any other thoughts? Bad coil?

I would suggest looking for a bad ground on the gauge or a tach wire starting to short out/bad connection. Don't think it would be the coil because that would show up more so at higher rpm

 

[achris]

........ Any other thoughts? Bad coil?

You have 2 engines. Swap the tachos over and see what happens.