Sounds like both tachs are marine tachs, since they have the option to set the pointer on the back at the number "6". That is the correct setting for that OMC engine. Odd both tachs work differently. It is very possible that the rectifier is shot. Usually when the tach seems to act up (misread, not work at all, etc.) it is due to a bad rectifier. There is usually a second problem with a bad rectifier-the engine usually won't recharge the battery correctly. This is a little harder to check, unless you are running a voltmeter gauge on the dash. Normally, after running the engine for 20 minutes, you would expect to see the battery voltage creep up to perhaps 16+ volts. If the boat does not have a volt gauge, you can temporarily use a simple household type voltmeter to check for battery recharging. Since the ignition runs on it's own stator coils, the engine can run for a long time without recharging the battery. The only time the battery is used is when the engine starts, or when the trim motor is activated.
