my tach doesint read right i think it says the motor is only getting 3400 rpms i have the back of the tach set to 8 cyl its a 7.4 with a bravo 3 outdirive on it and its a 1974 wellcraft nova 25, on the back of the tach it has numbers like 864 and also pulse settings to what would i set it to its a mercurser tach any help would be appericated thanks james