i hava a 1977 150 hp merc. outboard. but my rpms are not registering on my gauge. i need to get this working. im not sure if i am getting enough rps from my moter. i ran it yesterday with 2 people . i have a 1976 renken sport 19 foot long. and i was only gettin 30 mph. i dont know my rpms. i have run a 21 foot privateer loaded dwon with seafood and it had a 90 hp evinrude i got 30 mph atleast. i dont know any info about my prop there is no markins on at all please help me soooon 