1986 Riviera Ebbtide 4.3L OMC Cobra. Have a 14 1/4 X 17 prop. When idling the tach reads between 600-700 RPM. The engines runs like a dream. Starts right up, no problems. Checked wiring on Tach. Grounds are good, hot is good, sender good. Get it on the lake and when I go to almost WOT the Tach will read about 3000 RPM and the engine is reving loud. The boat is probably moving about 40-45 mph (can't decipher to knots). I looked on the back and put the dial on 6 cyclinders is this right for my 4.3L OMC Cobra? Maybe a new Tach? Help please. Any other way to test the Tach for accuracy? Thanks.