I have a 18' go-devil with twin 23hp vangaurds. Out of the water they turn 4090 RPM in the water they are only turning 2400 RPM. had a recent tune up done and motors are running good. the boat is only reaching 14 MPH. Before the start of the season i put new 9X6 Hill Marine props on it. Need help trying to figure out why its not reaching RPM under a load and any help or suggestions would be greatly appreciated.

Your prop may be pitched more than it is supposed to be. Send it back and have them check, or take it to a local prop shop. I had the same issue. make sure they also check for balance.

Both motors doing the same thing?


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How old are the motors? When was the last time the spark plugs were changed? You might want to check the spark plug boots for cracks because they can arc off the motor too. You also may have trash in the carbs. Save the misery and get carb kits. They are 100 buck a piece but well worth it. Also recheck the magneto's on the flywheel. A piece of manilla folder paper is the gap distance.

Have ya checked your governor arm and linkage?

Who did the tune up? Did they put way to much oil in both these engines? That's wierd they are doing the exact same thing...