I just took my boat (and engine) out for the first time today. There seems to be a major malfunction with the drive system, at least that is my best guess. The engine didn't have much difficulty starting up at first launch, however it soon quit on me out of nowhere on the middle of Pistakee Lake. It kept quitting on me throughout the day, forcing me to restart it time and time again. But back to the story, after some finaggling I was able to get it restarted, and putted along back into port. The gentleman who ran the boat rental shop was kind enough to come take a quick look, his opinion was that the engine itself was running okay, so I decided to try topping off the buoyancy tubes and keep with air, thinking that may be the issue. I had called a Zodiac dealer to get his thoughts, that was his best suggestion.
It did not really solve the problem. My first suspicion there may be a problem was the high speed sound of the prop whirring/chopping through a mixture of air and water at full throttle, as if there was a major cavitation or ventilation problem. Though, ventilation is unlikely, seeing as how the boat is designed for a short shaft motor and the keel/tubes were verified to correct pressure by my Bravo pump. The starter motor was also acting up on me quite a bit throughout the day.
As I continued my test of the boat, I found that full throttle would produce enough thrust to raise the bow, but not actually get up onto plane and speed. The best I can describe is that I was getting 1/4th the amount of thrust I should've been at full throttle 80% of the time, and a rev to 7000rpm and very quick planing out 20% of the time. In essence, I was either going 6mph or 20mph at full throttle depending on what the motor was doing at that time. The tach is set to 4P, as per Mercury specs, I had a Mercury on the phone with me as I set up the tach. The prop chopping/whirring noise could be from it overspeeding in the water, turning too fast. The strange thing though is that the RPM at full throttle setting was seemingly correct when (getting way too little thrust from the motor), at around 5200rpm. Only when I finally started getting some power did it jump way up to 7000RPM.
Just as an aside: Fuel was automobile gas, with 50:1 oil mix of Quicksilver Premium Plus, along with one ounce each of Mercury Quickcare, Quickleen, and Quickstor. Fuel line is equipped with Attwood fuel demand valve (this is a factory pre-assembled Mercury fuel line). Also, I am not sure if there is a thermometer-metered water cooler, but the pee stream varied from a halfway decent stream to just kind of a dribble. If there is a thermometer, I would like it yanked out, not seeing a decent stream is disconcerting. I can turn the prop by hand in the same direction the prop would be turning in forward gear, while it is in forward gear. It makes significant audible (and felt) clicks when turned. I cannot turn the prop by hand either direction when gear selector is in reverse. Not sure if that's relevant information, but the Zodiac dealer I was able to get a hold of had me do that as an on-water inspection and said it was his assertion i shouldn't be able to turn it at all by hand in gear.
I took a video, which I uploaded to YouTube, to show the problems I was having. The link is below.
Please let me know what you think about what is going on with this motor... It's still under warranty from the shop I bought it from, they will apparently cover any costs for repair of defects 90 days on from first use.