Have a 1985 115 Mercury, inline 6, on a 1985 astroglass 175v fish and ski. I've just purchased this boat before Thanksgiving, so I've never ran it before. I have, however, performed maintenance items and have replace: starting battery and both trolling motor batteries, water pump, spark plugs, fuel water separator, and cleaned the carbs. The fuel was treated before putting it away for the winter. Starts ok when cold, maybe turns over for about 1 second when warm. Engine seems to run great in the driveway, although this is my first boat. When I put the boat in the water, starts up great (I've warmed it up just before going in) and I can start going. Out of the hole, excellent. Shoots up to top speed in about 10-15 seconds, running at around 4500 RPM. Does great until about 3-5 minutes into a trip and engine will start to lose engine speed. If I try take it back to idle and then go back to full throttle, it acts like its bogging down. If I keep doing that, and keep it at full throttle, it will just eventually die. If I take it back to idle and then I just keep speed up enough not to flood the stern, I can take it back to idle once its safe and it may or may not die, usually doesn't die at this time. Although, if I keep trying to throttle up, it will die. If it dies at any time, it will become hard to start. My first thought was I was sucking air, but it doesn't do this in the driveway, only when underway. So I kind of went a different direction and decided to chase down the electronics. I've been reading so many posts on here and most seem to think that most of the time it is a spark issue.
I haven't tested for spark during the time this is happening, but I've gone through some of the troubleshooting from CDI Electronics website and have a few things that came up out of spec. First, the resistance readings from the Blue, Blue/White, Red, and Red/White are all good, but the voltage readings were not. Checking the voltage for the Red and Red/White wires came up with a negative voltage readings (I am a diesel mechanic by trade and I know how to use a meter). I check battery voltage at the battery positive post to the previously used ground (starter case) and came up with 12.5 volts. The spec for this is 20 volts or more.
Second, I also tested the rectifier. It was putting out about 12.9 volts at the battery wire the short time I had it running. Testing voltage from the post with the Yellow AND Gray wire on it was 6.7 volts. Spec says I need to be over 8 volts. Tachometer is working.
The troubleshooting really isn't clear what to replace or what is bad with these readings. I'm also not 100% sure these specs are accurate and I am still suspecting a fuel issue. As far as troubleshooting the fuel system, bulb is not hard when this problem occurs, but I've read that it isn't hard when engine running. Bulb is not collapsed as if I have a fuel restriction. Fuel tank have open vents and are clear. Fuel water separator has been replaced.
I'm not really sure where to go on the electrical side, and I don't want to just throw parts. As far as the fuel side goes, I think I will remove the tank and check for intermittent debris, maybe a possibly cracked suction tube, loose fittings, etc.
Any other suggestions are very much welcomed and appreciated.
I haven't tested for spark during the time this is happening, but I've gone through some of the troubleshooting from CDI Electronics website and have a few things that came up out of spec. First, the resistance readings from the Blue, Blue/White, Red, and Red/White are all good, but the voltage readings were not. Checking the voltage for the Red and Red/White wires came up with a negative voltage readings (I am a diesel mechanic by trade and I know how to use a meter). I check battery voltage at the battery positive post to the previously used ground (starter case) and came up with 12.5 volts. The spec for this is 20 volts or more.
Second, I also tested the rectifier. It was putting out about 12.9 volts at the battery wire the short time I had it running. Testing voltage from the post with the Yellow AND Gray wire on it was 6.7 volts. Spec says I need to be over 8 volts. Tachometer is working.
The troubleshooting really isn't clear what to replace or what is bad with these readings. I'm also not 100% sure these specs are accurate and I am still suspecting a fuel issue. As far as troubleshooting the fuel system, bulb is not hard when this problem occurs, but I've read that it isn't hard when engine running. Bulb is not collapsed as if I have a fuel restriction. Fuel tank have open vents and are clear. Fuel water separator has been replaced.
I'm not really sure where to go on the electrical side, and I don't want to just throw parts. As far as the fuel side goes, I think I will remove the tank and check for intermittent debris, maybe a possibly cracked suction tube, loose fittings, etc.
Any other suggestions are very much welcomed and appreciated.