Since my 4th of July was spent fixing a problem on my boat, and since I use this forum alot for advice (mostly silently), I figured I'd describe both my problem and the ultimate solution in case it helps somebody else out there avoid 3 days of troubleshooting and repair.
I have a 357 Mag from 2011. These engines are made by mercury remanufacturing. For the first 3 years of its life, the engine ran like a top. At the end of last season, on the last long trip I took (Probably 200nm+) the engine started acting up on our return leg. I normally cruise around 3300 RPM. WOT is about 4500 RPM. On the way back, the engine dropped in RPM and sounded like it was going to stall. I pulled the throttle back, and we cruised the rest of the way back at about 2300 RPM. I winterized the boat, and figured I'd do some work in the spring.
In the spring I got the boat ready, replacing fluids, etc. On my lawn with earmuffs on, the boat would run to WOT without any sign of trouble. I launched the boat, exited the harbor, and tried to come up. When I got to 3000 RPM, the engine bogged down. I returned to around 1200 RPM and it was fine. I tried again, and still no joy above 3000 RPM. Sometimes it would stall completely, sometimes I could catch it. Under no load, I could go to WOT fine. It was also idling more roughly, bouncing around by about 200 RPM over several minutes and occasionally stalling when transitioning from fwd to detent and rev to detent. I returned to the harbor, moored up my boat, and left.
There are two things that folks always blame for boat problems, gummed up carbs and crappy fuel. So I bought a carb rebuild kit for it (Weber). About 2 weeks later I returned and rebuilt the carb, then replaced the fuel filter. Again headed out. Again couldn't go above 3000 RPM. NOT THE CARB.
I decided since everyone says it's always that damn E-10,15,etc. fuel, it must be fuel. I removed my dip tube from the tank and stuck it in a 5 gallon can of gas. I replaced the fuel filter and pumped all of the fuel out of the carb using the accelerator pump. For good measure, I squirted in some carb cleaner and pumped that through the accelerator pump too. I hooked everything up, left the harbor, and still couldn't go above 3000 RPM. NOT THE FUEL.
I figured it might be ignition. I decided to follow the Thunderbolt IV/V ignition diagnostics chart on this board. If you read it carefully, it starts from 'NO SPARK'. I had spark, but followed it anyway. It indicated the sensor module in my distributor might be gone. I have the plastic sensor. I went to the marina and the mechanic there told me he's never seen one fail, but he replaces 2 fuel pumps a week. I decided to buy a fuel pump, and for good measure a cap and rotor.
I installed all three things. Started up the boat (which was getting harder to start without a lot of throttle while cranking), and got almost out of the harbor before the boat completely quit. I was pissed. After being towed in, I started to threaten my boat that it had a promising future as an artificial reef. It became more difficult to start. NOT THE FUEL PUMP. NOT THE CAP. NOT THE ROTOR.
Not knowing what to do next, I was ready to call the mechanic. However, I figured I'd run the Thunderbolt diagnostic test once more, right after the engine quit. There is a part of the test where you hook the coil to a sparkplug and touch the green/white lead to ground to repeatedly make spark. Instead of looking blue like the last time, it looked pink. I decided that I would replace the coil, and if that didn't work I'd drop the keys off at my mobile marine mechanic's place.
I put the NEW COIL in and it now runs like a top again.
Moral of the story: the easy answer is fuel, it's probably wrong.
I have a 357 Mag from 2011. These engines are made by mercury remanufacturing. For the first 3 years of its life, the engine ran like a top. At the end of last season, on the last long trip I took (Probably 200nm+) the engine started acting up on our return leg. I normally cruise around 3300 RPM. WOT is about 4500 RPM. On the way back, the engine dropped in RPM and sounded like it was going to stall. I pulled the throttle back, and we cruised the rest of the way back at about 2300 RPM. I winterized the boat, and figured I'd do some work in the spring.
In the spring I got the boat ready, replacing fluids, etc. On my lawn with earmuffs on, the boat would run to WOT without any sign of trouble. I launched the boat, exited the harbor, and tried to come up. When I got to 3000 RPM, the engine bogged down. I returned to around 1200 RPM and it was fine. I tried again, and still no joy above 3000 RPM. Sometimes it would stall completely, sometimes I could catch it. Under no load, I could go to WOT fine. It was also idling more roughly, bouncing around by about 200 RPM over several minutes and occasionally stalling when transitioning from fwd to detent and rev to detent. I returned to the harbor, moored up my boat, and left.
There are two things that folks always blame for boat problems, gummed up carbs and crappy fuel. So I bought a carb rebuild kit for it (Weber). About 2 weeks later I returned and rebuilt the carb, then replaced the fuel filter. Again headed out. Again couldn't go above 3000 RPM. NOT THE CARB.
I decided since everyone says it's always that damn E-10,15,etc. fuel, it must be fuel. I removed my dip tube from the tank and stuck it in a 5 gallon can of gas. I replaced the fuel filter and pumped all of the fuel out of the carb using the accelerator pump. For good measure, I squirted in some carb cleaner and pumped that through the accelerator pump too. I hooked everything up, left the harbor, and still couldn't go above 3000 RPM. NOT THE FUEL.
I figured it might be ignition. I decided to follow the Thunderbolt IV/V ignition diagnostics chart on this board. If you read it carefully, it starts from 'NO SPARK'. I had spark, but followed it anyway. It indicated the sensor module in my distributor might be gone. I have the plastic sensor. I went to the marina and the mechanic there told me he's never seen one fail, but he replaces 2 fuel pumps a week. I decided to buy a fuel pump, and for good measure a cap and rotor.
I installed all three things. Started up the boat (which was getting harder to start without a lot of throttle while cranking), and got almost out of the harbor before the boat completely quit. I was pissed. After being towed in, I started to threaten my boat that it had a promising future as an artificial reef. It became more difficult to start. NOT THE FUEL PUMP. NOT THE CAP. NOT THE ROTOR.
Not knowing what to do next, I was ready to call the mechanic. However, I figured I'd run the Thunderbolt diagnostic test once more, right after the engine quit. There is a part of the test where you hook the coil to a sparkplug and touch the green/white lead to ground to repeatedly make spark. Instead of looking blue like the last time, it looked pink. I decided that I would replace the coil, and if that didn't work I'd drop the keys off at my mobile marine mechanic's place.
I put the NEW COIL in and it now runs like a top again.
Moral of the story: the easy answer is fuel, it's probably wrong.
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