Sudden decrease in RPMs at cruise

I was in my 1999 ProGator with 150hp Mercury on it yesterday. Haven't used it much in the two years I have owned it but it's always started reasonably easily and ran real good. Yesterday, first crank, it spun and spun and would not hit one bit and, then, suddenly, it started and didn't miss lick for the two hours I fished.

When I quit fishing, I decided to just see what top speed is, as the wind was down. I went a couple miles at max speed, 53mph, slowed down, turned around and headed to the launch. I was about halfway there when, suddenly, there was a def change in sound and I noted speed reduction. What the heck? I was thinking what the heck, something is caught on my motor, like fishing line or a net or something. I slowed down and checked in and found nothing. Got underway again and it still sounded different and was running rougher and didn't have as much speed. I slowed way down and made it to the launch. It went dead at one point and was hard to start but did. Eased in. Went dead at launch. Hard to start again to get it on trailer. Just running rough. Trying to start it, it would not spin fast and freely, it would just spin like one blip and stop, over and over. Anyway, it finally limped to life and I got it home.

It was an unpleasant experience, I feared something bad happened. I was thinking, heck, it's not hitting on all cylinders or something. I don't know much about motors. I hope it is not something terrible, lol. What could it be to make it run rough and not have as much power? Ugh. I will probably need to take this to a mechanic unless there is something I could troubleshoot.

I had same issue. Water in gas from sitting up. I had to empty my tank and E fuel/water separator filter and fuel filter on motor.

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Originally Posted by Osnola

I had same issue. Water in gas from sitting up. I had to empty my tank and E fuel/water separator filter and fuel filter on motor.

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WOW, you have given me hope! I didn't even consider that! Since it ran well the day before, I didn't even think of that! Could this just come from condensation? I tell ya, more and more, it seems that fuel sure is an issue. Having issues with my old 40hp Tracker (Mercury) as well. Regarding this 150hp Merc, I use non-ethanol gas (for all boats) but the guy I bought it from used e10 gas and added seafoam and I haven't even run a tank of gas through that motor in two years. Just used it for a few short trips.

EDIT: I am wondering what would be the best way to test my gas to see if there is water in it. How should I source it? What's odd to me is that I added five gallons of e-free gas before boating yesterday. That gas should have been been clean unless it came bad from the gas station.

Also, how could one drain ALL of the fluid in a bassboat gas tank. I tried to do this very thing on my Tracker boat. I siphoned the gas out but it still left about 3/4 inch in there; wondering how you could get everything out. Might be some sludge, trash, water left.

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Originally Posted by livemusic

WOW, you have given me hope! I didn't even consider that! Since it ran well the day before, I didn't even think of that! Could this just come from condensation? I tell ya, more and more, it seems that fuel sure is an issue. Having issues with my old 40hp Tracker (Mercury) as well. Regarding this 150hp Merc, I use non-ethanol gas (for all boats) but the guy I bought it from used e10 gas and added seafoam and I haven't even run a tank of gas through that motor in two years. Just used it for a few short trips.

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I would drain that old gas out of the tank and the carbs if possible. Then fill the tank with fresh non-ethanol gas and add stabilizer to the gas before you fill it up. Even 100% gasoline with stabilizer in it will go bad over time......8-12 months for me and I pump it out and add fresh. You can also add an in-expensive wter/fuel separator somewhere in front of the outboard and drain the moisture out every month or so.

I am draining the fuel. I have a little fuel-it water tester and it hasn't totally settled out but it's looking like 6%. I suppose that means 6% water in the fuel. Is this level a problem for motors?

Would a water test done at beginning of draining fuel be different from toward the end? I would think water would be higher percentage on the bottom of the fuel tank but that's just a guess.

I note than looking at the spark plugs, there is what appears to be motor oil remaining on the seat where the threads begin in the three spark plugs I have looked at thus far. Is having oil there normal? That doesn't seem good to me but I am certainly not that knowledgeable about motors. Sure hoping it is just water in the fuel.

As for disposing of this gas, could I just pour off the top and use that in my truck? V8s are pretty robust, lol.

EDIT: I do not see a water/fuel separator in this boat. Also, I did not see water droplets when I removed those three spark plugs. I will check the other three.

6% is pretty bad. Can you drain the carburetors? Usually there is a drain screw in bottom of each carburetor. The fuel and water is burned by the time it gets to the spark plugs. I would not use this fuel in any engine you need to run....Water is heavier than gas so the bottom of the tank will have most of the water.

you might also want to change out your fuel line and bulb !!! had the same thing happen to me once ,and the fuel line was coming apart from the inside clogging it up making the engine run rough starving for gas and hard restarts !!!

Also check your tank vent. I fixed a fellas pontoon boat after he had the motor worked on 3 different times and it kept losing rpm and was hard to start. His tank cap had cracked and he covered it with a plastic bag to keep water out. A friend asked me to take a ride on it to see if I could figure out what was wrong. Full throttle for about 10 mins and it fell on it’s face and didn’t want to start. I went back to see if it was anything obvious, primer bulb was sucked almost flat. Removed bag, opened cap to relieve vacuum and away we went. Best that motor had run all year. I recommended a new tank cap or take the bag off when using it.

Welp... I'm no mechanic... but I related that I feared something catastrophic and now, I am more worried. I borrowed a compression tester. It's a Harbor Freight, so, I dunno how reliable it is. But on this 1999 6-cylinder 150hp motor, I have about 105 reading on all cylinders except the bottom one on the left and bottom one on the right and both of those give no reading. The fact that not one, but both of those two read zero, what does that indicate, anyone have any idea? Blew a piston or something? But both of them?

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Originally Posted by livemusic

Welp... I'm no mechanic... but I related that I feared something catastrophic and now, I am more worried. I borrowed a compression tester. It's a Harbor Freight, so, I dunno how reliable it is. But on this 1999 6-cylinder 150hp motor, I have about 105 reading on all cylinders except the bottom one on the left and bottom one on the right and both of those give no reading. The fact that not one, but both of those two read zero, what does that indicate, anyone have any idea? Blew a piston or something? But both of them?

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Never heard of that before. That is very strange.

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Originally Posted by Barnacle Bill

Never heard of that before. That is very strange.

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It's strange that I get a reading of zero or strange that the two out of six are like that? Sorry for ignorance.

If you have 3 carburetors I’m afraid that it may have been the bottom carburetor has stopped fueling and the rings at minimum are gone. It sounds more like a lean seize to me now. Last engine I had that did that was a Mercury. I hope I’m wrong.

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Originally Posted by cattoon

If you have 3 carburetors I’m afraid that it may have been the bottom carburetor has stopped fueling and the rings at minimum are gone. It sounds more like a lean seize to me now. Last engine I had that did that was a Mercury. I hope I’m wrong.

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Yes, three carburetors.

What did spark plugs look like in those cylinders?

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Originally Posted by Mbsbeek

What did spark plugs look like in those cylinders?

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I didn't notice anything different from the rest but I don't recall scrutinizing them closely like I wish I would have. Gapless plugs.