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My buddy has a boat that has a 50 hp Mariner 4 stroke on it. Last week he was out fishing and said it ran great all day until when they got ready to come in, the motor would not go over 3000 rpm and would not get on plane. We put it on a water hose at the house a few days later and it ran fine. Took it back out again today and it would not go over 3000 rpm in the water. Ran fine if it was out of gear, just not when in gear. The spark plugs look nearly new. Anyone have any suggestions as to what may be causing the problem or know what I should check to try to solve the problem?

How's the lower unit? Plenty of gear oil? No cracks?

May be a spun hub in the prop.

When you say it will not go over 3000, what do you mean? Does the engine seem to lose power and 3000 is the max you can get out of it? If it is starving for fuel, it can run just fine at idle, because it is not pulling the fuel hard, but when you shower down on the throttle, the flow can't keep up with the demand.

I would check the fuel filter and then lines to be sure there is no ethanol issue. If it feels like it is starving for fuel, I think ethanol first and have been right more than wrong. What kind of fuel lines? Connections? Filter? Tank?

The lower unit is in good shape, no cracks, and a believe the oil was changed a few months ago.

Can a spun hub cause the engine to max out 2500 rpms under what it should be? Seems like that would make it hit higher rpms quicker. I'll tell him to pull the prop and check it out.

onefishtwofish wrote:When you say it will not go over 3000, what do you mean? Does the engine seem to lose power and 3000 is the max you can get out of it? If it is starving for fuel, it can run just fine at idle, because it is not pulling the fuel hard, but when you shower down on the throttle, the flow can't keep up with the demand.

I would check the fuel filter and then lines to be sure there is no ethanol issue. If it feels like it is starving for fuel, I think ethanol first and have been right more than wrong. What kind of fuel lines? Connections? Filter? Tank?

Agree with this post. Mine did the same thing, and the line from the water separator to the motor had deteriorated and had crud all in the fuel pump. Replaced the line, rebuilt fuel pump, and back to 100%. (If the line is gray with RED writing on it, I can almost guarantee that is the issue)

If it is the line, replace it and the Bulb with a a Silverado 4000 line. From what I understand that can take 100% ethanol with no issues...

BTW, I was 10 miles out when mine did it...NOT a good feeling. But I knew I had SeaTow if I had needed them. I was able to limp back in safely at a low RPM....

Dubble

The engine seems to lose power and all it will do is 3000. However, when not in gear it runs like a top. I know all the fuel lines are ethanol rated and the fuel filter looked clean. Getting fuel was the first thing I thought of we he described the problem. I'll have to check the bulb next time we run it to see if it is maintaining pressure
.

Take the fuel line off. Look inside for the liner. See if it is brown and flaking. I have seen ethanol rated line still go bad.

These line flakes will enter the fuel pump and get stuck under the reed valves and cause the fuel not to pump. When you say you checked the filter, do you mean the water separator, or the little screen just before the carbs. Just because there is nothing in the water seperator does not mean the line between that and the engine is ok.

Dubble

I checked the screen right before the carbs. I'll have to take the fuel line off tomorrow to see if that is the issue.

On our little motor, it was actually the bulb going bad, not the lines. We had to take apart the fuel pump, the carb and clean everything twice, cause we thought it was just the gas in the tank. Once we figured out where it was coming from, easy peasy.

take the fuel line off the motor and pump some of the gas into a clear bottle, if there is any debris yolu will find it.

Oh yeah, check your jets in the carb.

My Bass Tracker with a 50 Mercury did that exact thing a couple of times. Both times I had to have a carb kit put on. It would run fine on a hose but not with a load in the water since the carbs were gunked up. If you can you will have to thoroughly clean the carbs, especially the jets, and probably replace the fuel lines.

My buddy took his boat to a mechanic and said that his carbs were clogged and he needed a new fuel pump. He said the quote from the guy was $1100 . Is this a normal price for a carb cleaning? Seems a little steep to me, but I haven't had any work done on my motor by a professional in a long time.

I would shop around on that price, seems steep to me.

Try running it using a portable gas tank with new gas and with different hose, bulb etc. so you are totally off the normal fuel supply sytem, if that clears it up it's in the fuel line/filter side, if it doesn't then you are likely dealing on the carb/engine side. Quick easy way to troubleshoot a potential fuel supply/quality issue without spending big bucks. Running it on the hose won't work because there is not enough fuel flow to make the problem show up.