Mercury 50hp keeps losing rpm

[gingerbreadman68]

I have a 2009 2 stroke 50hp mercury outboard. Last fall with just me in boat ran fine but all of a sudden when I added another person it wouldn't go over 5k rpm and wouldn't get up on plane, (5500 is max). Waited till spring and after a few weeks it wouldn't even get up on plane with just me in the boat. (btw its a 17ft smokercraft), then brought to marina. mechanic tells me I must have wrong prop. I ask him if it is wrong prop then why did it work all last summer just fine. He said " I don't know about that but if your rpms are low you need a diff prop". He changed the prop but it didn't do as much as he thought it would so he cleaned the carbs and changed all the fuel lines and the bulb. Now its back to where it was last fall and the mech tells me that's as good as it can run it gets full rpm and that's the tell tale sum of it all. Still no answer on how it ran fine last summer with a boat full of people and now wont get up on plane with more than me in it. So I used it all summer like it is and decided I will buy bigger motor next spring. All of a sudden ive lost rpm and speed again its back to that 5k rpm with only me in it. I took it to a different marina and had it checked. I'm told I must have the wrong prop on it... any input u guys may have because I cant believe its normal to change the pitch on your prop every few months.

 

[SoCalBoatGuy]

Is this always on the same body of water? Altitude can have a big effect on max HP.

 

[Silvertip]

You also need to find a different mechanc. One that tells you a new prop is needed without more diagnostic information is not helping you. An engine that has a weak or nonfunctioning cylinder may run well enough to get you alone on plane but cannot do so with more load. Fact is, if the boat ran fine at one time with the prop you had, then something else is wrong. Most likely an engine issue which can be any one or more of many. compression test, fuel system check, reading the plugs can tell you a little about the fuel system. Just don't start throwing money at it. Troubleshoot, diagnose, and then repair.

 

[gingerbreadman68]

thank you for the response. The first answer I am always on Oneida Lake NY. I have been to both marinas on the lake and the only boat mechanic who is bonded and will come to my home marina which is small and doesn't have a mechanic on duty. The first place tried to sell me a prop and had no answer for how it could have worked with the old prop. pitch. The travelling mechanic did run a bunch of tests and cleaned the carbs and replaced all fuel lines and the bulb. he said its running great and smooth with no misfires or anything like that so it had to be the prop pitch. he said if not getting enuff rpm and motor running good has to be the prop. still doesn't answer how it worked good once. he put a new prop on it with diff pitch then with just me in it I can get on plane but with extra people no way. he said motor just not big enough. Once again worked before but no answer for that. it has been 1 month now it will no longer get on plane with just me in it. the rpm at wot is now 5100 instead of 5500. the plugs are new by me but weren't fowled or gapped incorrectedly. I can by the meter to check compression but not sure how to check the fuel system except for the bulb which I checked while under way by squeezing it to see if that raised rpm. it didn't. the fuel guage works correctly and the fuel filter is brand new. is there something else in fuel system I can check? Also I'm not sure what compression numbers should be so ill see if I can buy a repair manual. and other ideas? I might as well try myself since no1 else is helping and my labor at least is free.

 

[racerone]

The moptor may be running on only 3 cylinders.---Test the compression.----Check for spark on all leads .----Spark that can jump a gap of 3/8" is considered as " good "---Test those items first.---If that is good I can post some more " no money spent " items to check.