problem mercury 50hp 4-stroke problem

charknes

Recruit
Joined
Feb 9, 2003
Messages
1
Engine is 50HP Mercury, 4-stroke, carbeurated, year 1999.<br />It will run okay up to about 3500 RPM. (This is about max displacement speed for the boat.) Upon applying more throttle the engine starts running rough and will get no more than 3800 rpm, no matter how much throttle. If I apply WOT from standing start it will start running rough earlier, but will still max at 3800 rpm. Max should be about 5000 rpm. Out of gear, it will run on up past 3800 with no problem. I did change plugs and fuel filter, but those were no help. Based upon other threads I've found here this sounds like carbeurator problem. Is that something I could try to fix myself? Are there other things I should check?<br /><br />I do have the service manual and in my younger years I repairs my vehicle engines, but I've never tinkered with outboards. Wouldn't mind giving it a try though.<br /><br />Chuck H.
 
D

DJ

Guest
Re: problem mercury 50hp 4-stroke problem

Chuck,<br /><br />It sounds like you are not running on all cylinders. An outboard will typically rev. up to or beyond its max rated rpm, even when not firing on all.<br /><br />This leads to two things to check.<br /><br />1. Check spark on each cylinder. Spark checkers are realtively cheap and availble at automotive stores.<br /><br />2. One or more of the carburetors high speed jets may be plugged.<br /><br />You can spray some gasoline into the carbs from a spray bottle to see if one or more carbs. reacts to the spray.
 

BillP

Captain
Joined
Aug 10, 2002
Messages
3,290
Re: problem mercury 50hp 4-stroke problem

Chuck,<br />I bought a new Mariner 50 4s in 96 which is also same as a Merc but made by Yamaha. Could be close to yours. Mine was eating the factory NGK DPR6EA-9 spark plugs every 50-75 hrs and running similar to yours...(3 sets of NGKs in 125 hrs and they WERE running the correct colors, temps and gaps). They were breaking down under load. Anyway, I changed to the same heat range in Champion (810C aka RA8HC)plugs and the problem ceased...several hundred hours later I am still running the Champs. I've had the same problems with NGKs on motorcycles and experimental airplane engines too. My local boat mechanic (who I bought the Mariner from) was not surprised when I told him about the NGKs.<br /><br />So, my questions are: Did your Merc come with NGKs and have you changed the spark plugs? By the way, I have NEVER seen an engine more sensitive to plug gap either. It shows up at idle first.<br /><br />There is one other place that may cause missing problems...at least with mine it did at about 30 hrs. The round main harness connector (mounted under the cowl on the aft end of the block) has prongs with a split in the center of each. Mine needed to be opened up with a small screwdriver to make good connections. This showed up as hard starting and one time in particular NO starting until I opened the prongs.
 
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