25 hp Merc only getting 1/2 power band

[smokin 2 strokin]

Hello,<br /><br />Im fairly new to the boating world but a veteran to mechanics. I have a 1986, 25 hp Mercury 2 stroker. Went fishing the other day and it ran like a bat out hell after one pull. It ran like this for about three more starts/stops and then it seemed to only have 1/2 power but still ran very smooth. I could see the butterfly was wide open (when running), the plugs were good and clean, fresh fuel, had plenty of spark, 50 to 1 mixture it seemed to be all gravy!!! However it seemed to idle extremely rough when this inermittent problem would occur. Took home checked all of the (above) once again and everything was fine and it seemed to run great on the stand.<br />Further more-<br />Took out next day and did the same thing!! It seems to run smooth (at full throttle) with this problem but its just not hitting the powerband.<br />I tryed to choke the engine while running at full throttle and nothing happened at all. It still poked along like a four stroke! HAHA <br /><br />Any suggestions? <br />Possibly Idle/Flooding?<br />What is the idle mixture screw set at from the zero position (all the way in).<br />Possibly a fualty power-pack?!?!?!?!<br /><br />Any help would be greatly appreciated!!

 

[eurolarva]

Re: 25 hp Merc only getting 1/2 power band

Sounds like water in the fuel. You mentioned new gas but did the tank have any old stuff left in it. YOu can disconnect the fuel link to the carb and pump some gas in a mason jar and look for water in the gas. Water is heaveier than gas and settles to the bottom of the gas tank. I would start by rebuilding carb and put a new diaphram on fuel pump if it has one. Also check for any air leaks or the opposite that tank is venting. When problem happens try giving it choke and see if it smooths out. If so it is gas related. also try loosening the gas cap on tank and see if that helps.<br /><br />Good luck and welcome to iboats.

 

[jag306]

Re: 25 hp Merc only getting 1/2 power band

i have an 88 merc 25hp that was doing the same thing ---- its always a good idea to eliminate everything---- mine ended up to be the switch box it was running on one cylinder sometimes (1/2)power. just something to try if all else is good

 

[jim dozier]

Re: 25 hp Merc only getting 1/2 power band

Sounds pretty much like you are running on 1 less cylinder. Find out why. If not spark, then its carbs or fuel pump. If the bad cylinder is the one with the fuel pump attached, check the fuel pump for a leaking diaphragm. If not the fuel pump you probably have crud in your main jet. Rebuild the carbs.

 

[Dman]

Re: 25 hp Merc only getting 1/2 power band

I would run it on the muffs and pull one of the 2 plug wires off while running. If it's not running on one of cylinders, there will not be any difference in idle speed. If both are firing, you will get a drop in RPM's. Start there and determine what cylinder is not firing. Or spray a bit of pre-mix in the carbs and see if it'll smooth out at idle when you do this. Check the main jet on the cylinder that is not firing and it's probably not getting gas. Don't rebuild the carbs yet. Give them a good cleaning first. <br />Start simple. <br />No need to dump money into a suspected problem and then find out that it's not the power pack etc.<br />Dman