Low power, 25hp Merc

nola mike

Vice Admiral
Joined
Apr 22, 2009
Messages
5,519
This season I had been having trouble with my 2004 25hp merc. It's been a low speed problem; I found water in my tank/filter. Cleaned out, ran better but still had an idle problem. Once it got going, it was fine. Yesterday I decided to adjust the idle and low speed mixture. I set the mixture screw on land initially before starting. Started the boat, still had the idle problem, but now I couldn't get on plane. If I hit the primer, I initially get a bump in engine speed, but then nothing. The motor runs smooth with no misfiring at full speed. I removed the carb, did a quick disassembly/clean and didn't notice anything. Everything looked clean. Tried to compression test, but my tester needle was bouncing and returning to zero (I assume it's a problem with the gauge; I had compression on both cylinders when I put my finger on the plug hole). Unrelated (?) problem, the starter wasn't cranking. I had been having an issue with a sticky neutral kill switch. I checked the fuse, and it fell apart when I opened the holder, so I don't know if that was a problem before I messed with it. No spare available. I tried to check the timing, but my light wasn't working--wasn't sure if this was because the fuse was blown. Anyway, after reinstalling the carb and adjusting idle, it now starts and idles great. Still no dice at throttle though. So what are my next steps? Going to replace the plugs, gas is fresh and no water now. Still seems like a fuel issue, but the main jet was definitely clear. Didn't see a way to adjust the float, but I'd think that there would be more sputtering if it was starving because of delivery to the fuel bowl. Planning on getting a rebuild kit and doing it proper anyway. Any other suggestions, anything I'm missing? I'd also think that if this was ignition, I'd be able to hear a miss in a 2 cylinder engine. Haven't worked much on outboards or 2 strokes bigger than a lawnmower, so any help appreciated...
 

flyingscott

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Joined
Apr 8, 2014
Messages
8,171
You need to check for spark and need to do proper compression test. You could be running on one cylinder.
 

nola mike

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Joined
Apr 22, 2009
Messages
5,519
Yeah, I at least have spark on cranking, don't know what's going on at high speed. Just remembered that I swapped out the fitting on my compression tester (that I think has the check valve in it). Would it not sound obvious if I was only running on 1 cylinder, esp at idle?
 

nola mike

Vice Admiral
Joined
Apr 22, 2009
Messages
5,519
Well, I swapped out the plugs and it's running great now, both at low and high speeds. Looks like I had 2 issues going on, and it looks like I probably was running on 1 cylinder. Strange that it felt so smooth once I got going. The girl at the FLAPS told me that the plugs I wanted (NGK BP8H) didn't exist and sold me BPR7HS instead. I can't find that as the proper plug, so I guess I'll get a new set for next week. The fuse is 20A btw.
 
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