25hp merc 4 stroke problem

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gahunter706

Petty Officer 3rd Class
Joined
Jun 3, 2013
Messages
82
I recently bought a 1648 aluminum boat with a 2008 Mercury 25hp 4 stroke efi, and when I took it to the lake it doesn't seem to have any power when in gear. I didn't test drive it before buying (probably should have), but started it up on muffs and it seems to run great in neutral. When I had it at the lake it still runs great in neutral, and will hit the rev limiter like it should in neutral, but when in gear it doesn't hit the rpms that it should. I dont have a tach on it, but I dont think it is getting anymore than 2500 rpms. With one 135lb guy driving it would get to about 14-15mph, and with 3 guys in it we might hit 10mph. I have seen many similar boats and feel like this boat should easily hit 25mph with one guy in it. Any ideas on what could be causing this? I am going to put new fuel lines and a fuel filter on it today, and take it back to the lake and mess with the tilt positions some to see if that has anything to do with it.
 
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gahunter706

Petty Officer 3rd Class
Joined
Jun 3, 2013
Messages
82
Put a new fuel line, fuel filter, spark plugs, and a tank that I know is good on the motor and took it back to the lake and not much difference in the way it runs. Brought it back home and put some seafoam in the tank, thinking the injectors might be clogged up, and ran it for about an hour on muffs. Took it back to the lake and ran some more gas with seafoam through it and it is starting to run better, but still not very steady as far as rpms and speed at full throttle. If I stop and let it sit for a few minutes, then restart it and go, it will get up to about 24mph and then start slowing down and hold steady at about 18-20mph. Is there any way to take off the injectors and manually run some cleaner through them? Or does that even sound like it could be my problem? Thanks for any help.
 
G

gahunter706

Petty Officer 3rd Class
Joined
Jun 3, 2013
Messages
82
I went back to the lake and ran some more seafoam through it, so far I have ran 3 gal of gas with 3/4 can of seafoam in it through the motor, and it is still not holding steady rpms at full throttle. While I was on the water, I pulled the plug wires one at a time and ran the motor, and with the top one pulled it will still get on plane and run about 18mph, but with the bottom two pulled it hardly runs at all. This makes me think that there is a problem with the top cylinder. I swapped the top and middle coil, and ran it again with pulling the plug wires one at a time, and got the same result. Any thoughts as to what could cause this?
 
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