Outboard won't exceed 2000 RPM under load

Question

I have an older Mariner 115 outboard that's got relatively low hours, it's the 6 cylinder so I suspect it's a 1986 or 87. This could be somewhat long so please bear with me. Basically I purchased the boat three months ago, Changed the fuel filter and drained the fuel tanks (suspected stale fuel) in this time I was running on a 6 gallon external tank, all was well.

Decided to go and fill the internal gas tank with 30 gallons of fresh fuel (great, because it's a 30 gallon tank), so I suspect I was able to get everything out of the tank when I drained it. Took the boat out while still using the external tank, we were moving along great. Decided to switch to the internal tank and within 30 seconds the engine wouldn't go over 2000 RPM which is roughly 8 miles an hour. Switched back to the external tank with no improvement in performance.

Took the boat out of the water, put carb cleaner thru the engine, changed the water seperator, changed the fuel lines (kind of suspect this was the first time the boat has been used with the main tank since ethanol was introduced) ran it out of the water and had no problem exceeding 3000 RPM's in neutral(please note I did this in a tank). Launched the boat, once I put it under power I still couldn't get it over 2000 rpms, but in neutral it will exceed 3000 rpms.

Does anyone have any experience with this? As a precaution I'm going to pick up new spark plugs and order a fuel pump.

Answer 1

Likely your main tank put c**p into one of your carbs. To see-take the front cover off and run the motor up. Then use your hand -cover each carb inlet one at a time and see if one makes no difference.  This test will sometimes solve the problem. Keep in mind the secret mind of kiekoffer's ghost loves to make stuff happen all at once, so you might just put a timing light on all the plug wires and see if you've been fooled.  

Answer 2

I agree with William M, It's ether water or trash sucked into the carbs.  Go back to the 6 gal tank, and go through the carbs, looking for blockages.  Just clean the plugs, I assume you have primer ball, I'd guess it is colapsing, I'd bet the fuel pick-up in the main tank is plugged with c**p.  Good luck!