60 hp Bigfoot problems

[Pbtman]

I recently bought my 1st boat. (I’m 67). A 2004 Sun Tracker 21’ Party Barge. It has a 2004 Mercury 60hp Bigfoot 2-stroke. It’s had the oil injection system removed so I mix the gas/oil 50:1.
I took the boat to the local marine shop and had spark plugs, lower end oil and the water pump & seal replaced. Also had the compression tested. They said that the motor was in excellent shape. I took it out for the 1st time today 03/26/2019. It started and idled great. When I try to throttle it up, it tries to stumble and die. If I get the boat moving and finally get it to run at about 1/3 throttle, it jumps from around 2800 to 5300 rpm.
The only thing I was unsure of was the fuel in the tank. (1/2 tank when I bought it. We filled the rest of the way up on the way to the lake) My guess would be bad gas or fuel filter. The primer bulb stayed pretty firm to squeeze.
Any help or advise would be so very appreciated.

 

[QBhoy]

Hi. Just thinking in simple terms and easy mistakes to make (we’ve all done them)...was the vent on top of the fuel tank open ?
Failing that, did it feel as if the engine was still driving the propeller when it jumped in rpm ? Very unusual or unlikely for it to suddenly and quickly jump that much rpm quickly. If it was driving the prop and on load, it would be a gradual thing.
Check the prop and it’s rubber hub. Make sure it’s in tact.
All the best

 

[Pbtman]

Yes, Sir. It was driving the prop for sure and the tank cap vent was open. We even took the cap off to make sure I could blow through it. If this were on an old carbureted v-8 automobile or truck, I would swear that the choke buttery fly was stuck almost closed.
Now, one time when it actually did pick up and ran great, we (my son and I) got her up to 16.5 mph (using an app on my cell phone). I trimmed the motor and it got up to 19.2mph. Now that isn’t much but I was pleased for a 21’ pontoon with just a 60 on it.
It didn’t last long. I starved out, lost rpm’s and died. It started right back up but would get too far off of idle for quite a spell.

 

[Faztbullet]

Sounds like carbs need attention...

 

[QBhoy]

Yeah. Thinking carbs now too. Lazy way would be to mix a whole bottle of seafoam with only a couple of litres in the fuel tank and run it through as neat as you can. Should see a load of smoke and crap come out the exhaust. Amazing stuff and totally sold on it after being so sceptical at first. Known it to work wonders on a dozen engines or more now.

 

[Pbtman]

I saw some article on the new Seafoam Marine. It’s supposed to keep the fuel system clean, plus, it conditions the fuel for longer storage. Sort of like Stabil or products like that.
I’m in favor of draining all of the fuel, put in fresh gas/oil, changing the fuel filter and running some Seafoam. The marine shop is gonna put it on the dyno. Maybe they’ll come up with the same conclusion.should know by this coming Tuesday.
I sure wish to thank each one that responded. It was appreciated. I love these forums. There’s such a wealth of information.

 

[racerone]

If carburetors have low speed mixture needles, open them 1/8 turn at a time and test run.