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Haven't been the owner of a boat for very long, and its my first as well..

Havin some trouble with the engine. I thought it may have been running the fuel too rich and fouled the plugs, but I'm starting to second guess that. I've changed the spark plugs 3 times and ran seafoam through the motor twice. My Problem: Motor does not want to perform under load. I'm limited to about 5 miles per hour. My motor is a 1976 johnson seahorse 40. It may be a problem with my carburetor, I really have no idea as I'm not extensively knowledgeable about boat motors. It gets to a certain speed, and any more throttle past a certain point, and the motor shuts off. Maybe a water pump? I really have no clue. I'd like to fix the problem myself, but with no real knowledge of the motor, I may have to take it in. Any good shops around here that won't cost me an arm and a leg?

Any ideas?

Could be a reed valve.

Could be plugged carb jets. This usually sounds like it runs good at idle and bogs down when trying to speed up.

Harold Dyke does good work & is very reasonable. He is on Lincoln Rd.

awesome. When I have a free moment, I'll take it down and have him look at it. Do you by chance know the name of his shop?

Harolds Marine I believe

I would try cleaning the carb real good and see if that works before you bring it somewhere.

Hi. Sorry you're having troubles. I'm NO pro marine tech, but I've been around 2 strokes all my life. ( family used to own a Yamaha bike dealership) and I've rebuilt more than a couple of OMC engines.
So take my advice for what it's worth... :)

Here is how I'd approach your problem, based on what you posted:

1. I'd do a compression check on it. Go to autozone and buy a cheap gauge. You want both cylinders to be nearly equal...and over 90 lbs...as I recall. But dont quote me on that number. If one is more than a few lbs ( 5-8) lower than the other, there is one of your problems.
2. I'd check the fuel line and all connections carefully....especially if you've been running ethanol laced fuel. It eats away at the older lines from the inside and generally mucks everything up. Nevertheless, the fuel line and bulb will need to eventually be replaced with the "ethanol rated" type.
3. Check the fuel filter. My guess is that you'll find some gunk...which tells you the fuel line is toast and that your carb needs a rebuild. ( probably needs that anyway)
4. Make sure both plugs are firing by starting at home with the ear muffs hooked to your waterhose and use an inductive timing light on each plug wire. You could have a coil breaking down.
5. Not related to your current issue....but change the waterpump after you get her running right again. Waterpumps are cheap. Powerhead rebuilds are not. I swap mine every year.

Like I said earlier... I'm no pro by any means. But without hearing it run, inspecting the plugs myself, etc....these items are what came to me first.
Hope it helps !

P.S. I try to always run a little Seafoam AND Marine Stabil ( green/blue type) in my tank at all times. I havent had to rebuild my carbs in 10 years...and no...I dont run the gas out of them at the ramp. That only causes a lean condition which can fry rings. ( not the kind you eat with ketchup, either !)