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1997 mariner 40 magnum powering up

B

biglakeboaterguy

New member
I was running boat 3/4 throttle on plane for approx 4 miles with not a problem and suddenly it slowed down and will not rev up or run right past half throttle. It sounds like a rev limiter kicks out 1 or 2 cylinders at that point. No overheating problem, (new impeller, lots of flow) new plugs and it starts and idles all day and runs good to half throttle. The last owner ran it for 2 tanks of gas with no issues whatsoever across a big lake with GPS at 42mph. (he is a friend of mine so I know he is not B.SSS. ing me) I have checked all linkages, throttle butterflies are opening (together) and no obvious wiring issues. The oil injection was disconnected and it is running on mix gas. Quicksilver synthetic 50-1 mix. This was a sudden unexpected event and I put new plugs in to try again with same results. Anyone have some help out there? Thank you.
 
Portable gas tank or built in?

In either case make sure the vent isn't blocked (cooties, bugs etc).

Check the fuel line pick-up in the tank - may have sucked crud and got blocked up. Also, the tanks anti-siphon valve may have gone flooey.

Ditto any fuel filter(s) - OEM or add-on's.

Did you try pumping the bulb on the fuel line - it should flatten out a little while running, but a single pump should stiffen it back up. If not could have a leak somewhere, or a toasted bulb. If a pump of the bulb causes the motor to pick back up, you probably have a fuel pump starting to die (tear in the diaphragm etc).

That's the easy stuff to check. If none of that makes any difference, then it's on to more serious trouble shooting.

As an aside, there is a chance that you have just developed a clog somewhere in the fuel system. While there is no "magic in a can" you might add some Seafoam to the tank (according to the directions on the bottle) and take it for a run. If that solves or partially solves your issue then you may have/had carb cooties. A slight improvement after running Seafoam is usually an indicator that the carb(s) need to be pulled, cleaned and possibly rebuilt/partially rebuilt.
 
Thank you- you hit the nail on the head- crud in the fuel pump and carbs. The previous owner put fuel in with 2 older part full jerry cans and thinks they may have had some crud in the bottom :( I took the carbs and pump apart and found bits of rust/dirt in everything but no varnish in the carbs. I'm surprised the filter did not stop the carb contamination but it is after the fuel pump-odd. Anyway, thank you for the info and will try out the motor tomorrow. Cheers
 
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