I have a 1990 260 overnighter with a 454 merc. I'm trying to diagnose my engine faltering at full throttle. If it doesn't fall on its face and sputter, it will rev up to 5000-5500 rpm and then rythmically drop 800-1000rpm. Up and down. The problem started in July and I've been trying things ever since with no luck. New items are: cap, rotor, plugs, wires, electronic pick up, coil, fuel pump, fuel filter, carb rebuilt. Engine seems to run fine everywhere except full throttle--good up to 4500rpm but the boat itself doesn't seem to move. The boat was a 40 mph boat, but now I can't get 25mph out of it. Please help. I don't have much hair left to pull out.
power loss
- Thread starter marty
- Start date
Alex D
Active Member
I would check for any other restrictions in the fuel line. Replace the fuel hoses too, they sometimes can grow shut or collapse internally without a hint on the outside. Could there be any exhaust restrictions??
JeffM
Active Member
If the engine revs but the boat doesn't move or will not go very fast, you could have a spun prop hub. If the boat is kept in the water, there may also be growth on the bottom which can reduce your speed.
Jeff
Jeff
Marty,
Welcome aboard! I think you will get some good help on this board, really you already have some. :thumbsup:
I think 5500 RPM is too fast for your engine. I don't know the specs on yours, but my 1996 7.4 liter, carb, is rated for a max RPM of 4200 to 4600. The higher horsepower rated engines of the same size have a max high end of 5000. Unless your prop is really wrong, I would not think you should be able to do more than 5,000 RPM. IF you really can, then it makes an even stronger case for a spun prop hub.
Be aware that a tach that gives a falsely high RPM reading is also not unusual, and the reading may vary between a correct reading and one that is too high. I just made a post about mine being wrong a few days ago under Engines/Drives/etc... So I would ask whether you can also hear the engine change speed when the tach says it is doing so.
Then, does the engine rev to a much higher RPM now than it did back when it was working properly? If so, then sounds like a spun hub. If not, then sounds like growth on the bottom or some such thing.
I think Alex and Jeff have steered you in the right direction.
Take care,
Dave
Welcome aboard! I think you will get some good help on this board, really you already have some. :thumbsup:
I think 5500 RPM is too fast for your engine. I don't know the specs on yours, but my 1996 7.4 liter, carb, is rated for a max RPM of 4200 to 4600. The higher horsepower rated engines of the same size have a max high end of 5000. Unless your prop is really wrong, I would not think you should be able to do more than 5,000 RPM. IF you really can, then it makes an even stronger case for a spun prop hub.
Be aware that a tach that gives a falsely high RPM reading is also not unusual, and the reading may vary between a correct reading and one that is too high. I just made a post about mine being wrong a few days ago under Engines/Drives/etc... So I would ask whether you can also hear the engine change speed when the tach says it is doing so.
Then, does the engine rev to a much higher RPM now than it did back when it was working properly? If so, then sounds like a spun hub. If not, then sounds like growth on the bottom or some such thing.
I think Alex and Jeff have steered you in the right direction.
Take care,
Dave
