mercury problem

[langaz]

hey guys,
went out the harbour today, and had a great day. New pb flatty!
but on the way back to tomago ramp i had a bit of a problem. Was flying along at full throttle when the motor gave a bit of a jump and reved up. I thought id hit something! couldnt see anything wrong so off i went again. about 100m on it did it again. Stopped for a bit and checked, nothing seemed amiss.
after a bit of fishing under stockton bridge, off i went, but when leaving, as the boat began to plane, the motor would rev, but the boat would slow down.
as i backed off the throttle it seemed to regain drive again. It got progressively worse, until the boat could do little more than a walking pace before the motor would over rev, and lose drive. It seemed similar to a car with a buggered clutch, would rev while the clutch slipped, but as the revs dropped it would regain drive.
After an hour of slow putting along i got back to tomago ramp (thank god), but have no idea what is wrong.
I thought either spun prop, or rooted gearbox.... but im assuming the later as im guessing a spun prop would result in absolutely no drive at all???
the motor is a mercury 20hp, 1996 model. only had gearbox serviced about 6months back, new waterpump impeller, new plugs etc... there was no evidence of an issue that i was told about.

any suggestions on the issue would be appreciated.

[jameshallam]

check ur spark plugs i have a 40 the the plugs were really oiled up

[danh124]

check ur spark plugs i have a 40 the the plugs were really oiled up
I am prtetty sure this is not the cause of this particular problem, i would say that the spun prop or gearbox theory is most likley.
Just my opinion not by any means an expert.
Good luck with it mate.

[langaz]

its def not an engine problem. Engine runs, revs well and smooth. Just lacks drive at higher revs.

[forsterfisho]

Is it revving normally or really over revving, ie cavitating? I'm not mechanic, not by a long shot but I had my 25 start to cavitate a while back and it was something to do with a stuffed water pump (for some reason?) which was causing the motor to over rev and therefore over heat - I could limp back to the ramp but that was it.

Is your tell tale ok and the motor not getting crazy hot?

[langaz]

tell tale flows as normal, and is not any hotter than normal.
All indications are spun prop. cheers for the feedback guys.

[evilnev]

g`day mate, check your plug leads are on properly, had the same thing happen to me on my old motor, driving along & felt like i had hit something, then it was fine,happened next time out, got home looked over the motor & 1 of the leads was slightly loose, & that fixed it, then some grub flogged it.

[blueh2o]

Im not to buffed up on engines but when I did an Injector on my Etec it did the same thing, you could give it full throttle but all you got was higher revs and walking pace speed , they call it Limp home mode and it prevents any further damage to the engine, a pain but atleast i didnt need a tow in:o sounded pretty cool though coming in on 2 cylinders instead of 3

[bender]

Sounds like what happened to me on my 1996 25hp merc...i hit a log and its damaged the rubber insert in the prop then as i made my way back to the ramp it started slipping so bad that i ended up on idle for the last 2 km..

It wouldnt matter what motor you had but if like me you hit something to break the rubber insert then thats why the revs went right up while your speed dropped right off

[Hayden]

Sounds like what happen to me mate macca85 on here pm him mate he might be able to help you

[Mandurah Marine]

Take your prop off and visually inspect it where the rubber should be. If you cannot see evidence of a spun prop, mark the bronze shaft tube directly adjacent to a mark you put on the ally hub of the prop. Rev the motor in gear until you make it slip like before. Remove the prop and check to see if the marks are no longer directly adjacent. If moved, you can get a prop re-bushed, but depending on the condition of the prop you may be better off replacing the prop altogether.

And always carry a spare prop! We all carry a spare tyre in our cars. . . . .