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2006 Mercury 9.9 Bigfoot with 4 blade prop on 24 ft sailboat. Low hours on motor, prop perfect never hit. Engine revs and boat slows down if more than half throttle used. Sounds like prop is slipping on shaft but nut tight and cotter pin in place. Can it be a problem with the cushion hub?
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Seems like your are describing a outboard motor. If so, the Boat section of Fixya may have more experts that you need. You would want to avoid turning any screws on the carburetor body. The externals on the linkage have some adjustments like throttle stops.
The linkage on boat shift mechanisms are tempermental. They usually combine 2 functions on 1 control lever. They are synchronized that when the transmission shifts from neutral to forward and neutral to reverse the idle speed is low. This keeps the gears from grinding and takes the shock out of the shifting process.
Another thing different than on a car engine is the effect of the propeller on engine rpm. You can change the "in gear"
rpm for both directions by changing prop size and/or pitch.
This also means you can over-rev the engine by having the wrong prop on the boat. There are engine rpm "stops" for the high end too, but some just rely on using the correct prop. As what you may be experiencing with a "stall" can be brought on by
A word of caution. The inboard/outboard engines can have the same problem as outboard engines. The shift levers are synchronized with a pivot system and adjusting the throttle can interfere with the shifting.
You may be able to view youtube videos by searching for these topics.
The amount of RPM's on a boat motor depend on a number of things...
1. Height if the motor in relation to the bottom of the boat. - too high and it over rev's.. Too low and it won't rev..
2. The correct size (O.D.) of the prop. Too large and you have mechanical interference or too large for the motor to spin...- ..Too small and the motor will over rev!!
3. The correct pitch of the prop. - Too large/big of pitch and the motor can't spin it..-.. too small and it will over rev the motor.
..... these are the big three...
Now, Make sure the carburetor butterfly(s) are opening exactly flat or vertical ...and not under/over opening..if this is a fuel injected motor...have the dealer check it out..
Maybe the transmission went out. Take the bottom plug from the lower unit and see if it is full of water or lots of metal in the oil. THEN take a magic marker or a punch and hammer and mark the hub of the prop opposite the blades of the prop and go run it again. If the marks move from one another, you need to have the prop repaired.
you could try a lower pitched prop.2" would up the revs by 400 revs.,which would up the revs ,but give you more low down thrust,and/or tucking the motor under more[if you have electric trim/tilt].thats a big boat and fair bit of weight for a 40hp..my 40hp merc is on a 13ft saracen,and with two of us it takes a while to get gooing.so i bought a ProPulse prop,with adjustable pitch.been god to try different pitches
I have 1969 5hp Johnson. Starts and runs good, but will only push a 14ft row boat slow at low to medium rpm. When at higher rpm the the motor just revs up and the boat slows down to almost nothing just like it has seaweed around prop.Any help would be great.
the inside of the prop has a rubber between the prop itself and the inside splines to act as a shock absorber,say if you were to hit a rock. that rubber had spun out and the splines are turning inside the prop. the prop is screwed. seen it 100 times before.
hi, thats a little motor for a big boat. how big is the boat? you can try changing the timing by loosening the distributor cap and turning it slightly to see if that helps or you can get a different prop that has less pitch on the blades which will spin the prop faster in the water and keep you up in powerband better. maybe call a marine boat shop and tell them your problem, maybe they have a trick or two. happy boating J
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