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Troublehook

Another o/b motor question

17 posts in this topic

Well I am having a little problem with my 4 horse mercury o/b. When I am driving it at or near its highest speed, it will sometimes make a louder than usual noise that sounds like the rmps are going way faster. The boat doesnt though, so I know its not just accelerating or anything. It kinda sounds like it is going into nuetral. All I know is that its not a good/normal noise, and it has prevented me from going full speed because I dont want to break it. Any thoughts on what the problem might be? I am thinking about having it tuned up if it is not too expensive. Do you know how much a tune up might cost? Any help is appreciated cool.gif, and I am sorry if I am not providing enough information. I will try to answer any questions that would help troubleshoot for answers on the problem.

Thank you. TH cool.gif

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It sounds like it might not be trimmed to where it needs to be. I don't know the specific term, but it gets into an "air pocket" and basically free spins the prop. Another thing might be weeds. If you get a heavy or even a little amount of weeds on the motor it can create this same affect. I would first try and trim the motor down as far as it will let you go (if it has a pin to trim it) and try that out first. I had similar issues with my motor when i first got it.

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Fishin, you meant "cavitation". Doubt it on a 4 horse.

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Yup thats what I meant. Yeah probably not, but its worth a shot before he takes it in.

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The prop could be spinning on the rubber mount.

To check this, first pull the spark plug wires so it doesn't start. Then put it in gear and use a rag as a cushion and try and turn the prop with your hands. If it moves that's the problem and the rubber mount is bad.

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Well Im going to take it out very soon tonight so I wond be able to check the rubber mount yet, but I will lower the trim because that does seem possible, and also I will check for weeds when it happens. thanks for those tips, and I will go out and see what happens tonight. smirk.gif

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I have a feeling you have a short shaft motor on a 20" transom. You are probably getting up to X speed and your prop is starting to come out of the water.

If it is a newer motor and it could be a spun hub(prop), or a slight chance its a shear pin if its an older motor. Sometime the broken pin can bind and hold the prop until there is too much torque and it will slip. I dont think either of these are the case because your boat isnt losing speed(prop is still spinning).

I think I was right in my 1st comment.. your lower unit is too short.

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To test for a spun prop use a magic marker and draw a line on the prop to the prop nut. When the motor surges again look at the line you drew on the prop. If it is not straight the hub is spun and you will need a new prop or have to get yours fixed.

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heres a pic of his boat from a previous post. I think the motor is long enough for the boat.

IMG_0294.jpg

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I agree with Fishin4Life, drop your trim.... go all the way down with it, then gradually experiment with higher and higher.

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Well I dropped the trim down, but only by one setting. Had I gone any further the prop would be at a very odd and badd angle. I want it to be able to plane out so I cant have the angle of the prop be where it would completly keep the bow down. maybe this drawing will help explain what I mean.

prop.jpg

Even at the angle I set it at tonight it wouldnt plane. But the good news is that it didnt make that noise tonight. Hopefully that was the problem, but I will have to take it out another time to really be sure.

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I seriously doubt you will get your boat on a plane with a 4hp.... grin.gif

Keep your motor down will help a lot, as I said you can study the right angle with time.

As other mentioned watch for weeds, if they get tangled in motor when you are going forward they will slow down your propulsion, put motor in reverse for a few seconds then forward again, it will clean it up.

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That boat is not going to plane with a 4 hp with a person in it. I have a raft here(zodiac) that will plane with a 5 hp, but I have to hit full throttle on the motor, then shift all my body weight forward and it will plane out on calm water.. the raft is only 9 1/2' long and floats high.

The lower unit looks to be the proper length for your boat. If you get 5 or 6 mph from a 4 hp motor, you are doing good.

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I go way faster than that on it, and I think it does plane, at least the bow gets out of the water and thats planing isnt it? It goes at least 10mph but I am pretty sure it actually goes fifteen. well it made that noise again tonight when my mom was in it as well. I think I will try looking at those other tips that were mentioned before.

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Its not a bad rubber mount because I put it into gear and it spun just a tiny bit and then it did stop. I may be going boating again today so I will check the spun prop idea. The nut on it is one that I havnt seen. Its a lot different than the one on my I/O(stern drive) boat's prop.

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Quote:

Its not a bad rubber mount because I put it into gear and it spun just a tiny bit and then it did stop. I may be going boating again today so I will check the spun prop idea. The nut on it is one that I havnt seen. Its a lot different than the one on my I/O(stern drive) boat's prop.


The nuts will be much different you I/O is a much bigger motor so it will have a bigger prop than you 4 horse. On a small boat and motor I would put it on the bottom or second to bottom hole. Start there. I have a 9.5 that I have on the bottom hole all the time.

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Quote:

To test for a spun prop use a magic marker and draw a line on the prop to the prop nut. When the motor surges again look at the line you drew on the prop. If it is not straight the hub is spun and you will need a new prop or have to get yours fixed.


I think that is the problem. I tested it yesterday and they were not allingened after it happened. I will take the motor to a marine mechanic and tell them the deal and see what they can do for me. thanks eurolarva, and everyone else for the information.

TH cool.gif

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