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outboard motor vibration question?

post #1 of 23
Thread Starter 
I hope someone can give me some guidance here. I have a low hr. 2001 mercury 30 hp. 4 stroke on my 16.5ft tin boat. I went fishing last night and after drifting around for a while and using the electric trolling motor to move from spot to spot I decided to fire up the O.B. and head in. It started fine but when I went to slowly pull away I felt alot of vibration, the more gas I gave the less I felt the vibration. Tried it in my driveway tonight and same thing, with boat in gear low rpm alot of vibration, high rpm almost none. Now I have ocassionally run the prop into a barely submerged sandbar and even hit a log the trip before, but these were all relativly soft and resulted in little more then some paint loss and some very shallow gouges in the edges. My question is, are these symptoms of a damaged out of balanced prop or have I damaged my lower unit. Any help/opinions would really be appreciated.
Thanks,
Dave
post #2 of 23
I would have to say its your prop. Props are designed to the tightest of clearences. You mite only think its paint loss but it may be bent a few mm's, and thats all it takes for a vibration. Vibrations are not good by any means and will cause premature wear on other parts/seals/bearings. Im not 100% sure but I would have it checked out before heading out again. Good luck
post #3 of 23
I would say its the prop too. Might have hit something and it could be bent out of shape a hair and it will give vibrations. Is the prop stainless or aluminum?
post #4 of 23
most likely bent prop but you might have bent the prop shaft a bit??? pull the prop off and check the shaft.
post #5 of 23
Thread Starter 
Thanks for the replies, it is an aluminum prop and I was hoping it the problem as opposed to a lower unit issue. I guess I will go out and get a new one today and give it a try.

Thanks again for yuor help,
Dave
post #6 of 23
Pull the prop, run the engine and see if it vibrates. If it does, it's not the prop. If it's the shaft, you probably won't feel the vibration with the prop removed. You would need a dial gauge to check it if it is slightly bent.
post #7 of 23
Quote:
Originally Posted by Dave T. View Post
Thanks for the replies, it is an aluminum prop and I was hoping it the problem as opposed to a lower unit issue. I guess I will go out and get a new one today and give it a try.

Thanks again for yuor help,
Dave


Hey Dave,

I am fairly certain that this is a problem with the prop just like the others have said. Having a second prop is a good idea but do not chuck your old one if it's bent. A lot of gys don't seem to know that you can have them repaired for very little money. We have precision propeller dowb here. I am sure you have someone up there where you are. Souldn't cost more than $50 to straighten that prop back to new.

Tight lines and let us know what you figure out. The posters advise to run the engine without prop is exactly what I would do!
post #8 of 23
These guys are probably right.......because they usually are

Just check to see if anything is wrapped around the shaft too.
post #9 of 23
Thread Starter 
Well ran short on time today but I did purchase a new prop just didnt get a chance to put it on and give it a whirl. I did however run the motor without the prop and didnt notice any vibrations, so i am hopeful the new prop will fix the problem. If it does I do plan on getting the old one refurbished to keep as a spare, so far everything I have hit has been relativly soft, but I do like rockpiles so it is probably just a matter of time.. I will put the new one on tomorrow and let everyone know how I make out. Thanks again for all the excellent advise.
Dave
post #10 of 23
The way I read the original post would indicate it is not the prop. I thought Dave went out, and all was well. Decided to come in when the shakes hit. Unless he trashed the prop between leaving and returning, I would suspect something went sour while he was out there - like a coil breaking down.

Or did I read it wrong. Or did he even leave the doc using the trolling motor.
post #11 of 23
I bet wqhile he was using the trolling motor he was in shallow water. The boat dropped off a wave and the prop hit unknown to anyone. When the outboard was started and used the vibration showed up.
post #12 of 23
Thread Starter 
Billy A,
That is exactly the scenario that occured, upon leaving the dock, I didnt notice any vibrations when I started fishing I used my trolling motor to reposition the boat when needed. It was not until going in that I noticed a pronounced vibration at low RPM's. I tested it out of the water and same vibration is present, unfortunatly even with the new prop . I also am very confident that I didnt hit anything on this last trip. The water was flat calm and I was in an area I know pretty well. So if it isnt the prop what am I looking at? Pardon my ignorance but what is a coil, and how would it break down? Again I would appreciate any advise.
Thanks,
Dave
post #13 of 23
Well, if you eliminated the bent prop scenario, a coil is an ignition part. It provides high voltage to the spark plug. If a coil quits, you will have a misfire and it will cause a bad vibration as the engine has to overcome the compression drag of a bad cylinder. If you are not mechanically inclined, its time to head to the mechanic. You need some test equipment to diagnose the problem. Coils are not cheap.
post #14 of 23
I would assume you could get a coil for $50 - $75. Maybe as high as $100.

Check your plugs first for fouling (not so likely with a 4 stroke.)
post #15 of 23
Thread Starter 
A friend also suggested a possible fouled plug, my question with that is why dont I feel the vibration at idle. The vibration only occurs when transmission is engaged at low speeds. I am really hoping for a simple solution, and will check the plugs tommorrow, if not fouled off to the shop it goes.
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