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Charles
Charles, Marine Mechanic
Category: Boat
Satisfied Customers: 3342
Experience:  US Navy (Ret) Certified Mercury Marine Tech with over a decade of repair experience.
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I have a 2005 mercury 15 HP short shaft salt water engine.

Customer Question

I have a 2005 mercury 15 HP short shaft salt water engine. It starting having a bad vibration at low RPM and gets worse at higher RPM. I put a new prop on but the vibration is still in the motor. What else can cause the motor to vibrate>
Submitted: 10 months ago.
Category: Boat
Expert:  Charles replied 10 months ago.

Hello, my name is***** will assist you today.

Is the motor out of the water currently?

Customer: replied 10 months ago.
out of the water. It is on a 14 foot Zodiac on a trailer.
Expert:  Charles replied 10 months ago.

OK, With the motor OFF, spin the prop while you watch the center of the prop shaft, it is bent?

Customer: replied 10 months ago.
I put an indicator on the prop shaft, it is running within .002 true. The prop does not seam to spin as freely as i think it should.
Expert:  Charles replied 10 months ago.

When was the last time the gear oil was changed?

Customer: replied 10 months ago.
The gear oil was changer about 5 hours of use ago.
Expert:  Charles replied 10 months ago.

So it was good, no water?

Expert:  Charles replied 10 months ago.

Ia this a two stroke?

Customer: replied 10 months ago.
I was free of water however, the oil very dirty with with about 25 hours of use.
Customer: replied 10 months ago.
No it is a 4 stroke.
Expert:  Charles replied 10 months ago.

Alright, do you have the tools to do compression and spark on it?

Customer: replied 10 months ago.
No. The motor starts and idles great. The spark plugs were changed 75 hours ago used NGK.
Expert:  Charles replied 10 months ago.

This engine will start and run on 1 cyl. Hard to tell that it is not. But it will be lacking power on top end.

Customer: replied 10 months ago.
OK. I will locate a compression tester and get back with you.
Expert:  Charles replied 10 months ago.

Sounds good. You can normally rent one from an auto pats store. A spark checker as well.

Make sure that you have the throttle opened as far is it will go with the engine in neutral before starting the test.

Customer: replied 10 months ago.
Chris
The Spark test was good in both cylinders, The compression on the top cylinder was 38 pounds and the lower cylinder was 35 pounds.
Expert:  Charles replied 10 months ago.

Ok, How does the engine oil look?

Does it vibrate even in Neutral or just in gear?

Will it also vibrate if you run it on the hose?

Customer: replied 10 months ago.
The engine oil was very dirty after 25 hours. No vibration in neutral, just in gear under load. Can't hook up a hose the model as the water intake in under the cavitation plate.
Expert:  Charles replied 10 months ago.

Well you could put it on a flusher, you just need a special one and remove the prop.

Did you check that gear oil recently? Like in the last few days or just the last time you changed it?

Customer: replied 10 months ago.
After five hours of use the oil is a little dirty. I can remove the prop and put it in the lake on the trailer and run the motor. what are we trying to do at this point? what was the compreshion when new?
Expert:  Charles replied 10 months ago.

The compression spec is no more than 55 LBS at cranking due to the compression release.

We are trying to see if the vibration is just the prop or something else.

While you have it in the water, use a good insulated pliers and remove one plug wire at a time to see if both cylinders are firing the same.

Customer: replied 10 months ago.
Hi Charles
Put the motor in the water, ran it at about 3/4 throttle pulled top plug wire and the motor started running bad replaced wire and motor returned to running ok. Same thing happened with the lower plug wire. Removed the prop and ran the motor it, vibrated in neutral as well as full speed. It did appear to be vibrating more from the motor area than from the lower unit.
Expert:  Charles replied 10 months ago.

Have you recently hit bottom of had the engine stop very suddenly?

Customer: replied 10 months ago.
Nope
Expert:  Charles replied 10 months ago.

Replace a timing belt?

Customer: replied 10 months ago.
How would the timing belt cause the vibration? Could it be a flywheel out of balance?
Expert:  Charles replied 10 months ago.

If the motor was not out of time, that would do it. Was the belt changed, or has it been off?

Customer: replied 10 months ago.
I bought the motor new. Have never had any problems with it with exception of the pull start assembly being replaced.. The belt is as it was from the factory.
Expert:  Charles replied 10 months ago.

Has it ever run hot or run with out oil?

Customer: replied 10 months ago.
No. I am a little anal about checking oil level every time I used the motor. I also check the water stream at least one time while on the water. The water has always been cool.
Expert:  Charles replied 10 months ago.

Do you idle the engine allot?

Are the spark plug tips really black?

Customer: replied 10 months ago.
I do do a lot of trolling but when I have changed spark plugs they have always been brown/bronze never black or any sign of running rich.
Expert:  Charles replied 10 months ago.

Ok, did you buy or rent that compression gauge. I misquoted the spec.

Customer: replied 10 months ago.
I bought a new one.
Expert:  Charles replied 10 months ago.

Ok,I need for you to do it again. Make sure that the throttle is as open as you can get it in Neutral. Crank the engine over until the gauge maxes out.

Customer: replied 10 months ago.
OK Will do this tomorrow/
Expert:  Charles replied 10 months ago.

Sounds good. It should be around over 150.

Customer: replied 10 months ago.
Hi Charles
I tried to find out what the compression on the net. I finally called mercury direct. They told me that engine does not have a specific compression ratine. They recommend preforming a "Leak down" test that falls no less than 10% over 10 min. I preformed the leak down test. Both cylinders held at 60LB for 10min.
Expert:  Charles replied 10 months ago.

Would you please poist the serial number on the engine?

Customer: replied 10 months ago.
The serial # ***** *****
Expert:  Charles replied 10 months ago.

Thanks. Been looking around, but I cant find anything either. I will grab the manual tomorrow from the shop. It has the spec in it and I will let you know what it is.

Expert:  Charles replied 10 months ago.

Ok,

I have the manual here with me. the compression should be 60 MAX. This is due to the compression release on the cam. When doing a leakdown test, the result will be off, unless the valve cover is removed the the compression release deactivated.

Also, when you did the test, did you use the hose from your compression tester?

Customer: replied 10 months ago.
I used the hose and fitting that came with the compression tester. Sounds like the compression is good and it holds.
Expert:  Charles replied 10 months ago.

Before you did the leakdown test. Did you remove the valve from the hose?

Did you also use a ratchet to roll the engine over to the compression stroke of the cylinder being tested?

Customer: replied 10 months ago.
The gage has a female quick disconnect on it, the hose has a male disconnect plugged on one end and a threaded fitting to connect to the spark plug hole. I screwed the correct fitting into the spark plug hole and then attached the hose to the fitting, I then attached the hose to the gage with the quick disconnect.s. I turned the engine over using the pull start rope to get the max compression.
Expert:  Charles replied 10 months ago.

Ok, that is a compression test. How did you do the leakdown test?

Customer: replied 10 months ago.
As per Mercury, I obtained the max. compression on the gage and started the clock. I let the compression stay on the gage at about 60 pounds for ten min. The compression only dropped about 1 or 2 pounds over that time .
Expert:  Charles replied 10 months ago.

Now I understand. What you actually tested was the holding of the valve in the end of the hose of your compression gauge. A leakdown test requires that you put compressed air to a special gauge set, then put that pressure into the cylinder, roll the engine over using the flywheel nut to TDC of the cylinder being tested. The percentage of leakdown is shown on your test tool. This article describes how to do it.

https://mobiloil.com/en/article/car-maintenance/car-maintenance-archive/how-to-do-a-leakdown-test

Customer: replied 10 months ago.
It took a while for the local marine mechanic to preform the test. He was not sure what the test was going to prove as related to the vibration but I paid for the test. The top cylinder showed 8% leakage and the bottom cylinder showed 12 % leakage. The mechanic said that was not bad for the specific engine. The mechanic thought the vibration might be coming from a bad power shaft bushing in the motor. He suggested dropping the lower unit and running the engine. I did drop the lower unit hooked up water and ran the engine, the vibration was still present.
Expert:  Charles replied 10 months ago.

The test was to see if each cylinder was functioning correctly.

With the running without the gear case, that has eliminated it. Remove the pull starter, see if you can wiggle the flywheel at all. Also try and lift it up. This will check the bearings around the crank.

Then remove the plugs, slowly turn the flywheel by hand. Insert a screwdriver into the cylinder and turn the flywheel until the piston is at the top of the stroke. UIsing the screw driver, press in on the piston, there should be no movement at all.

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