Motor vibration

[Bob/Mn]

 I have a 2013 700CC Mercury 4 stroke 40 HP motor with three cylinders.  The motor runs well but at idle the motor vibrates causing vibration noise in the  bow casting platform. The noise is quite loud and annoying and has done this from day one. 

Any info on a fix?

Thank you.

 

 

[delcecchi]

Have you tried changing the idle speed?

[Bob/Mn]

The idle speed was factory set at 800 RPMs

[RebelSS]

I wonder if you're getting a harmonic "distortion" that's just right to reverb your boat...I'll guess that it's aluminum? Does it go away if you bump your idle up a tad with the throttle lever?

[delcecchi]

That's what I was thinking. Perhaps a resonance.

Did you complain about it while under warranty? 

Edited September 20, 2015 by delcecchi

[RebelSS]

Mine will do that sometimes around 5000 RPM while planing out....on the sides of the boat. Blip the throttle a tad either way, and it goes away. You might want to make sure it isn't your prop; that will really do it sometimes. Make sure all blades have equal/same pitch; no whonks out of it to set it out of balance.

This is from a good tech page on boat vibration:

If the whole boat is vibrating, ask to test the boat with a "clean" prop before you buy. This will verify that it is indeed the prop that is causing the problem. Wouldn't want it to be a loose motor mounting bolt, lower unit......

If it's a few small things vibrating, I would still test with a clean prop but, your going to have vibrations no matter what you do. All things on a boat have their own harmonics. My fishbox lids that do not lock in place, they are just held closed by gravity, vibrate at different RPM's. The port and starboard are generally the same (shape, weight...) but vibrate at ~3600 (port) 3800(starboard). My center lid vibrates at idle. I guarantee you that there will always be something vibrating on your boat at just about any RPM. The thing to do is find the RPM's that mean something (cruise, trolling...) the RPM's you will use most often and make sure nothing vibrates in those ranges. If something does, see what you can do to quiet it down. Little rubber feet, the kind used in drawers work well for hatches and such, or changing out to locking units on hatches will work also. Just make sure nothing is loose and everything is attached securely.

__________________
 

[Bob/Mn]

I wonder if you're getting a harmonic "distortion" that's just right to reverb your boat...I'll guess that it's aluminum? Does it go away if you bump your idle up a tad with the throttle lever?

The boat is a Lund Fury. The vibration does go away when I bump the idle up and I would say it is a harmonic distortion.  Since the boat is set up for back trolling I really need the low RPM.

 The motor is under warranty.

[RebelSS]

800 RPM is bit high IMO. I'd take that down to about 650 r's..especially since it's a 4 stroke.

Edited September 21, 2015 by RebelSS

[BoxMN]

Maybe try a 4 bladed spitfire prop...?... it is awesome prop for trolling or reverse as  well.

[marine_man]

I'm not sure if your 800 RPM idle is in neutral in gear - if it's in gear, that seems pretty high for a 4 stroke to idle while in gear.. I'd consider seeing if they can adjust that down.

Does it vibrate when the engine is in neutral (as well as when it's in gear?)  If it's in neutral also, you can rule out the prop.  If this is the case, you're stuck with the harmonics of the engine - your best bet is to address the symptom - the rattling casting deck - can something be tightened up / padded with rubber / etc to make it more acceptable?  On mine, while in gear at idle my rod box rattled like crazy.. I ended up tightening the latch to the point that it's a little more difficult to open it, but the noise is finally gone...

marine_man

[Bob/Mn]

I'm not sure if your 800 RPM idle is in neutral in gear - if it's in gear, that seems pretty high for a 4 stroke to idle while in gear.. I'd consider seeing if they can adjust that down.

Does it vibrate when the engine is in neutral (as well as when it's in gear?)  If it's in neutral also, you can rule out the prop.  If this is the case, you're stuck with the harmonics of the engine - your best bet is to address the symptom - the rattling casting deck - can something be tightened up / padded with rubber / etc to make it more acceptable?  On mine, while in gear at idle my rod box rattled like crazy.. I ended up tightening the latch to the point that it's a little more difficult to open it, but the noise is finally gone...

marine_man

The vibration happens when the motor is in neutral and gets worst when in reverse. Mercury changed the 40 hp power head from a 700cc three cyl. to a 900cc four cly.

 I wonder if the reason could be low end vibration?

[marine_man]

That rules out the prop / lower unit combination.  You don't see the same vibration in forward?

marine_man

[Bob/Mn]

That rules out the prop / lower unit combination.  You don't see the same vibration in forward?

marine_man

Vibration in forward also.

[marine_man]

At this point, I'm not sure where to send you... if it were me I'd be inclined to take it back if you have warranty and get their take on it, with two possible resolutions: 1) They fix what's causing the boat to shake / rattle (motor)  2) They adjust the symptom (rattling casting deck).

It could be a motor mount out / etc, but it seems odd that you'd have that after only 2 years.

marine_man