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1999 Sunsetter shaking at low RPM

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I've got a 1999 Sunsetter LXI and last summer I noticed the boat start to shake at lower RPM(<1500).   Once I bring boat up to speed it seems to be perfectly fine.  This spring, it seemed to be a bit more noticeable.   I checked the prop, and there was a small bump(not a chip, but more of a bend on the end of one blade on the prop).   I replaced the prop.   Shaking still happens.   I've read on other posts about replacing the IAC, but not sure if that is cause.   Anyone have any ideas on other things to check?

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Possible strut bearing too loose or worn out . I would also check tightness of prop, prop shaft to engine coupling, and jam nuts for each motor mount. I had one of the jam nuts loosen up and cause a different vibration at low speeds , then went away at higher rpms or more torque from engine.  The engine actually would physically drop down 1/2 " at low rpms and then raise as you added more power on one side .  Hope you find it 

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could you describe the shaking in more detail?  You mention the IAC which makes me wonder if the engine is surging or running rough at low speed?

Edited by Bozboat
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@Bozboat the only reason I mention the IAC is because that's the only thing that I could find on discussion boards that resembles my situation to some degree.   It doesn't feel like its surging at idle to me, just running rough at low speed.   

I was thinking maybe if I rev the engine in neutral without prop spinning, does it still yield the same shaking sensation at a particular RPM.   Would that help narrow down the scope of potential issues? 

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Have you laid eyes on engine while this happens? Run with hood up to see if issue is with engine or with drive train?

rev to 1500-2000 in neutral might give you an indication, if it’s rough in neutral we can start figuring out what to check 

are you describing a rough running engine that is shaking at low rpm?

Edited by Bozboat
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@MadMan shaking still occurs when turning rudder at low speeds. 

@Bozboat  I tired revving engine in neutral and there is no shaking at all which leads me to believe there is something in drive train

@937footer1 I checked the four engine mounts and the tow bolts that hold each mount to the hull all seem fine.   there is a larger bolt as part of the mount, but I didn't want to mess with that as I think it may be used when adjusting alignment...not sure I've got the expertise or tools to mess with that.

I haven't checked rudder yet, but will do that tonight.   Any other thoughts?

Thanks for all of the help thus far.   

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While you are checking the rudder, go ahead and look at the drive shaft and strut.  A bent strut can cause a shake because it forces a bend in the shaft, making the thrust uneven on either side of the prop.  Bad alignment can do it too. 

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Did you try opening the cowl and take it through the RPM that you feel the vibs at while going down the lake ? You may see what is vibrating . Also I would check how tight the lower jam nut on each mount is . They should be tight and you shouldn't get any turns of the jam nuts. 

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I checked the rudder and shaft...and it all looks fine.   

However, yesterday I noticed a new symptom that may be a better indication of root problem.   We were towing some young kids on a tube and when bringing boat through the lower RPM's where vibration was happening, it felt like engine power was dropping at times..not that it was necessarily going to stall, but losing a bit of power and the normal growl of the exhausted sounding like a cough.   It would do that a few times then as I transitioned to higher RPMs it was fine.   

Could this be an indication of a bad fuel injector?  Transmission?(we had that replace several years ago and I'm familiar with what that feels like when it is slipping due to lack of pressure, but this feels different)  

Thanks again for all of the help thus far.   

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What rpm do you get at wide open?

have you checked the fuel pressure at Key on, idle and Wide open?

When were the cap and rotor last replaced?

When were fuel filters last  replaced ?

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  • 2 months later...

Update here:  I replaced the cap and rotor.  No change.   I purchased a dial indicator to check for a bent prop shaft.   Everything looks fine.   I called a mechanic who specializes in Mastercrafts up in Plymouth, MA to see if they could check out the boat.   Unfortunately they are jammed up with work and don't want to take a look.   However, he said to check out the damper plate as that can cause vibration at idle and low speed.   Thoughts?  Anyone have any place they would recommend in New England area to have someone take a look?

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