Rattle on decelration

[Equis]

Hello,<br /><br />I am new to boating, I just got a merc 115 hp and it is great (but noisy) <br /><br />its sounds fine on the plane but when I back off it rattles for a few seconds, is that normal?<br /><br />Its only when backing off<br /><br />Thanks<br /><br />Rhys

 

[outboard tech]

Re: Rattle on decelration

You might be hearing some "decel knock"; would need to know how old is the unit--if antique, you could try higher octane fuel. But if you're throttling off quickly but not returning to neutral; you may be hearing the forward gear ratcheting the clutch dog in the gearcase, normal.

 

[Laddies]

Re: Rattle on decelration

might haue a cracked flywheel fhex plate

 

[DHPMARINE]

Re: Rattle on decelration

I throttle back quickly,but DO return to neutral.<br /><br />What octane should I use?<br /><br />can I adjust my decel?<br /><br />1997 115hp<br />DHP

 

[outboard tech]

Re: Rattle on decelration

'97 is tuned to run fine on regular. 87 octane. So I'd forget the decel knock and prop ratcheting. Almost all 2strokes have a reed flutter at around 2500 to 3000 rpms, but you'd hear that accelarating also. Ya got me on that one. Any chance the motor has been at some time overheated and you're hearing some piston skirt noise? There are many possibilities, and noises (even from the gearcase) transmit through the motor and can be hard to pin down. If the motor ever injested any water, you could have some rod or bearing damage, and that can be fatal. When in doubt, always start at the beginning--see what you have for cylinder compression. 15% or more diviation among the 4 is maximum, and they should all be very close--plus or minus about 10psi. Also, are you running a stainless prop? If so, what kind?

 

[Equis]

Re: Rattle on decelration

Hello,<br /><br />It's a 2002 115hp, just got it.<br />Yep, Its got a Stainless prop on it, a "Vengence" (think its stainless)<br /><br />All other times the engibe just purrs and has heaps of power<br /><br />Thanks for all the replys <br /><br />Rhys

 

[Hooty]

Re: Rattle on decelration

De-cell knock is caused by a high vacuum, lean fuel/air mixture condition. If it is a de-cell knock, you should be able to bump the primer switch and stop it. <br /><br />c/6<br />Hooty

 

[bubbakat]

Re: Rattle on decelration

This may or may not play into this action but my little evenrude did the same thing and I chased it all summer. Found out the other day that the prop was a little crooked on the hub. Sent it to a prop shop for re hub and all is well.<br />My prop man said I was hearing the virbation which was in turn making the engine viberate when it slowed in rpms. Making it rattle.

 

[Equis]

Re: Rattle on decelration

bubbakat, did your engine only do it while decelerating? did it do it under power also?<br /><br />Thanks

 

[Equis]

Re: Rattle on decelration

Hmmm, just thinking what "outboard tech" said about the octane level in fuel, I just got the boat with 100 litres of more than likley 9 month old fuel in it, I wonder if this may have something to do with it?<br /><br />I wonder if I should pump the tank and refill with new fuel..<br /><br />Rhys

 

[bubbakat]

Re: Rattle on decelration

Mine did it only on deceleration. You can always try a smaller portable tank with fresh 89 octane fuel and see. If the fuel is 9 months old and wasn't treated for storage I would put it in my truck and run it and get fresh fuel.

 

[gss036]

Re: Rattle on decelration

I would go for the small tank idea to see if it is the old fuel and while doing that add a can of SeaFoam or Chevron Techron to the small tank to clean the possible junk out of the fuel system on the engine.<br />One thought, you do have the "thrust washer" on the propeller installed proper don't you?