25 hp rude knocking on idle

[BugEyes]

Hi,<br /><br />I bough an old 25 hp Evinrude, it's a 1980 model. Got it cheap and I did not expect it to be perfect.<br />I have it running and idling good, the comp is 100 PSI + and I can't feel any play in the bearings by moving the flywheel or pushing on the pistons trough the plug holes.<br />Tried it on the boat today and it runs fine but I have one problem. On low revs and idle (in neutral or in gear) I can hear a distinct knocking sound from the powerhead area but I can't make out exactly where it comes from. The knocking is relative to the RPM and goes away in higher revs. I thought the engine felt a little raw on higher revs as well (more vibration than my even older Evinrude 18 hp).<br />Any ideas before I tear the powerhead down?<br /><br />Thank's in advance

 

[rodbolt]

Re: 25 hp rude knocking on idle

hello<br /> try lifting up on the flywheel. a tiny amount of lateral play is normal but no up and down motion is permissable on that motor. other than that they all rattle some at low speeds.<br />without hearing it I cant say if its normal or not<br /> good luck and keep posting

 

[the machinist]

Re: 25 hp rude knocking on idle

It might be piston slop. I have a 15hp that I recently bought that has about .008 cylinder wear & about another .015 piston wear. Yet the compression is 100/105 & it runs with no noticeable problems, but does have the low speed noinse you describe.

 

[BugEyes]

Re: 25 hp rude knocking on idle

Thank's for the replies,<br /><br />I can't detect any play in any direction by pulling, pushing, lifting and rocking the flywheel so I guess it's not the crank bearings causing the noise. It's certainly not the normal rattle, I compared it side by side to my ancient 18 hp yesterday and the 25 has a distinct knocking sound not present in the 18.<br />Might be piston slop or something with the wrist bearings etc. I'm more and more sure it's something within the powerhead. I have ordered a Seloc workshop manual and when it arrives I will tear the powerhead down and check it.<br /><br />Thank's again

 

[G DANE]

Re: 25 hp rude knocking on idle

Hi Pikefisher<br /><br />I too experienced the 76 and up blocks can rattle a lot more that the 18 - 20 - 25 older types. Seems they are a lot noisyer than the small block. If it is piston slop, it will be much less if you run it at WOT for, lets say 5 minutes and rapidly return to iddle. Now both pistons will be heat expanded and rattle less. One thing I would check is if magnets touches stator, that can give a rattling sound. Turn flywheel slowly by hand, and watch stator plate. It will move some by magnet power, but if it rubs, you can hear it by wiggling with stator at the point. Another thing that can produce a rattling sound is lower main bearing, if someone has been beating on cranckshaft to get flywheel off, but thats another story.

 

[G DANE]

Re: 25 hp rude knocking on idle

Forgot to say, I ran my rattling motor for 2 years like that, it didnt get worse, the new owner uses it A LOT, no problems.

 

[BugEyes]

Re: 25 hp rude knocking on idle

Hi Guys,<br /><br />EDIT/ Started a new topic with the questions I had here to make them more visible, removed them from here.<br /><br />BTW/ It's not the flywheel touching anything and it's not a rattling sound, more a distinct knocking sound. Like when a valve is very loose on a 4 stroke engine.