1985 75 HP knocking at idle when cowling / covers on, no knocking when cowling is off

[BrettNC]

I have confusing situation. The motor runs fine at all speeds. If I have the cowling on, I can hear a minor slap at low speeds. I can't hear it with the cowling off of the motor. I can't find anything interfering with the cowling. All brackets are fastened down tightly. Can an internal knock show up with the covers on but completely disappear with the covers off?

Thanks.

 

[Barnacle_Bill]

Re: 1985 75 HP knocking at idle when cowling / covers on, no knocking when cowling is

My 94 does that. It is definately something in the cowling but I can't find it. Weird huh?

 

[Texasmark]

Re: 1985 75 HP knocking at idle when cowling / covers on, no knocking when cowling is

You sure the noise is coming from the engine and not the prop? With plastic one piece housings and tight clamps with rubber seals, there isn't anything to rattle. The flywheel has a shroud around it, so it's not that rubbing on the cowling.

If the innards of the engine were making noise (piston skirt slap, combustion noise type things), you'd think the cowling would muffle that; however, if the noise were low frequency, installing the cowl would tend to attenuate the higher frequencies more than the low and it could be that this would allow you to hear what could be a regular engine noise.

Clutch Dog Rattle (lower unit gear system) usually occurs at speeds below 2000 rpm's. Might try shifting to N and see if it disappears.....CDR will if that's it.

Get back with your answer as I have more if that's it.

Mark

 

[BrettNC]

Re: 1985 75 HP knocking at idle when cowling / covers on, no knocking when cowling is

Thanks for your replies. I only notice the noise when it is idling out of gear, around 900-1000 rpm's.

 

[Texasmark]

Re: 1985 75 HP knocking at idle when cowling / covers on, no knocking when cowling is

Do a PM to Silvertip as he has a 75 but a different year model and he has engine moxy.

Mark