1970s-80s ? merc thunderbolt ingition. 80hp 800series knock knock knocking !!!

[WaveJumpingMaxum]

Ok my cousin has a 70s-80s merc, Black with blue stripes, thunderbolt iginiton. 800 series 80hp. Put a new prop on like a week later were motoring out and it started knockin. Ruled reeds cause it wasnt spitting and sputtering, runs like a top just makes a taping noise. any suggestions? i told him to do a compression test but he is greener then i am when it comes to boats so he didnt wanna hear it he just wants to replace. tried turning the fly wheel and its difficult. i kno the powerhead on my force is only 8 bolts how different from a merc?? any suggestions on what it can be? ive heard mercs have a jeweled powerhead is this one?!!?

 

[79Merc80]

Re: 1970s-80s ? merc thunderbolt ingition. 80hp 800series knock knock knocking !!!

Try tapping on top of the flywheel with a light hammer, if it rattles it's the pressure plate. I had that happen on our old 1250.

The powerhead is held on my 2 bolts under the belly pan and (I think 8) hidden by the lower cowling under the belly pan.

Hope this helps. Craig

 

[WaveJumpingMaxum]

Re: 1970s-80s ? merc thunderbolt ingition. 80hp 800series knock knock knocking !!!

Thanks thats a cool trouble shooting trick. Gonna head out try it in about 30min with a rubber mallet.It does sound like its comming from the topside of the engine but we would really rather not run it making that noise. It looks like the merc in your picture is that a 79? They way it didnt lose power can i rule out bearings, pistons and connecting rods? (sorry if wrong terms there really green to outboards using what i know from automotive)

 

[WaveJumpingMaxum]

Re: 1970s-80s ? merc thunderbolt ingition. 80hp 800series knock knock knocking !!!

ok the fly wheel rattled. pulled the plugs and put a screw driver in while turning the fly wheel and theres no play in any of the pistons. any advice on how to take the flywheel off? its a 78/79 btw, says 79 on front of engine under cowl and 78 on some electrical panel.

 

[*EdC*]

Re: 1970s-80s ? merc thunderbolt ingition. 80hp 800series knock knock knocking !!!

ok the fly wheel rattled. pulled the plugs and put a screw driver in while turning the fly wheel and theres no play in any of the pistons. any advice on how to take the flywheel off? its a 78/79 btw, says 79 on front of engine under cowl and 78 on some electrical panel.

There is a special puller for this job. About 30 bucks.