1978 Merc 20hp

[freddyray21]

got this for next to nothing. Guy said he hit a rock the the prop stopped turning and it overheats now. My guess is broken driveshaft, but have not torn into it yet. Just wondered if anyone has run into the same thing and what they found. I just picked it up tonight so haven't even gotten a manual for it yet.

 

[achris]

Re: 1978 Merc 20hp

Sorry, my crystal ball is at the repair shop at the moment...

 

[freddyray21]

Re: 1978 Merc 20hp

Sorry, my crystal ball is at the repair shop at the moment...

I realize you cannot tell me what is wrong as that would require a crystal ball, but in the 30 years this motor has been around I am sure this has happened a time of two before and you can hazzard a guess. I am usually an OMC guy and you don't see this happen on them.

 

[achris]

Re: 1978 Merc 20hp

...I am usually an OMC guy and you don't see this happen on them.

REALLY!!!

I doubt it... or you haven't seen too many OMCs.

I owned a boat shop in the 90s and saw more OMCs with overheats than all the other brands put together, and we were a Merc agency!

Yes, it could be a broken driveshaft... or more likely the engine was run dry and the water pump impeller cooked....

You need to pull the leg and see what's broken or not broken.

Chris..........

 

[hamit59]

Re: 1978 Merc 20hp

You might also see if the flywheel key is ok .If it is almost sheared it will change the timing and that can cause it to run hot as well.
I with Chris, The water pump would be my first though.

Hamit
BT1 USN ret

 

[achris]

Re: 1978 Merc 20hp

Crankshaft is continuous splined taper on these engines... Not a woodruff key like OMC.

C.

 

[freddyray21]

Re: 1978 Merc 20hp

the OMC comment was just to get a response from you. It does happen on them also, but I usually deal with the real old ones and they would have spring loaded shafts that would fail when a rock was hit and so that is what you looked for then you pulled them apart. Didn't know on these mercs if there was something similar to look for. It's clear to me the shaft is not turning the impeller. I do hope they did not run it long. It doesn't show the usual signs of over heat or new paint. Thanks for the response. I do know I need to pull it apart and will once I get a minute. Got to work today unfortunately and we are running motors tomorrow. I've got a couple of old ones I am playing with. Well one old one a 1953 and an almost new one a 1967.

 

[hamit59]

Re: 1978 Merc 20hp

Just did the check on the flywheel key, it does have one woodruff key for it listed. I had to make sure. I knew the Merc 200 20hp 75 model had one as I just sent it out this week with a new pump in it.

 

[freddyray21]

Re: 1978 Merc 20hp

I'll have to pull it and check. Don't think it is the flywheel key as I told the motor runs, but have not checked that either. As cheap as I got this I can't be hurt in any event.

 

[achris]

Re: 1978 Merc 20hp

My bad... It does have a key... The later ones don't... Sorry.....

 

[Chinewalker]

Re: 1978 Merc 20hp

Hi Fred,
I would check both the driveshaft and also the upper driveshaft splines. Not uncommon for a previous owner to have replaced a waterpump and neglected to grease the splines. No grease results in the crankshaft splines rusting out, which can result in it losing its grip on the driveshaft.
Also, I think that motor may have a bi-metal driveshaft, made of carbon steel and stainless steel. I've seen a few sheer off at the juncture, particularly if the carbon steel section has started to rust significantly...
- Scott

 

[achris]

Re: 1978 Merc 20hp

Hi Fred,
.... Also, I think that motor may have a bi-metal driveshaft, made of carbon steel and stainless steel. I've seen a few sheer off at the juncture, particularly if the carbon steel section has started to rust significantly...
- Scott

Agree, but the fusion joint is below the water pump... would still get water when the engine was running...

Let's just get the leg off and stop the speculating... Pull the leg and let us know.