1979 merc classic 50 hp cooling problem

[61harleyman]

hi
i am new to the forum.
i have a 1979 mercury classic 50 hp.
when the engine is idling or trolling just above idle it cools fine. there is a pretty good stream from the pisser.(it does not seem to be quite as strong as it used to be) it does not have a thermostat.
after i get on plane for 30 seconds or so the water coming from the pisser turns to steam. if i go back to trolling along it gradually cools the motor down and the water from the pisser goes back to normal.
it is a short shaft motor on a short shaft boat.
i have replaced the impeller and had my marine mechanic replace the entire water pump.there is no pitting on the drive shaft,an air hose when i had the lower unit removed and blew air threw. there did not seem to be any blockage.
the water pipe is seated properly in the water pump and engine block.
this is driving me nuts.
any help would be greatly appreciated.
thanks
scott

 

[allymoss]

Re: 1979 merc classic 50 hp cooling problem

attach a hose to the cooling pipe going into the block, put some pressure through it and maybe it can blow out a blockage.

Sounds like its blocked around the head. Has it been used in saltwater and has it got worse?

 

[61harleyman]

Re: 1979 merc classic 50 hp cooling problem

I had the lower unit off. I attached an air hose to the fitting on the water jacket were the pisser hose goes. I blew it out with air pressure. the air seemed to move threw quite easily. It has been in fresh water only.
since the new water pump went in it seems a little better but still not right.
scott

 

[floatingwoody2006]

Re: 1979 merc classic 50 hp cooling problem

Cavitation?

 

[mthieme]

Re: 1979 merc classic 50 hp cooling problem

Good suggestion.
Where is the cavitation plate in relation to the bottom of the keel ?

 

[emckelvy]

Re: 1979 merc classic 50 hp cooling problem

Maybe you have a leaking exhaust manifold gasket blowing exhaust into the water passages at higher speeds.

Or, you have silt/sediment buildup in the block which is impeding flow.

If you are 100 percent sure the water supply is not the problem, it stands to reason you have a circulation problem.

The last time we tackled one of these there was a chunk of impeller lodged partway up the copper water supply tube, partially restricting flow. Not bad enough to even show when blowing air up the tube, but significant enough to make the motor run hot when underway. Something you could check by running a wire up the tube.

I'd recommend pulling the exhaust cover and cyl block water jacket cover (the one over the back of the block) for inspection and renewal of gaskets. Also check very carefully the inner exhaust plate for corrosion/perforations. This is a standard replacement item and your motor is old enough where it may need it anyway.

Don't think you're gonna solve much until you do this. You may have to go so far as to pull the powerhead for inspection.

But I'd try the back cover first, then exhaust covers if you see nothing in there, then pull powerhead if you can't find anything else.

And before you pull everything apart, go back and inspect that pump assy again. Also inspect the inlet chamber of the lower unit for obstructions.

Never assume anything has been done correctly unless you see it with your own eyes.

These motors aren't rocket science, they have no thermostat, they just get water pumped thru them. So either the supply or the item being supplied has the fault.

Providing of course, as suggested by previous posters, the height on the transom is correct and you're not starving the motor for water once on plane.

HTH & let us know what you find.........ed