75hp unknown blockage?

75hp has sat for years. It's a 1983 as near as I can tell. Got it running great however after running WOT it will pump out steam from the 2 holes in the back of the midsection for about 30 seconds when you throttle down. I'm guessing a cooling blockage somewhere. I have ran water into both holes under pressure, blocking the other hole but same results. Where should I be looking?

It is a Johnson and has a new water pump BTW

It's not a good idea to be shooting a garden hose into either one of those exhaust relief holes... unless you need to take a chance of washing out the bottom cylinder and the crankcase with water.

(Exhaust Relief Ports - Exhaust Housing (Inner/Outer)
(J Reeves)

The long housing between the powerhead and the lower unit is called the exhaust housing. There is a inner housing within it that has a heavy duty seal around the bottom of it, or heavy duty seals around a inner extension between the housing and the lower unit.


The red hot exhaust travels down thru that inner tube and out the propeller with a somewhat supply of water to cool the propeller hub. A good amount of water surrounds and fills the space between the outer and inner tube, otherwise the outer housing would get so hot that the paint would burn off.


Some water pumps, for some reason (differing even when new) exert a great amount of water pressure, and if the exhaust housing seals are in perfect condition, the water fills the tube to a point of overflowing.

This brings into play those two holes or slots, whichever the engine might have, at the top rear portion of the exhaust housing just below the powerhead.

Now, if those two holes/slots weren't there, water would continue to flow up into the cylinders. Water not flowing out of those holes is no concern for alarm UNLESS that outer housing suddenly becomes extremely hot..... the warning horn should sound long before that happens.


The main reason for those holes being there (exhaust relief holes) is that when at an idle, there is an extreme amount of resistance encountered by the exhaust trying to escape due the fact that the outlet via the propeller is now blocked by a wall of water. The escape route in this case is for the exhaust to escape out those two holes, otherwise the engine would slow down quite quickly and die. If exhaust cannot escape, air/fuel cannot gain entrance to the engine.

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Ok Joe, now I have an understanding of their purpose. Thanks. Would that mean there is not enough water getting into that area fast enough? I also had changed the coolant exit to a 45 degree angle and a slightly larger exit diameter...more water is passing through the system faster now. Did I create a deficiency in he exhaust housing?

I just took it out for a bit and it appears to have an ample amount of water exiting the 2 holes. However it still steams when I throttle down. Is this a concern? My 60hp Evinrude didn't do that.

Frankly... I don't know. I've never had a engine do that and have no idea if the modifications you made have anything to do with it or not.

Test compression, Is engine overheating?Check with infrared thermometer,hold hand on head for six second count,no burn running ok. Hold hand in telltale stream,Does it burn??Recheck your water pump installation,was proper keyway installed? Was sealer used on install?

Compression is good...135+- on all 3. No overheating. I can keep my hand on heads almost indefinitely. Stream is lake water temperature. Water pump was properly installed and sealant/new gaskets and seals where needed. But another problem has popped up. At WOT it barks, burps and farts to the point where I lose speed. Doesn't feel fuel related. Back it to 3/4 throttle and runs great. I put a new power pack on it Tuesday....that's when the WOT problem began. Before doing so I could mash the throttle from idle and not any hesitation whatsoever

new power pack may have rev limiter. do you have a tach?