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Exhaust relief

J

Jay1234

Regular Contributor
Hey guys,
Been doing a lot of reading this morning but didn't really find anything definitive on this. I noticed that my 1968 55hp has never had anything come out of the above water exhaust relief holes. Not a drop of water or exhaust... It idles fine and trolls fine at slow speed. I did have the power head off when I first got it a few years ago to replace exhaust cover gaskets.. and replaced the base gasket. I also installed a pee hole.
Not sure why I thought of this... But anyway any input would be appreciated.

Thanks
 
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(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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Thanks Joe I did come across your article. Excellent info. I'll go back to anxiously waiting for the launches to be free of ice...

Thanks again.
 
Jay1234 said:
Thanks Joe I did come across your article. Excellent info. I'll go back to anxiously waiting for the launches to be free of ice... Thanks again.
Click to expand...

Wondering just exactly what I could have possibly left out of that article since you had already looked thru it and still had a question. I can easily edit it.
 
Jay1234 said:
I was just curious because even at idle nothing comes out of the holes. That's all.
Click to expand...

Okay.... What's left would be:

The large seal (313656--> 324937) at the bottom of the tuning tube, the seal that is installed in the top area of the lower unit before it is installed.... that seal may be faulty, worn, improperly installed...... OR... The water pump is not functioning properly for some reason.

I'll work that entry into my database entry.
 
Sounds good thanks again. I'm servicing the pump soon I'll replace that seal while the lower is off.

Appreciate the time as always.
 
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