70 hp Johnson Question

[steveo32]

HI, I am new to the forum and new to boating.

Motor is a 1975 70 hp 3 cyl Johnson Outboard

I bought a motor flush or Muff so I could start the boat to make sure eveything was working. Water comes out the 4 holes that are a couple of inches above the water inlet holes. And water comes out of the pee hole on the top right side of the lower unit. But no water comes out the two exhaust holes on the top back side of the lower unit. Is this because I am using muffs and not actually in a lake? I have seen videos of similar motors running with water comming out those holes. The motor does start and run fine has good throttle reaction and will rev up great. I was just concerned because I don't want to overheat the motor. It is not to hot to touch or anything like that.

Thanks, Steve

 

[Silvertip]

Re: 70 hp Johnson Question

With the motor out of the water there is no backpressure on the main exhaust which exits through the center of the prop. When the motor is in the water and at idle, exhaust back pressure would cause idle issues so the ports at the back of the engine are called relief ports and they provide a path an easier path for exhaust at idle. Once you are at speed the exhaust goes out through the prop but you may see a spray of water coming out those ports as well.

 

[Joe Reeves]

Re: 70 hp Johnson Question

(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

 

[steveo32]

Re: 70 hp Johnson Question

Ok guys thanks for the information, I think this is a good place for information and quick responses. From what I gather the water pump is working fine and everything is being cooled. Also Joe you mentioned the warning horn, I assume this is for everheating and I was under the impression that they didn't put those on until '76, this is what I have read on the internet.

 

[tashasdaddy]

Re: 70 hp Johnson Question

welcome to iboats.

 

[Joe Reeves]

Re: 70 hp Johnson Question

Steve..... The two terminal (green metal) warning horn came into existence in 1971. The self test two terminal with built in ground wire (black plastic) came upon the scene at a later date.