70 H.P. Johnson little bit of smoke coming out of outer exhaust housing

[softshoe]

Hi guys, I have a 1984 J70TLCRD Johnson outboard engine. First time boat has been out in over a year. We went camping for 4 days and the boat ran great pulling the kids on the tube Wide open throttle, and as well when trolling at 1000 r.p.m. The last day was switching riders while tubing and the boat got up to about 3500 rpm, and was kinda stuck there. I shut the boat down and restarted and all was well again, and it would run at 5200 rpm like it always had. when pulling back in from tubing I noticed some smoke coming out of 2 holes (exhaust ports maybe?) on the back of the outer exhaust housing. I let it idle for awhile and it started to go away. The outer housing was pretty warm to the touch, I could leave my hand on it but it was very warm I thought. I took it out by myself and it ran like a top, but once again when I would idle it down coming in, those 2 holes had smoke coming out and would eventually start to get less and less smoke. I am pretty mechanically inclined on vehicles but when it comes to boat motors it is all new to me. First question is I haven't really noticed this before is this usual or is something possibly getting very hot? Boat has no gauges of any type on it as far as water temp or any other temp. I do know that all the usual places where the water comes out of engines is still coming out very well. If this is something to be worried about any ideas where I should start looking at to find the cause for the smoke. Thanks in advance.

 

[Joe Reeves]

Re: 70 H.P. Johnson little bit of smoke coming out of outer exhaust housing

(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.