'72 Johnson 50 hp exhaust leak under cover

[burntchevy]

I have a '72 Johnson 50 hp model 50ESL72 that is running a lot better now thanks to the advice of participants in this forum. I have a remaining problem in that there is quite a bit of exhaust gas rising up from the starboard side of the motor under causing the motor to run poorly unless the cover is removed. Is the culprit likely to be the gasket that lies at the base of the powerhead or something else? And if so, is the only way to fix it to remove the power head and lower unit in order to get full access to the exhaust system?

 

[ezeke]

Re: '72 Johnson 50 hp exhaust leak under cover

Is the exhaust gasket between the mid section and lower unit in place?

 

[burntchevy]

Re: '72 Johnson 50 hp exhaust leak under cover

Hi ezeke,<br />Thanks for your response. Pardon my ignorance, but would that be what the Johnson Service Manual calls the "Gearcase inner exhaust seal" that sits right in the middle top of the lower unit? We have recently had the lower unit off and on several times trying to fix a shifting problem with the hydroelectric shift. I suppose it is possible we did not get something back right. Or are you referring to one of the other exhaust seals?

 

[ezeke]

Re: '72 Johnson 50 hp exhaust leak under cover

You shold have a exhaust seal between the two units which seals the exhaust from the rest of the mid section. If you drop the unit down about a half inch you can probably see it well enough to find out if its damaged, or just out of place.<br /><br />I usually temporarily stick that to the lower unit with some kind of adhesive so that it will not flop around when I'm closing.

 

[ledgefinder]

Re: '72 Johnson 50 hp exhaust leak under cover

That's a hydroelectric shift, and I'll bet it's leaking exhaust gas up through where the electrical cable goes down into the lower unit. It's starboard. Just seal it up with some good marine silicone sealant (like what you use for through-transom bolts when you install an outboard). <br /><br />If you don't pay attention to that seal when you pull the lower unit to change the impeller, it leaks exhaust gas up into the cowling at idle. Dont' ask how I know this...

 

[burntchevy]

Re: '72 Johnson 50 hp exhaust leak under cover

Many thanks ledgefinder. That theory makes sense. I am going to try it before dropping the lower unit again for the umpteenth time in the last month. I degreased the hole where the wires come up as well as I could and then filled in the hole around them with blue RTV. Aim to try it tomorrow after the RTV has had time to set up and see if that was it.<br /><br />I am guessing that the way you know this is the same way I am learning about boat motors - by making many mistakes, sometimes expensive ones.

 

[burntchevy]

Re: '72 Johnson 50 hp exhaust leak under cover

Sorry I have not gotten back on the results. I went to check whether the exhaust problem was fixed and the motor would not start - no spark at all on either cylinder. Looks like the problem is the powerpack. So, I won't know if the silicone fixed the exhaust problem till I am able to get the new powerpack and get it installed, hopefully in the next few days.

 

[burntchevy]

Re: '72 Johnson 50 hp exhaust leak under cover

At last, with new powerpack and new coils, the motor starts great and the RTV around the wires stopped the exhaust gases from entering under the cover. Many thanks.

 

[gt5001968]

Re: '72 Johnson 50 hp exhaust leak under cover

I have the same motor on my boat, and I have found that the exhaust problem was due to the electrical wire going to the lower unit, there are 2 exhaust prts on the motor, thru the prop, and 2 slots in the back of the shaft, check and see if they might be clogged!

 

[burntchevy]

Re: '72 Johnson 50 hp exhaust leak under cover

Thanks for the tip gt. I will be getting a look for the first time at the whole exhaust system pretty soon since the top piston came apart around the top edge last weekend so I am going to be having to take the powerhead off and take it apart to see what is up. It would not surprise me in the process to discover that something is also haywire in the exhaust causing the motor not to breathe right and perhaps contributing to the piston failure. Any tips on what to look for when a piston goes bad around the top edge? The engine now has new leaf valves and the carbs have been thoroughly rebuilt. The piston pieces hammered the cylinder head so I guess I am going to have to replace that at least.