1967 80 hp Johnson Super Seahorse After Overheating

Last fall I was running the boat near WOT. The motor had been running fine nearly all summer. (had to put a new coil in it) I noticed the rpm seem to fall off just a little bit and immediately looked down to find my overheat light on. Great. Pulled back the throttle and turned off the motor within a few seconds. Not sure how long the light was on for before I saw it. I usually make sure to check it at least every minute or so. I parked it, let it cool (it was very hot), and briefly tried starting it about 5 hours later. It fired right up but I noticed it sounded a little different and it looked like some of the exhaust was coming up behind the motor inside the cover. I didn't have time to deal with it so I winterized it, and reluctantly stored it for the winter. Now boating season has nearly arrived here in the UP of Michigan. I splashed it at the boat ramp yesterday and it started okay, seemed to be spitting the usual amount of water out of the exhaust, but was still smoking up between the motor and back cover. I did not take it out on the water as I did not want to cause further damage.

Here is my tentative plan. I will pull the lower unit and check impeller and key. I will look for damage to water uptake and exhaust as I suspect if there are any rubber seals they may have been damaged by overheating. For good measure I will likely put a new impeller in even though the current one is only 2 years old.

It is important to note that I let the boat idle for at least 15 mins yesterday and aside from a few small coughs it ran pretty well. No overheating as far as I could tell.

My question is what do I need to look for after a pretty severe overheating. Head gaskets? Ignition damage? What could be causing the exhaust issue? The motor is obviously not seized but I will check compression to make sure there is no piston damage as well. There is no visible damage to wiring etc.

Thanks for the help guys!!

As far as the smoke coming out under the engine cover, that could be an overheated gasket where the exhaust housing meets the bottom of the power head, just a guess!
As far as the motor itself, I would perform a compression test and if the compression is on the low end, I would pull the head and look for cylinder scoring. If no scoring is visable, replace the head gasket, thermostat, by pass valve and spring, and also a new water pump impeller. Overheating does not normally damage the ignition unless it is run almost to a melt down state.
If the cylinders are badly scored it is either time for an over haul or new motor. You can also use a bore scope to check for cylinder scoring if you have access to one!

Thanks for the help! I'll report back on the compression test and look into exhaust/head gaskets. Any other advice is greatly appreciated!

Doesn't your engine have a rubber tube that directs water out of the rear of the exhaust housing? Maybe that tube got damaged and is leaking exhaust.

Did it get hot enough to burn or discolor the paint on the powerhead? If so, any engine that gets that hot needs to have all the head gaskets and exhaust gaskets replaced. It may run even if you don't, but it probably will also leak some water into the cylinders, causing great damage over time. When you get the exhaust covers off, you will be able to look into the ports to see if the pistons and rings are scuffed.

Replace the heat sensor in the head while you are at it. It probably won't work a second time. And consider replacing that hot light with a hot horn.

Thanks for the help guys! Will I be able to get the rear exhaust housing off without removing the motor from the boat? Is this going to require removal of the block as well? I have a seloc manual but am not sure if I have to tools required to pull the motor. I will be tearing into it tonight and will report back. Didn't have a chance last night.

And no I don't see any visible discoloration or anything that leads me to believe it was a meltdown caliber overheat.

Okay sorry for the delay but I had another project surface and this had to go on the back burner for a couple weeks. I was also waiting on a new impeller and head gaskets.

Anyway, I installed new head gaskets and impeller as well as a few other lower unit gaskets as I was getting some water in the gear case. I did a compression test by pulling all 4 spark plugs and cranking the motor with the tester in each of the 4 cylinders. Is that the correct method? I have compression values of 64, 70, 70, and 78. Oddly enough the only cylinder that had visible scoring was the one with the highest compression value. The scoring was bad enough to see, but I couldn't really feel it with my fingernail. Anyway are these compression values too low? I put the motor back together and splashed it again and I must have installed the impeller improperly as I was not spitting any water. It did start right up however. In addition it is still leaking exhaust from somewhere under the block. I thought it was coming from the rubber boot and spring that directs water and exhaust out of the back of the exhaust cover as it was loose enough to pop off when I pulled the exhaust housing.

My next plan of attack is to again pull the lower unit off and figure out why I am not getting water. I have done the impeller more than once before so I am not sure what I did wrong. Additionally, I am thinking I should pull the powerhead off and replace the main exhaust gasket.

If these compression values are too low I may consider purchasing a new motor but that will really break the bank. I don't mind working on this motor but if you think the block could be shot I'd like to know before I waste anymore time on it.

Thanks again for all of the advice! Any guidance on the next step from here would be greatly appreciated.

Those compression readings are too low. However, it starts right up, etc., which makes me question the validity of the numbers.

A common cause of exhaust coming up from below the powerhead is the foam seal on top of the water pump housing. There are more seals (O-rings) on the drive shaft tube extension, if it is a long shaft motor. If any of those seals leak, exhaust goes up through the drive shaft tube and out a vent hole at the top, below the powerhead.

It is a 20" shaft. Not sure if that is considered a long shaft or not in that HP range... I was wondering about those numbers as well. I have read that it is best to lubricate the cylinders with some 2 stroke oil first which can raise the values by as much as 20 psi. I did not lubricate them. I didn't use a cheap harbor freight caliber pressure gauge either. I used a rented one from auto-zone which seems like good enough quality.

As far as exhaust goes, I was thinking maybe I could run it with the rear exhaust cover off which might help me identify the source of the leak. I was thinking it would be best to get the exhaust and water pump problem figured out first so I can do a more effective trial run to see if it is down on power.

Is it safe to run the motor with the rear exhaust cover removed? It seems like it would be okay because the only thing that mates up to it is that spring and upper exhaust boot as well as the large rubber ring down near the bottom.

I suppose it would be "safe", however you may not be able to see what you are looking for, depending on just what that might be. The vent hole I mentioned is not visible without dramatic disassembly. But no need to see it anyway, because the problem would be the seals I mentioned. A couple more exhaust leak possibilities are the big exhaust housing seal ring, and the shift rod access cover plate.

Showing the parts that will blow smoke out the vent hole if bad or missing.

Okay I will inspect these seals tonight. If they look to be in questionable condition I will post pictures on here. Thanks so much for the help. These diagrams are also very helpful!

I took it apart last night and found the drive shaft spacer did not sit correctly the last time I put it together. I have a parts motor and swapped that spacer in. I also found the cause of my overheating was an impeller key that had fallen out when I was reassembling. Not sure how. I fixed those issues and I think I found the source of my leaking exhaust. The electric shift wires run through the exhaust housing and are sealed off with a thick rubber sleeve. A small portion of that was damaged (melted) likely by the overheat. To remedy the problem I slid that sleeve up the wires a little bit so it would make a good seal with "shift rod access hole". Took it to the launch last night and have no signs of exhaust leak or overheating.

I had one unrelated problem of a slight water leak in one of the two rear hoses that run to each of the cylinder heads. But I swapped those with ones from my parts motor as well.

I briefly ran it wide open last night and it seems to have just as much power as it did before the overheat, so I am optimistic that the compression is still okay. I will return to the lake tonight now that I have the water leak fixed and do a few more speed tests.

I was also curious if someone could point me in the direction of where to find an overheat warning horn to wire in....Thanks!

It does have a light...but I was hoping to add a buzzer of sorts under the dash for extra peace of mind. I also wont feel like I have to keep an eye on the light all the time.

Took the boat out again last night and it is running really well. I only have two issues left so sort out. I very slow water leak on the back of the motor where the rubber hoses go into the heads. I am thinking I can remedy the problem by replacing the old spring loaded clamps with some traditional pipe clams.

The other issue I am having might be best to start in a new thread. When I shift it into gear, it seems like something is slipping for about 10 to 20 seconds before I can feel the prop start to grab and then I'm on my way. It engages somewhat at first and doesn't seem to do it in reverse. I think it has always done that to some extent but it is getting steadily worse. I thought it may have been because I was using the electric shift gear lube but now I am.

Any likely causes? Thanks for all the help guys!

Whoa, getting the cart before the horse here. The first step in diagnosing electric shift problems should be oil inspection. That apparently has been done, so second step should always be electrical tests at the cable that goes to the lower unit. Low voltage will cause erratic or no shifting. Simple way is connect a voltmeter to each wire individually and see what the voltage is (key on, control shifted into gear). Shift the control several times for repeatability. Do not disconnect the terminals while doing the test. And never, ever connect a voltage to both wires at the same time while running. That will bust something.

I'm a bit concerned about the cable going down, from reading previous posts. Is it burned?

I carefully inspected the wires going down to the gearcase where I mentioned it had melted. They seem to be fine. I will do a voltage check today and make sure all of the connections are good. That is for the tip!

Slipping / not engaging clutch spring is fairly common.---------Need to take it apart to inspect bore of the clutch spring.---Low voltage / bad wiring could also be the problem.