Engine slows then dies at WOT- 1960 18hp Johnson Seahorse

Guys,
1960 18hp Johnson Seahorse-pull start/electric start-tiller model # FD-FDL14. I got a new carb kit, all hoses, and plugs done about a year and two months ago. Ran/idled fine then about 2 months ago i hit a log while WOT down in the bayou. Then the following is what's happening:
Motor on initial start takes a couple of cranks with the choke out, and then will idle fine. I can get full rev in idle, but once i idle down to shift into forward the motor dies. I can get it to start while in forward gear, but sometimes it takes a few tries. Once I'm up and running i can go for a long while on WOT, but sooner or later the speed decreases from 24mph slowly decreasing to 12mph then eventually the motor dies. It does not spit or surge when this happens, it just does as described. After that happens i have to give it a good while (20-30 min.) of rest before i can restart it. I hope this is enough info, if not then let me know what else you need to help diagnose my problem.
Thanks as always,
Adam

Did you check your coils when you worked it over, if they are cracked you need new ones. Sounds like you might be losing fire on one then the other as they heat up, then once they cool down they start working again.

First and foremost - I hope you are running this engine on a 24:1 mixture. All OMC engines prior to 1964 required a 24:1 mixture. Does not matter how good the oils of today are in comparison to "back then".
Give me a time frame on how long it takes to go from 24 to 12mph. When you let off the throttle finally at the 12mph mark does it shut off? Feel like there is a brake of some sort on the flywheel when it shuts off?
What have you checked? Compression should be checked and gearcase oil should be checked at this point. Stick your head back there at WOT and see if there is ample cooling water. Run with the cowling off when this is happening and when it does and the engine has shut down place your hand on the top of the cylinder head and see if you can hold your hand there for a few seconds. Pull your plugs when this happens and take a look at plug color and let me know what that looks like as well.

bbstacker1 said:

Did you check your coils when you worked it over, if they are cracked you need new ones. Sounds like you might be losing fire on one then the other as they heat up, then once they cool down they start working again.

Click to expand...

this: ^^^^^
had a similar thing happen. ran fine untill it got warm and then started acting up. it was one of the coils that was causing the problem.
would definately check them.

A hot coil failure is usually immediate. His statement is that it slowly goes from 24mph to 12. Even if it was coming and going the engine would be surging which was not mentioned. Who knows if it has been described accurately though!

A little more info on my first post.

The reason I mentioned this, and it may not be the same, is I told a boat repair shop that I was running up a river and the motor just died, not shut down, but sounded like it was loading up and just wouldn't go back to full power. started twisting the throttle back and forth, from full to idle, and it would take off again, and then go back to no power. finally it wouldn't go to full power at all.
they charged me $200 to work on it, and when I took it out it did the same thing, took it back, and that's when they found it was a coil, another $110.
they said, you didn't tell us it was doing this when it was hot. what the heck do they think the motors doing when I told them I was running it up the river; getting colder ?
worked fine after a coil was replaced, but haven't gone back to them since.

First off let me say thanks to everyone. Now to answer some questions:
Pappy- I've been running 50:1 on that engine for the last 8 years. That was told to me by one of the trusted mechanics at the Art's Marine here in North Little Rock, AR. Although, I would be more than willing to experiment with the oil ratio.
My throttle does not have a spring back, stays in whatever position of throttle i turn it to. It will usually begin to decelerate after running WOT for about two minutes, then will slowly decrease in speed. The throttle position does not change i've got it wide open. Once the motor does finally die it is not with a surge, it's more like how an engine dies when it's flooding. As for compression i plan on checking it this weekend time permitting.
She's spitting water fine at WOT, I changed the LU oil and it was still good in color although it did seem to be a bit cloudy, plugs are not black they are more light brown in color. I will have to check the cylinder head next time i'm out. As for coils, what is a way to tell if they have gone bad? I do fancy myself to be somewhat mechanically inclined, but I've been reluctant to tear apart my boat motor. I do have the original repair manual for reference.

Thanks for all your help,
Adam

I concur with what Pappy said earlier about coils they usually just cut out when they go. But it would not hurt to check them if you haven't, it is easy to do. Pull your recoil starter assy off, then you will see the top of the flywheel, there will be an inspection plate on it. Pull the plate, then look at the coils thru the inspection hole, if they are cracked they are bad, if they are not cracked they are probably good but you should still get a spark checker and check you fire, you should be able to jump 1/4 inch at minimum.

Another vote for coils. The vet I used to work for had that same motor. It did the same thing. Coils.

Thanks, will wait for the compression numbers. Based on what you are telling me it does not sound like ignition. If you are still running the pressure tank have you tried pumping the plunger on the tank when the issue occurs?

found one obvious problem. the choke will get stuck in full when engaged and the valve door will not open once choke is released. apparently there is a spring or metal arm missing (will fix that mcgyver style for now). I used a thick metal key ring that i straightened out. See pic
The plugs did appear to be somewhat fouled due to this issue, will put new plugs in.
tried to pull off flywheel to get a better look at coils, points etc., but apparently you need a special tool- flywheel puller? I looked through the inspection port and coils looked brand new absolutely no signs of wear. Everything else looked good too.
I did notice when i turned the flywheel there was some grinding/friction noise coming from the LU. I will change the oil in it and see if there are any pieces, if so then i will drop entire unit for further inspection. Oil looked fine, not exactly clear but doesn't appear too bad. I use Penzoil Marine Gear lube which has a slight green tint to it. See pic.
Compression test- upper cylinder at 107psi, lower at 105psi. So that looks fine. It did look like there was some moisture on the plugs and compression tester once removed from the cylinder. Is that possible or am i confusing little beads of moisture on the ends for oil?
Off to the lake I'll let you know what happens.
Thanks again everyone.

Just got back, and still having the same problem. The motor did start right up after fixing the carb choke. I quickly got WOT and shortly (45 sec-1 min. WOT) the engine begin to decelerate. This time it did not stay at 12mph. The speed got down to 8mph and then would go back to 11/12, then back to 8 mph. I shut the engine down and fished for a while (hour or so), and then prepared to leave. Motor at WOT would only get 8 mph, then it gradually built to 13 mph by the time i go to the ramp.
I did like Pappy asked and put my hand on the cylinder head after WOT and burnt the ever loving crap out of my hand. Got two nice blisters! The motor was still spitting water fine though.
I've got the boat back in the garage at home and i'm definitely hearing a grinding noise when i turn the flywheel by hand. It sounds like it's coming from the lower unit. I'm wondering if the impeller is messed up. It doesn't sound like metal on metal. It almost sounds like the water pump pumping and maybe something hindering it.
I forgot to answer one question that was posted concerning the gas tank. It is not the original metal one, it's a 6 gallon standard red plastic one with primer bulb that i've had on it for the 8 years i've owned it.

i had the same issue with my 56 evinrude and it turnd out the nuts for my high and low speed where not tight anuf and where leaking fuel so it would run out of fuel at wot so its worth checking out

Well you have at least two issues so far. First issue. Your gearcase is full of water and oil. That milky oil is that color because it is mixed with water.
Another related issue. Water pumps do not grind. Pull the lower plug in your gearcase and see if there are any metal shavings in the oil coming out of it.
Second issue. Your engine should actually cool down a little once at planing speeds. If you burned your hand your engine is overheating big time, regardless of visible water stream and I would be looking at that as your main power issue. Would be interesting to see what your compression numbers are when the issue is occurring. I suspect they are much lower. You may even be able to feel a difference if you try to start it immediately. Be honest on this answer so no time is wasted on troubleshooting.
You mentioned water droplets on the plugs? If you are getting water in your cylinders this can be a cause as well but the normal pattern on the plug is that they will be very clean looking.

who pooted, On your last post you said you have had your fuel line (tank to engine) for 8 years, check to make sure you are not sucking air at one of your fittings, change the o-rings and check to make sure your clamps or tie straps are tight.
You might get lucky with the noise you hear in the lower end, I have a couple of different engines that you can hear the impeller squeaking on when you turn the engine by hand out of water, but the foot oil does look like you are taking in water.

Good point however the engine should crank hot with a fuel line issue....and that engine is running way too hot.
Suggested he pump the plunger on the two line tank and hopefully he has tried pumping the primer bulb while the issue is occuring. If not.....it may be worth a try.

now that the "VERY HOT" has been found, I would assume that it has more to do with that than anything.
any engine that is overheating, piston to wall clearance is diminished, and at a certain point will seize.
Don't think I'd want to run that motor too much untill problem found, and fixed.

Concerning the LU oil. I agree that i must be taking in water. I'm going to order some seals and new bolts to remedy this. Perhaps when i hit that log it knocked something loose and caused water to seep in.

Gearcase plug- Pappy where might i look for this? I'm sounding a little green here, but your not talking about the oil in the LU are you? When i drained it there were no shavings, just milky.

Compression- did you want me to pull the plugs and check compression after the deceleration problems? I would be more than happy to give you an honest answer on that next time i'm out. Trust me pride is not the issue here, my motor is.

Spark Plugs- I put new ones in on my last outing and they are burning clean. I just inspected them and i definitely have water in the lower cylinder. Those "droplets" i was suspicious of appear to be water. I noticed some liquid on the plug in the lower cylinder so i tapped it against my hand and the liquid that came off didn't smell of gas or feel like an oil mixture. I firmly believe it's water. CRAP!!!!!!!!!!!!!!!! How does this happen? How do i fix it?

Thanks again! - Adam

I forgot to respond to the gas tank issue.
I religiously replace the fittings to the tank and motor every 6 months. I get a new hose/bulb every 2-3 years. Pappy I did try pumping the bulb when i was having my problems, but it did nothing.

Concerning the water in the lower cylinder head- Can water get in there if there is water in the gas tank? i rarely go longer than 1-2 months w/out running my motor. I always use marine sta-bil and fill my tank full for any amount of time the boat will be sitting.

Nope, with the engine overheating I would suspect you are leaking water into the lower cylinder via the head gasket. Pull the head and check. Buy a new gasket first. Check the cylinder walls for vertical scuffing while the head is off. Put each piston at BDC and look at the circumference of each cylinder. Let me know what you find.
On the lower unit. At this point just fill it up. In order to properly find the leak it should be pressure checked. Lets see how the rest of the engine looks before you repair that issue.