Overheating 1970s 20hp Johnson seahorse

Hello, First thread so forgive me if I do it wrong

I have a 20hp Johnson Seahorse around the 1970s thats overheating. I have replaced the impeller, and when the foot was off, I spun the shaft and it was pumping water like crazy! Pees great! and still overheating... any help?
Thanks,
Thomas

Re: Overheating 1970s 20hp Johnson seahorse

Was the original impeller complete when you took it out, meaning, were there any missing chunks off the vanes?

Re: Overheating 1970s 20hp Johnson seahorse

Was it deep enough in the bucket to prime? These are not self-priming and MUST be submerged an inch or two over the pump bulge to start pumping.

Re: Overheating 1970s 20hp Johnson seahorse

oldcatamount said:

Was the original impeller complete when you took it out, meaning, were there any missing chunks off the vanes?

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I am not sure if it was the original one, but yes. the one that was in there was complete, not broken off.

Re: Overheating 1970s 20hp Johnson seahorse

Chinewalker said:

Was it deep enough in the bucket to prime? These are not self-priming and MUST be submerged an inch or two over the pump bulge to start pumping.

Click to expand...

Yes, it was. It did not over heat in the bucket. When we took it out and ran it for about 10 minutes is when it overheated.

Re: Overheating 1970s 20hp Johnson seahorse

Hi tombob14. I'm right down the road in Tallahassee and have a '66 20 hp myself. When you way "overheat", what exactly is happening? Is the motor shutting off after running for 10-15 minutes, or does it stop pumping water and "blow up"? I suspect, you may actually have a faulty ignition component that is getting hot, failing and causing the motor to run on only one cylinder, or die altogether. After the the motor and the component cool, they can then be used again for another 10-15 minute interval. Btw, what fuel/oil mix are you using? It should be 50:1 (about 2.5 oz of oil to 1 gallon of gas), but some folks run them a little richer to be safe. Also, what year is your motor? That will tell us if you have the much loved, old school OMC universal magneto ignition, or one of the "improved" electronic ignitions they came out with beginning in the 70's. Go Noles!

Re: Overheating 1970s 20hp Johnson seahorse

How did you check for overheating ?? What's the compression on it ??

Re: Overheating 1970s 20hp Johnson seahorse

kfa4303 said:

Hi tombob14. I'm right down the road in Tallahassee and have a '66 20 hp myself. When you way "overheat", what exactly is happening? Is the motor shutting off after running for 10-15 minutes, or does it stop pumping water and "blow up"? I suspect, you may actually have a faulty ignition component that is getting hot, failing and causing the motor to run on only one cylinder, or die altogether. After the the motor and the component cool, they can then be used again for another 10-15 minute interval. Btw, what fuel/oil mix are you using? It should be 50:1 (about 2.5 oz of oil to 1 gallon of gas), but some folks run them a little richer to be safe. Also, what year is your motor? That will tell us if you have the much loved, old school OMC universal magneto ignition, or one of the "improved" electronic ignitions they came out with beginning in the 70's. Go Noles!

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I could not find a year on it. How do I tell? It runs for about 10 minutes and starts smoking, get really hot, slows down, starts to choke and then dies. I am not sure to the fuel/oil mix we used. (I did this with my dad who is very busy this time of year.) I think it was the magneto ignition.

Re: Overheating 1970s 20hp Johnson seahorse

:facepalm: My dad told me we did NOT change the impeller. We just looked at it and turned it over so it would be bent the other way. :facepalm: I am pretty sure that would thus be the cause. Thanks for everyone's help!! :joyous:

Re: Overheating 1970s 20hp Johnson seahorse

That could definitely do it. The only way to be sure is to drop the lower unit and inspect the impeller. You can get a new one right here at iboats for about $15. If you look on the transom clamp, there should be a small metal tag riveted in place with both the serial and model numbers. It should start with FD-xx. If you don't see any external components/coils bolted to the power head, it is probably a magneto ignition, which is much easier to fix/maintain. Here's a great link to read in the mean time that may be of some use. In the mean time, mix up a fresh batch of fuel at 50:1 using regular 87 octane gas and 2-stroke outboard motor oil. You'll also want to use Champion J4C or J6C spark plugs as well. I use J6C in mine without issue.

Maintaining Johnson E & FD Serie

Re: Overheating 1970s 20hp Johnson seahorse

kfa4303 said:

That could definitely do it. The only way to be sure is to drop the lower unit and inspect the impeller. You can get a new one right here at iboats for about $15. If you look on the transom clamp, there should be a small metal tag riveted in place with both the serial and model numbers. It should start with FD-xx. If you don't see any external components/coils bolted to the power head, it is probably a magneto ignition, which is much easier to fix/maintain. Here's a great link to read in the mean time that may be of some use. In the mean time, mix up a fresh batch of fuel at 50:1 using regular 87 octane gas and 2-stroke outboard motor oil. You'll also want to use Champion J4C or J6C spark plugs as well. I use J6C in mine without issue.

Maintaining Johnson E & FD Serie

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Thanks so much!

Re: Overheating 1970s 20hp Johnson seahorse

When you get in to doing a new impeller if the vanes are broken make sure you account for all of them.