1989 Johnson 120Hp V4 Looper Overheating Problem made worse

I had asked a question earlier this year about my overheating problem. I got a water pump repair kit and installed it. UNFORTUNATELY FOR ME, when I re-assembled the lower unit, I "missed" putting the water pipe into the rubber grommet on top of the water pump. I drove my boat for about 3-4 minutes and shut it down when I noticed there was no pee stream. By that time I think it was too late. One cylinder got very hot and even melted the small plastic connectors for the rubber hoses, the heat gauge and the thermostat. There wasn't any fire but there was smoke from the melted plastics. I let it cool down for an hour and then tried it again. It started right up so I thought I was lucky and didn't seize a piston. I had to wait for the low tide to get at and change/fix the water pipe connection, and then when it was high tide again I tried it. It turned over but was very had to turn over even with a battery charger connected. I don't have a clue what to do now.
I'm "guessing" that I may have seized a piston ring but I need some help here.
Many thanks in advance to the guys who give me advice on here. I really appreciate it

The warning horn doesn't work?

Remove spark plugs and check the compression. What are the readings?

Metal on any of the spark plugs?

Hi Joe, Thanks for getting back so quickly!
No, the warning horn doesn't work and I have no idea why. I've checked all the wires and made sure they had a good ground. Maybe the horn itself is bad.
I have to wait until tomorrow to get a compression tester from the garage and then I'll post the readings.
I didn't see any metal on the plugs but I did pour about 1/2 cup injector oil directly into the "hot" piston to make sure it wasn't seized.
I'll post the compression results tomorrow
Thanks
Robert aka soapstoneartist

Well, I didn't get a compression test done as I had been busy all day. BUT some good news is the engine was turning over like it did when it was trying to start even though I had the plugs out. I'm going to try it tomorrow morning as it's sitting on a beach and when it's high tide the engine will be in the water. I feel confident that it will run. IF not I'll post the compression results as I will get the gauge and write down the results

OK the turning over was just a tease. I got a loader and brought the boat back to my yard. Got the compression test done and here are the results:
Starboard side top cylinder (where it overheated and cooked the plastic water hose clamps) max pressure was 90.
Stbd bottom only a bit better at 100.
Port side top was the best at max 150 and
port bottom tested at 130.
So what does this tell you?
I have the Johnson service manual for this engine from OMC too

Pull the starboard head off and have a look in there .-------Post pictures if you like.

If those compression readings are right, you have a difference between cylinders of 20 psi on the port bank (2 & 4).... and a difference of 60 psi between the port and starboard bank (1 & 2).

Hopefully that's just due to head gasket failure but it could be worse.

Best to remove both cylinder heads in order to inspect the gaskets, gasket sealing surfaces (heads/Block), pistons, and cylinder walls.

Let us know what you find.

OK Thanks I'll do that. I was looking at the pictures in the service manual and cannot figure it out. They look like 4 cylinders all in a row similar to my Toyota engine, But this is a V4 and the spark plugs seem to be on an angle. Does this make any sense? I'll start taking the heads off and see what happened in there and post some pictures after. Once I take the heads off, won't I need a new head gasket? Every engine I've worked on, you are advised to replace it What other parts are needed IF it's a piston or ring problem? I'm thinking I need to order a complete one side.... 2 pistons, 2 ring sets, and a new head gasket

You start by taking the head ( starboard ) off.----------------Examine what you see there.----------Then decide what needs to be done.---------If there is piston / ring / cylinder damage then the whole motor block will need to come apart.----Yes you will need new gaskets if the motor is going to be re-assembled.