1978 Mercury 700 70hp Overheated on the water-bad to run?

[bashunov456]

Hey Guys,

Just wanted some advise; This weekend I took the boat out, pulled off from the launch and about 2 min later the boat shut off on me and there was some white smoke coming from the engine (overheated). I took the cover off and splashed water on the engine block to cool it and troll-ed the rest of the day (didn't try to start it again). Took her home and pulled the lower unit- the water pump was shot so I replaced it with a new one. Put it back together and it started up for me with the muffs on! It sounded a little bit quieter than she usually does, almost like it was running better? I pulled the plugs and the bottom cylinder did have some aluminum deposits on it, the others looked good though.

I haven't taken it out on the water yet but plan on it this weekend to test how she runs. I ordered new spark plugs but they won't be here until Tue. My question is- is it bad to be running the engine this way ? Say the bottom cylinder did loose some compression during the overheat, would it cause further damage to run the boat as-as with just new plugs? This is an old outboard and I wouldn't consider doing any major engine work to it.

Please let me know what you guys think!

Thanks

 

[Chris1956]

Well, take a compression test and see what you have. Clean the spark plugs of all aluminum deposits. If the compression is good, run the old girl.

I will guess the lower cylinder will have very low compression, but, hopefully I am wrong.

 

[bashunov456]

Thanks for the reply. I will test the compression this weekend. Say the bottom cylinder has low compression, would it be bad to keep running the engine this way? I do not mind a little less power/bad fuel economy.

When I tested the compression last year it was 75 PSI in top 2 cylinders and 80/85psi in the bottom (this is a 3 cylinder) and it runs amazing.

 

[racerone]

Impellers are usually to be replaced every 2 / 3 years.-----Some of these motors do not have an overheat horn.-----The metal on the plug is not a good sign.-----Big $$$ damage appears to have happened.------If you can put up with lots of stalling I guess it will run for a little while longer.