1987 70hp Johnson overheat problem

[vinniesol]

Hello all,

Have a 1987 Johnson 70hp with a tune-up completed on it a few months back. Ran it the other day and was going along great and suddenly, at full speed, the overheat horn came on. I throttled back to neutral immediately and the horn went off right away. Water was coming out of the tell tale hole the entire time the engine was running. (I always check). Brought the boat back to the dock at slow speed but engine is not right. Idle is rough and has a skip while advancing throttle. Also notice more bubbling out of exhaust vent just below water line (not sure if that is normal). Any suggestions on what to check next?

Thanks

 

[psteurer]

Re: 1987 70hp Johnson overheat problem

Did the turn up include a water pump replacement? The fact you have water coming out of the tell tale is a good sign but it does not mean the water is cooling the engine. Could also be a bad themostat.

 

[vinniesol]

Re: 1987 70hp Johnson overheat problem

Thank you for your reply. No, the impeller was not changed as part of the tune up. If water can come out of telltale hole strong while thermostat is stuck, sounds like that is the issue. I'll change both impeller and thermostat. What about it running bad? Did I possibly blow the head gasket?

 

[vinniesol]

Re: 1987 70hp Johnson overheat problem

Also, I noticed the flywheel was much hotter than the rest of the motor after getting it back to the dock. I don't know if that helps.

 

[vinniesol]

Re: 1987 70hp Johnson overheat problem

Just an update. Ran a compression check and all cylinders are within same range around 90psi. Still runs rough at idle while getting spark on all three cylinders. Can anyone help with this?