Johnson 70 overheating at high revs

[corkpiper]

Hi
I have a 1984 Johnson 70HP motor which starts and runs very well with loads of water coming from the tell tale - until I increase revs and she starts to plane. She then stops taking water and I get steam. When I throttle down again she takes in water. The engine is as low in the water as I can get her and the intakes do not seem to be clogged. As I said, it works nicely with pressure at low to mid revs but when the stern rises a little she takes in air. Any info would be appreciated

 

[Faztbullet]

Post a pic of engine on transom and when was impeller changed? Just cause it
'Pees" dont mean it cooling...

 

[corkpiper]

Here you go.

 

[Faztbullet]

I suggest a new impeller and a good bottom cleaning. Growth on boat especially in front of engine can disturb water flow and cause engine to overheat .

 

[Sea Rider]

That's a 52 year oldie run in fresh or salt water to start with ? How old is the water pump ? Remove the thermostat and check its physical condition if extremely salted, rusted, seized whatever, without thermo go for a wot test run and check if went much better, if not can go from there as more troubleshooting is needed...

Happy Boating​

 

[tphoyt]

Yea a good cleaning and water pump service is the place to start.
Best

 

[corkpiper]

I suggest a new impeller and a good bottom cleaning. Growth on boat especially in front of engine can disturb water flow and cause engine to overheat .

Will do Faztbullet. Thank you.

 

[corkpiper]

That's a 52 year oldie run in fresh or salt water to start with ? How old is the water pump ? Remove the thermostat and check its physical condition if extremely salted, rusted, seized whatever, without thermo go for a wot test run and check if went much better, if not can go from there as more troubleshooting is needed...

Happy Boating​

Had the impeller out last year when I bought it and it looked perfect so I put it back in. Used only in fresh water as far as I know.

 

[corkpiper]

Yea a good cleaning and water pump service is the place to start.
Best

Thank you.

 

[tphoyt]

Looks can be deceiving!
If your not sure of the impellers age it’s best to replace it. They get brittle over time.
It’s a small price to pay for peace of mind.

 

[racerone]

Were impeller vanes straight or did they have a nice curve on them ??

 

[corkpiper]

Looks can be deceiving!
If your not sure of the impellers age it’s best to replace it. They get brittle over time.
It’s a small price to pay for peace of mind.

Sure. I'll change it. Thanks.

 

[corkpiper]

Were impeller vanes straight or did they have a nice curve on them ??

They were curved but so much that they gave no resistance to being inserted.

 

[Scott Danforth]

They were curved but so much that they gave no resistance to being inserted.

then the impeller was past its prime. impeller and rubber bits do not age by the engine hour meter, they age by the calendar

 

[racerone]

If they are bent like that the impeller is scrap.------See what a new one looks like.-----Then you can make the easy and correct decision.

 

[Sea Rider]

If the impeller springs their blades fully straight when removed from the cup, that impeller remains in good working condition, whereas with bent blades it's a shot impeller. For peace of mind & wallet install a new one in. If keeps overheating, run the motor at wot with removed thermostat and report if same...

If same under both scenarios that motor is full of fresh water buildups crap accumulated over 52 years of use...

Happy Boating​