1987 70 hp yamaha outboard

[znebel]

Help 70 hp 1987 yamaha overheat at low rpms
Oil breather. Thermostat. Thermo switch changed. jb welded a small leak near zinc anode at cylinder block.. Water impeller and water housing chnaged
2000 rpm or more runs good
Trolls at 850 to 900 rpms
Compression. ‪90 90 95‬
Top speed 38 mph
Buzzer does not work
Old impeller had all of its teeth so no parts in engine.
When at home testing with muffs or in large bucket of water. Engine does not overheat. Rpms around 2200.


How do I fix this problem. Overheat if less than 1800 rpms. seems when under load will overheat in 10 minutes.

 

[racerone]

The impeller in the picture needs to be replaced.

 

[Sea Rider]

To start with, fresh or salt water usage for that 33 year oldie motor. According to the OP already changed the impeller,right ? Compare both pics of a Yam 70 against a Tohatsu 18 middle legs, both are awful looking, full of debris, salt, filth you name it.

If after having spent so much time in hand labor along a significant amount of cash with no avail, that motor needs urgently a meticulous water passages clean out located on the cylinder head, thermo area, exhaust covers and lower crankcase, install new gaskets, torque to specs and voilá.

When you say the motor overheats on a large bucket of water, the overheat sensor triggers and lowers the rpm the motor was running at, what about at open water, does same ? Don't expect for any motor running that filthy to cool properly, doesn't matter if having installed a ton of new parts....

Happy Boating​

 

[znebel]

Ok thanks

 

[Sea Rider]

Ok thanks

Check the following link : https://forums.iboats.com/threads/why-is-the-ob-overheating.740657/

Happy Boating​