"OverHot" wrote in message
om...
This one defies (my) logic! Is there something I'm missing?

We are running twin Yamaha 200s on an 8 mtr twin hull in The Gulf with
a water temperatue of 30 degrees C. This problem has only developed
over the last 5 months.

The problem is that the buzzer on the *left* engine comes on. The boat
yard has checked *everything* including the thermostat, and don't know
what's wrong!! Impellers have been changed during the last 12 months.



Your Yamahas have a pressure relief valve to increase water flow through the
cylinder block at higher speeds. It is located at the lower aft stb. side of
the cylinder block. It consists of a spring, rubber grommet, and a plastic
"valve" underneath an aluminum cover connected to a hose. If the rubber seal
is distorted or debris is caught in the valve, not enough cooling water will
flow through it and overheat the motor at high speeds. Take it apart and
inspect the components.

Another problem could be salt or debris buildup inside the cylinder head
passages. Remove the heads and water covers to clean out any deposits. In
older motors that were not flushed often, salt would also restrict
passagways in the exhaust cover and inside the cylinder block.

Transducers, thru-hull fittings, etc. can aerate the water getting into the
pickups on the gearcase, and the "foamy" water will not cool a motor
adequately. There shoud be no protrusions within 14" of a motor centerline.

This is assuming that the water pump assembly is still in good shape.

Bill Grannis
service manager