I have a 91 searay with twin merc 350's. One motor was replaced 6
years back and the other I have replaced this year. After putting the
boat in the water this year I realize I have an overheating problem
with the brand new motor. I built it and have built many. Everything
was replaced on the new motor, manifolds, water pump. the only thing
that hasn't been replaced was the impellers in the lower units. I
didn't figure I needed to replace these since they didn't have 10
hours on them the year before when the engine went. The engine failed
because the manifolds let sea water in. Here is the question. What
kind of flow should I have coming up the lower units. I can fill a 5
gallon bucket in no time. No one I have talked to yet can give me a
definite answer. I think when you rev the engine the exhaust pressure
is overcoming the water pressure and causing the engine to begin to
overheat. It will idle for ever and the gauge never moves, but give
it a load and the temp climbs fast. Does anyone have a pressure spec
that the pump is going to have to put out?

wrote in news:1184463914.393105.183010
@n2g2000hse.googlegroups.com:

I have a 91 searay with twin merc 350's. One motor was replaced 6
years back and the other I have replaced this year. After putting the
boat in the water this year I realize I have an overheating problem
with the brand new motor. I built it and have built many. Everything
was replaced on the new motor, manifolds, water pump. the only thing
that hasn't been replaced was the impellers in the lower units. I
didn't figure I needed to replace these since they didn't have 10
hours on them the year before when the engine went. The engine failed
because the manifolds let sea water in. Here is the question. What
kind of flow should I have coming up the lower units. I can fill a 5
gallon bucket in no time. No one I have talked to yet can give me a
definite answer. I think when you rev the engine the exhaust pressure
is overcoming the water pressure and causing the engine to begin to
overheat. It will idle for ever and the gauge never moves, but give
it a load and the temp climbs fast. Does anyone have a pressure spec
that the pump is going to have to put out?



Are you absolutely sure the spark and valve timing are correct throughout
the range, not just at idle? I'm sure we're sure the head gaskets are
fine, right?

Got a pressure guage to measure cooling water pressure at speed? An
outboard runs about 35PSI at WOT against the thermostats. If there's
that much backpressure on the exhaust, I'd wonder why.

Larry
--
While in Mexico, I didn't have to press 1 for Spanish.
While in Iran, I didn't have to press 1 for Farsi, either.
It just isn't fair.

On Jul 14, 10:45 pm, Larry wrote:
wrote in news:1184463914.393105.183010
@n2g2000hse.googlegroups.com:



I have a 91 searay with twin merc 350's. One motor was replaced 6
years back and the other I have replaced this year. After putting the
boat in the water this year I realize I have an overheating problem
with the brand new motor. I built it and have built many. Everything
was replaced on the new motor, manifolds, water pump. the only thing
that hasn't been replaced was the impellers in the lower units. I
didn't figure I needed to replace these since they didn't have 10
hours on them the year before when the engine went. The engine failed
because the manifolds let sea water in. Here is the question. What
kind of flow should I have coming up the lower units. I can fill a 5
gallon bucket in no time. No one I have talked to yet can give me a
definite answer. I think when you rev the engine the exhaust pressure
is overcoming the water pressure and causing the engine to begin to
overheat. It will idle for ever and the gauge never moves, but give
it a load and the temp climbs fast. Does anyone have a pressure spec
that the pump is going to have to put out?

Are you absolutely sure the spark and valve timing are correct throughout
the range, not just at idle? I'm sure we're sure the head gaskets are
fine, right?

Got a pressure guage to measure cooling water pressure at speed? An
outboard runs about 35PSI at WOT against the thermostats. If there's
that much backpressure on the exhaust, I'd wonder why.

Larry
--
While in Mexico, I didn't have to press 1 for Spanish.
While in Iran, I didn't have to press 1 for Farsi, either.
It just isn't fair.

Yes valve timing is fine and the head gaskets. Yes I do have a
pressure gauge to measure water pressure. I don't have 35 psi against
the thermostats. That is the spec that I was looking for. All of the
exhaust pressure is coming from the hoses from the manifolds and no
the engine. Thanks for the help.