For a pressure controlled system Merc uses a spring loaded poppet valve.
When pump pressure comes up the poppet is pushed open and allows for greater
water flow. The thermo itself just doesn't flow enough water to keep things
cool at WOT.

If I remember right, the poppet is located in the same housing as the thermo
on your engine (it has been a while since I've seen a 75 Merc and at my age
memory is not always my friend). The other place Merc puts the poppet is at
the bottom of the block, stbd side, just forward of the head.

I agree with you, the setup has always struck me as unusual, but then again
it does work well.

Ken

"Andy C" wrote in message
...
thanks ken,
yes, the hose is on the back of the thermostat housing as described, so i
guess this is normal... i thought there would be some sort of bypass to
keep
the telltale going all the time the engine is running
it just seems like a strange set up... presumably in very cold water the
thermostat could open and close constantly and you would not be able to
tell
if the water pump was working or not...

It should, though, discharge
some when you rev them while cold. The system is also pressure based
which
should override the thermostat system.

i don't quite follow that, are you saying the water pressure will force
the
thermostat open above idle ?

thanks !
andy

"Ken" wrote in message
...
A lots of Mercs and Mercury-Force place the tattle-tale after the
thermostat. You can actually trace the hose to the back side of the
thermostat housing. With those setups the thermostat does have to open
before it will discharge a stream while idling. It should, though,
discharge
some when you rev them while cold. The system is also pressure based
which
should override the thermostat system.



"Andy C" wrote in message
...
mercury force '97 75hp
hi there,
does anyone know if it is normal for there to be no flow from the
tell-tale
until the thermostat opens ?
there doesn't seem to be any cooling problems but it seems weird for
nothing
to come out the tell-tale for the first 2 or 3 minutes... the local
dealer
thought this was normal but didn't seem sure...
many thanks for any comments !
andy