drop the LU and spin the shaft to see if water is pumped properly by the impeller
if ok ….. must be clogged above was the old impeller chewed up … maybe parts went up in the engine circulation
the fact that water does come out at the beginning may indicate a loose piece of something (rubber) flopping around inside the water circuit
try hosing some water up the water tube ………. while the LU is down for the above impeller test .
see if water makes it way out if not it must be plugged up somewhere inside the head
it may be a wild shot but try to pressurize hose water backwards (worse case would be to pull the head and back flush the PH with a water hose )
run a piece of stiff wire up the water tube with the end shape as a hook …. maybe you can grab something that is blocking the upper end of the water tube
you could ….try a high pressure washer to blast the water circuit … maybe the clog will work itself out if pushed enough
all suggestions… up to you now
btw… did you look BELOW the water pump housing….for clogs , rubber parts which may have dropped below in the cavity . Year ago my Merc 50 drove me nuts till we found some rubber part " below the wp housing. never did figure out how it got down there
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