Water circulation issue.

[Bob in Tennessee]

1977 Mercury 500
Two weeks ago telltale pipe was clogged with mud wasp nest. Stuck a wire up tube and cleared. Water was pumping. Got to house and covered tube with tape. Went yesterday. Removed tape and no water was flowing. Tried the wire through tube but still no water.
Here are the things I did after returning home. Dropped lower unit and blew air into copper water tube. No air at all went through tube. Dropped lower cowling to access tube. Removed tube from block fitting and blew out. No obstruction. Air flowed easily. Blew directly into block fitting and could hear air blowing out. No air was blowing through copper water tube. Tried blowing up through water tube again with air hose taped to tube to prevent air escape. No air at all blowing through tube.
Any suggestions?
thanks

 

[racerone]

Testing on a hose ?-----In a bucket with water 8" above impeller?----Or in the lake ?

 

[Bob in Tennessee]

Testing on a hose ?-----In a bucket with water 8" above impeller?----Or in the lake ?

Tested on the lake. No water. Tested at home with muffs. No water. At this point, I brought in shop and dropped lower unit to try compressed air. Cannot get air flow through copper tube and I'm wondering what I should remove next....or if I'm even on the right track. Water pumped fine a few weeks back.
thanks

 

[Bob in Tennessee]

I'm wondering if I need to remove my exhaust manifold cover.