1986 20 hp Mercury water pump impeller issues

I have a 1986 20 hp Mercury. I just dropped the lower unit and swapped out the water pump impeller, the housing O ring, and the nylon seals that sit on top and bottom of the impeller. Before this the water stream was weak at best but there was water running through it. Since replacing these parts I can't get it to pump water at all. I've pulled it back down and double checked that the shaft key is installed properly, the O ring was set properly and that the connection to the tube that feeds the top end appear to be good. I've hooked it up to the garden hose with muffs and nothing. I also took my compressor and blew air from the top end to see if I had anything blocking the tube but it appears to be clear. Anyone have any suggestions on what I should be looking for next?

Not knowing what serial group your motor is, I'll ask if you've checked the thermostat yet.

The serial number is 0A 989 788. Can you tell me where the therostat is located on this engine? Thanks.

If that's a group 2 serial motor, it should be located just above and very close to the upper spark plug. Look at item 41 in this schematic. https://www.crowleymarine.com/mercury-outboard/parts/274_110.cfm

Thanks for the info JMichael. I'll hit the Tracker Boat Parts counter on the way home and see if they have one in stock. that looks a whole lot easier than dropping the lower unit again so I'll try that first.