Last night I did a little bit more accurate flow test. I used a digital voice recorder and called out start/stop points, then used a stopwatch and recorded the times. Last time, I used the stopwatch and just kept it in remembered the numbers and averaged them in my head, roughly.
I opened the bypass valve and filled a 1/2 gallon pitcher over and over again.
With the pump shutoff valve closed about 1/4 of a 1/4 turn (it was operating at this level with water going over the spillway) I recorded 10 fill times and eliminated the shortest and longest times, and the remaining times averaged to 4.81 seconds per 1/2 gallon, which works out to 374 GPH
Then, I opened the pump valve all the way and let the system balance out for a few minutes, and did it again. I recorded 11 times and eliminated the shortest and longest and averaged the rest, which came out to 4.15 seconds per 1/2 gallon, or 433 GPH
This is still less than half of the flow rate I should be getting.
I cleaned the pump out recently, it was pretty gunky though, so maybe I should try soaking it in vinegar to make sure I got it all clean. The shutoff valve has always ran fully open until after I installed the ATS, and it was clean when I put it all back together.
Here is the bulkhead and fitting for the overflow and return:
This is an old pic from before I plumbed the overflow with PVC
to the sump. Just FYI. Return is on the left, and when I originally replaced the return hose I had a leak that I had to double-clamp and silicone because it was dripping about once a second. I think this indicates excessive back-pressure from the pump now that I'm thinking about it.
Thinking back now, I do remember the owner saying something about how they thought the pump had lost pressure, he said that there used to be so much pressure from the jet that you couldn't put your finger into it. That has not changed since I took over maintenance. I am now wondering if I should empty out the overflow tower and dis-assemble the entire return riser, jets, and bulkhead and inspect them for obstructions. I really wanted to add a Durso standpipe anyways, so I have more than one purpose in mind...