U really shouldn't glue the plumbing inside the overflow box. If it isn't glued what I would do as a test is turn the 90 on the open channel up to where it isn't receiving any water.
Set the water hieght in the overflow box to where the open channel is only taking a trickle of water. So basically around 1/8" to 1/4" above the bottom of the bulkhead. In your first picture it looks like the water level inside the overflow is about halfway on the bulkhead which is to high. It needs to be closer to the bottom of the bulkhead.
The slots in the syphon downturned 90 will end up causing more issues then they are worth on a setup like yours. U can keep the one on the open channel but I would replace the one on the syphon. I would also add a short piece of pipe to the downturned 90 on the syphon to where it is only 1/4" to 3/8" off the bottom of the overflow box.
Your main issue is the water level in the overflow box needs to be just above the top of the slots in the 90 for the open channel to be quiet. The problem is when u set the water level at that hieght the syphon will suck air because of the slots.
For a temporary fix u can cut a piece of pipe just long enough to fit so u can slide it into the 90 on the syphon. For example, if the bottom of the 90 is 1" from the bottom of the overflow box, cut a piece of pipe 1" so u can fit it to slide it into the 90. It would essentially be removing the slots. Then see if u can tune it in. If u can then u know u only need to replace the 90 because of the slots. If u still can't quite get it tuned u will know u need to replace the 90 & the ball valve with a gate valve
My internal 90's are glued, was under the impression everything needed to be glued. And unfortunately my bulkheads are slip slip so they need to be cut out and the plumbing from below the T's and up needs to be replaced. Will use threaded bulkheads moving forwards!
We are on the same page...I can't turn the open channel 90 but I can block it off! I covered it with a baggie and elastic band and allowed the overflow box to fill and turned the emergency channel into the open channel. My syphon channel worked! No bubbles. And barely any water coming out of the "temporary open channel". The water was nearly at the top of my overflow box as my emerge 90 is about 3/16" below the box top. So no air leaks in the syphon channel. That tells me (I think?) I was not getting enough volume into the syphon for it to properly purge, correct??? With the overflow box fuller I had more water entering the syphon and it self purged.
Based on the above test, I don't know if water at the height you suggest will work? I may need more for it to purge?
Yes, the slots are trouble. If I have more height in the overflow box the teeth won't be an issue. I am trying to come up with a way to cover the slots without having to completely cut out the top 1/3 of that channel (all glued). I have thought of your idea about sliding a piece of pipe up into the 90. With 3/8" clearance I'm not sure if it will be successful or not. I may have to slide 3 pieces on top of each other. I will try this weekend.
The bottom of the 90's are already about 3/8" off the bottom of the overflow box, however, the teeth create a problem.
***My thoughts moving forward: to replace the open channel 90 with a threaded bulkhead and then turn the 90 sideways to essentially be able to raise the overflow box water height before it enters the open channel. This will have more volume enter the syphon and hopefully purge itself. This would be similar to having raised the open channel bulkhead say 1/2" like many do, but I followed the original plans. I recognize this is opposite to what you suggest as far as overflow box water height. On second thought, my first step should be to try to slide a piece or 2 of pipe into the bottom of the syphon 90 to block the teeth and try to get it to full syphon.
Potential problem with turning the open 90 sideways: if the syphon gets blocked will the open become a full syphon or will it suck air from the surface of the water like a vortex? My guess is it will syphon but this will be easily tested by turning the 90 sideways and blocking the syphon to see what happens. Even if it doesn't full syphon, I think I am still covered due to the low water flow of 550gph (probably less than the charts due to friction etc). I would have 2 open channels (open channel plus the emerge to handle 550gph).
Your thoughts on these comments? My apologies for being lengthy but my wish is to be through and hopefully provide enough info to answer in advance questions you may have. Thank you.