I wouldn't do anything to mod it. I would thoroughly clean the pump even the slightest build up on the venturi will make the performance drop. BY raising the water and forcing it to skim most likely your reducing the skimming performance. I would lower it and see if you can get it to produce foam sound like your overpower the skimmer with things your doing. I know lowering it may seem counter productive but once it kick in you can then tune. If it doesn't then something is wrong that needs to be fixed, so any mod will only mas the problem and you will never know how well the skimmer will work.Yea - I have no complaints about the density of the skimmate, just the unorthodox methodology to get said skimmate. I shouldn't have to have the slider beyond the normal limits to get just a basic skimmate.
I'll try extending the slider this weekend with PVC, and see if I can get more control over it. It's just annoying that it was working the nuts and now it's barely working.
Ugg!!
I'm now considering a DIY gate mod to solve the problem once and for all....