moedip - Yup, that's what I thought. I tried getting one of these screens out when I did my injectors (I removed the injector lines for the job). Used a wood screw. I guess it was not thin enough, because it grabbed the side of the wall, and I did not want to ruin anything on the distributor, so I stopped. I'll try it again.
I bought some dental picks the other day - one of them looks like it would be perfect for the job - sharp point, yet a small curve at the end, so it could grab below the screen.
Whish these things could be ordered as spares. Does not seem so, unfortunately.
I think you are most likely correct about this being the problem. I can't wait to go in there again this afternoon and checking for those screens...