ORIGINAL: 2slow2matter
I'd bet air leak--around the backplate as suggested above. I have two of these, and on one, I can't keep the backplate tight for over 10 or 15 flights....
Agree. It could also be a leaking cyl head (check the head bolts), & as noted by Cyclic Hardover, it could be the air bleed screw backing out, or accidently opened.
If you can find one, the metal backplate from the old OS 40 FP will fit your LA & it will peremanently cure the loose backplate problem