Read through this thread: https://www.stovebolt.com/ubbthreads/ubbthreads.php/topics/1411879/2.html regarding how true the cover that fits over the diaphragm is, as well as the size of the hole in the needle & seat valve. I had similar issues with my 964S YF on a 216, and sourced an original needle & seat set off of eBay and it's running great.
