This one you will all love! Months ago when my FX started lobbing pellets after 4 yrs of great service I was told all FX Impacts old to new would except a upgrade 720 plenum by Huma air. So of coarse since I decided I was going to do my own rebuild I might as well add a 720 to my old FX for some more killing power even though I know nothing about and truthfully was intimidated by working on PCP guns , but anyone who has ever talked to me or used my services knows I am well educated in all other facets of all centerfire & rimfire firearms. Well after getting all my money back from all the extra parts & shipping about 500/600..$ & me educating them at Huma air that a 720 plenum kit does not fit old FX Impacts without lots customizing work. I did not' have to put up much of fight to get all my money back, so I am not a Huma Air Hayter at all. Well Silver you was close if you don't know the 720 plenum kit has a entirely different valve adjuster with a large spring that goes between the valve body and the knob. So me being a rookie on anything that comes to PCP Impacts I assumed after my FX sitting in my gunshop after the Huma air disaster 5months ago, I thought they all have a spring ahead of the C1 WRONG!!! took that left over spring out of my valve body turned valve adjuster knob to line 4 rotated my rubber ball for a unused area, took and turned the regulator pressure to 100 bar bottle pressure to 130 bar now it shoots like it did brand new I hit a quarter at 30 yds. dead center first shot. Fine tuning for perfect harmony should be pleasure for me. And I owe it all to each and every one that posted your knowledge to help me. Sincerely yours Master Gunsmith David