BobI am going to try to add as much of a spacer as I can to hopefully reach at least 40cc of plenum. The issue I may run into is the short WAR tube may not allow for much if any spacer to be installed so that's my next step is to see how much room there actually is between the end of the reg and the tank dropdown block to install a spacer. if there's no room then it will be increasing the reg pressure to compensate.So if no room for spacer then I should start with as reg pressure of 2000 psi and see how it works and increase as needed. If the tuning does not go as planned then I will just mod the stock valve as much as possible and save the Cothran valve for a big bore build for the future.Thanks again Bob.
IMO a 70 cc plenum is OK for a .25 pellet shooter, but for a .25 shooting bullets, or a .30 cal, larger would be better....Bob
MikeMy build has been off and on with the tuning and working out a few bugs. I had it set at 2000 psi reg setting with a 0.190 port from valve to barrel and got 825 fps with JSB 33.95 MK1 pellets. Then it started becoming erratic and would seem to get stuck in either the on or off status. I found the thimble with the small o ring was loose in the front mount that it is secured in by a screw. The screw was loose allowing the thimble to remain in one place in the front chamber of the valve so it was operating much like a conventional valve but very hard to open or when the thimble stayed stationary and the valve cycled properly. It took a few tries to get the thimble screw to stay tight, finally worked after I used a hardened washer between the screw and brass of the thimble to prevent the brass from extruding away from the head of the screw causing it to loosen but still allow for the thimble to self center in the front of the valve so it would not bind.I just turned up the reg pressure to 2500psi ( was shooting for 2250 to 2300 psi) and got 935fps with the same 33.95s. This is also with a new TP made from PEEK with a 0.205" ID and a snug fit in the barrel seat and the valve seat adapter that reduces the valves exhaust port from 0.375" down to the 0.205 it is now. So higher reg pressure and bigger ID transfer ports. I will be lowering the reg pressure to get the 33.95s in the 860/870fps range for a final tune. So I am thinking the 2250/2300 psi will likely get me to the fps range I want for a final tune. I have so many guns that the brod does not get the attention it deserves so never really started a new thread on it.Mike
Mine has a shorter Mrod tube with an early WAR dropdown with a ninja reg and 500cc 4500psi bottle also. The Cothran valve has a .375" exhaust but I had Don make me a reducer to use with a factory breech. I also drilled the bleed hole in the stem out to 1/32" or .0625" to allow a bit more tuning.It has a .200" exhaust port and PEEK TP of .205" and .190 port in the barrel so a bit of choke there. It has a 25 cal Marmot Militia (Jim Gaska ) barrel that's 20 inches long. I am running a 26 gram hammer with a TSS so its not to hard to cock with the valve running a bit below the plateau. I have yet to shoot a full string from full fill on the bottle as well. When I had it at 2000 psi regged before the valve thimble came loose I shot around 60 or so shots and was still above the reg set point so pretty frugal with the air. I can tell you the Cothran valve is not tunable with normal methods like hammer strike or valve springs changes. It requires pressure changes to raise or lower fps numbers. So basically it releases the same amount of air with each shot so the higher the pressure the more energy released per shot. Mike
My .30 Marauder built by Lloyd Sikes has a 24" TJ's barrel, Cothran Powerhouse valve and Huma Regulator it makes 102 FPE for 8 good shots. Lloyd and Bob Sterne had a long distance relationship during the build and the fruit of their collaboration is now my favorite big bore.