A few things to check:Ensure the piston is clearing the top of the slot when the pump arm is fully open.With an adjustable piston and wear it can be set so that it can't intake air fully.Pump the gun 10 times and open the pump arm.If it springs past 30 degrees there's too much dead air space.Since you're counterboring the barrel then do the same to the valve.You can then use .170" ID ice maker line for your TP.Here's my 1322 Medalist pump arm with 10 pumps.
Sounds like you're making progress.The pump arm on my 1322 opened almost 90 degrees after reseal and 450 FPS.There was a lot of dead air space with the fixed plastic piston.How far the pump arm opens is an indication of how efficient the gun is.Ideally all the air from each pump should go in the valve.In reality there's always some dead air that acts like a spring.Adjust the pump arm so you can just feel the piston touch the valve with about 1/2" before the arm fully closes.Replaced the piston and barrel support on my 1322.Bumped up velocity some.14.3 CPHPs, 18" barrel:05 pumps 439 FPS 06.12 FPE07 pumps 515 FPS 08.42 FPE10 pumps 604 FPS 11.58 FPE12 Pumps 650 FPS 13.41 FPE
Past experience with these guns has led me to a better way to tighten the breech screw, so it stays tight. After starting the breech screw,, place the comp tube / breech assembly, lengthwise, in a padded vise, tighten the vise. THEN tighten the breech screw. You'll be surprised at how much better the breech screw will hold. I believe it has to do with being able to fully compress the transfer port seal, with the vise, versus trying to compress it with that tiny screw.
Hi Ron, My 1377 with GMAC FT internals goes past the 30 degrees after pumping tem times and releasing the pump arm.Is that harmful ? inefficient ?How do I rectify it ? ManishQuote from: Tack Driver 10 on April 29, 2021, 10:16:56 AMA few things to check:Ensure the piston is clearing the top of the slot when the pump arm is fully open.With an adjustable piston and wear it can be set so that it can't intake air fully.Pump the gun 10 times and open the pump arm.If it springs past 30 degrees there's too much dead air space.Since you're counterboring the barrel then do the same to the valve.You can then use .170" ID ice maker line for your TP.Here's my 1322 Medalist pump arm with 10 pumps.