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Below is some further history on my Daisy 790. Sometimes the hammer springs can become fatigued and require changing. I used a spring from a dollarstore spray bottle. Prefect size and compression tension. Too bad these pistols were discontinued.
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Daisy Powerline 790 – Final Chapter
Summary
This Daisy 790 gave me good refresher training on CO2 pistols and I finally understand what was creating the inconsistencies with velocity and accuracy. There was no gremlin hiding inside the valve and the seals are fine. There is no extraordinary friction points and the barrel is in wonderful condition. I’m guess that the crown is actually better than when the pistol was new. The problem turned out to be the hammer spring and possibly the valve return spring. This hammer spring may not even be OEM but I have no way to find out. In hot outdoor temperature, the valve locked up until put in the shade to cool off. Even then, the pellet wouldn’t leave the barrel unless I fired a second blast of CO2. Finally, the pellet was leaving the barrel but velocity was under 300fps. After being returned to 23C indoor temperature, the velocity would fluctuate between 320 and 360 fps. The hammer spring was tired and unable to open the valve consistently throughout CO2 pressure changes that normally happen as a powerlet slowly empties.
This pistol started as a mess with leaking seals, seized hammer and a gouged barrel. I disassembled the pistol completely, cleaned it up, replaced the seals that were leaking and chopped approximately 5/16” off the barrel to remove the 3 gouges. Accuracy and velocity testing still indicated that there was some sort of problem. Velocity would increase and decrease each day. Then, as mentioned above, the pistol locked up in the heat. Finally I changed out the weak hammer spring with one I pulled out of a broken spray bottle (correct size). The valve spring was so stiff I couldn’t press the stem in with my finger tip. I knew this wasn’t a good balance for a CO2 valve. The valve return spring was changed to a mild strength spring that I cut from an old supply. With the velocity adjuster maxed out this pistol now barks! Velocity was clocked at 485-490 fps with 8.44gr pellets for about 35-38 shots per powerlet. Loud!
Final Results
Shot Count
Shots #1-5 = 478, 485, 485, 482, 482 fps
I noticed that the velocity adjuster screw was in the middle of the range so I adjusted it back to max
Shots #6-10 = 487, 489, 489, 487, 490 fps
I moved the adjuster screw all the way out (lowest power)
Shots #11-15 = 463, 464, 463, 462, 464 fps
15 minute break
Shots #16-20 = 447, 453, 466, 465, 467
Shots #21-40 = on targets
Shots #41-45 = 466, 470, 471, 476, 473 fps
15 minute break
Shots #46-50 = 470, 467, 469, 470, 465 fps
Shots #51-63 = 462, 457, 455, 452, 449, 444, 436, 433, 426, 419, 413, 404 fps
> The velocity sometimes starts a bit low for the first shot or two after a period of rest. This happens when the bulb is still relatively full and is likely due to a pressure increase from liquid turning to gas.
> Shots #11-63 were all at the lowest power setting on the adjuster.
> Shot count for the low power setting would be about 55 shots at approx 465 fps with 8.44gr pellet. The first 10 shots above were with med and high power settings.
> The velocity was slow to decrease at the end of the bulb. Previously I was seeing an 8-12 fps drop for each shot once they started dropping. The new springs help.
Accuracy
8.44gr JSB Exact (5-shots@ 10 metres)
.28, .20, .26, .22 --- Average = .23” ctc
> The 8.44gr JSB Exact was the favourite pellet prior to the fresh springs so this is what I used for the test groups.
> The test groups were shot at the 465 fps velocity setting (shots #21-40). This is a good setting with noticeably less bark than at 490fps.
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