Career/Sumatra help needed

[tinbum]

Could you attach a picture of the tiny poof please.

Don't you think you've caused enough trouble?

[Jonjon79]

Don't you think you've caused enough trouble?

Um, er, yes maybe ...... I retract that last request

[tinbum]

The good news is its kept its air overnight!

I've removed the adjuster nut so the whole valve-stem is exposed and increased the preload even more.

No improvement!

So its not the power adjuster, the valve does work with a wallop from a proper hammer, and works minimally with the guns one. Any ideas?

[rancidtom]

Does it have the correct hammer spring in or has somebody replaced it with a weak one? Very often you find a secondhand gun with some ridiculously strong spring in it but some people go the other way or simply stick the wrong spring in because they lost the original. I doubt that the hammer has been lightened but you never know who has been fiddling with a secondhand gun or exactly what they've done. Is the valve closing spring standard? When there is no pressure in the reservoir does the valve operate smoothly, opening with no problem? I know it opens and closes with pressure in but having to whack it with a hammer sounds suspiciously like there is an overly strong return spring.

[tinbum]

I got this from a good friend. One day it was working faultlessly, next outing nothing! I know that is without question the case.

I stripped it again this morning and removed 4 coils from the valve return spring. The valve when empty operates smoothly, but still with significant force necessary. I rebuilt and tested at various pressures.
At 50 bar there is now a pop, at 100 a bang, at 150 a pop again and at 180 (my bottle needs a refill) a disappointing pffft!

Its worth mentioning the manometers on gun and bottle agree, but it is going the right way.

Surely the hammer spring can't just have lost it's effectiveness so easily could it? How about if it had been left cocked? (Not that I know that's the case.)

[rancidtom]

Springs don't lose their spring rate if left cocked unless they've never been hardened and tempered properly, in that case the spring will fail very early in its life either breaking or staying compressed.
The symptoms you describe indicate that the gun has a classic "power curve" as demonstrated by most non-regged PCP's. The valve becomes harder to open as the reservoir pressure rises and effectively limits ME if the reservoir pressure is too high due to the hammer not being able to knock the valve open wide enough or hold it open long enough to accelerate the pellet sufficiently. Many non-regged .22's demonstrate a reasonably flat power curve between 70Bar and 130Bar with a peak in the 80-90Bar area, the exact width of the "sweet spot" depends largely on the speed of the exhaust valve return, this is dictated by the spring strength, friction and the mass of the valve/spring assembly along with any dynamic gas flow forces that can induce faster closing.
Due to the power drop being sudden I suspect that your hammer spring has possibly weakened but I don't know why, has it fractured a coil or two off the end or are both ends flat ground as the original should have been? Have you got or can you get a new hammer spring? I have fitted different springs and lightened hammers to try to eliminate hammer bounce to one of the Careers I repaired, I used a BSA S10 .22 hammer spring as I had one spare, it did lessen valve bounce making the gun far more economical on air.

[tinbum]

Yep, I've regulated several over the years, and power curves are one of the reasons PCP's suck!
But what's really baffling is the fact it stopped working at the exact same pressure etc that it had been working previously. Something must've changed, but finding out what is doing my head in!
I've ordered a new spring which should be here tomorrow, we'll see what happens then.