Career/Sumatra help needed

[tinbum]

I've been having a fantastic time playing with a non-working Sumatra 500 today. I've skinned knuckles, used more deductive power than Sherlock hunting for the vaseline and made three different tools to render it down to its component parts.

Once apart I could find nothing wrong anywhere!

So it went back together with a little clean and fettle to see what occured. I put a few bar in the tank and it all cocked, fired and made encouraging "Bang!" noises. Happy days!

However, when I put 180 bar in nothing happens? I took the hammer assebly out and made another tool to reach into the exhaust valve and tapped it with a hammer: Pop!

So the valve is working, but the hammer assembly can't overcome the air pressure in the gun. I've tried different hammer settings and preloads, but as it was working before it mysteriously stopped I thought I'd ask the collectives advice before slinging it in a skip.

[tinbum]

Ok, I've wound the adjustable nose on the hammer right in to give the most travel and increased the preload by a lot. All I'm getting is a tiny poof! Not enough to fire the pellet.

I've had enough for tonight, but any ideas most welcome

[rancidtom]

I've not repaired any Sumatra 2500's but have repaired a few Career 707's. The 707's have an awful hammer stroke adjuster. This may be giving you trouble and not actually allowing a proper hammer strike on the valve? The last 707 I repaired had the hammer strike adjuster "overwound" by the owner causing a similar problem to the one it sounds like you have.
email me a pic or two of the hammer and stroke adjuster set-up, I may be able to point out where your problem is? PM me for my email...

[rovercoupe]

With a standard spring you should be able to preload the hammer spring by about 5mm with washers so might be worth trying to see if the spring is weak, i had to upgrade the spring after only getting 8ftlbs out of a standard spring no matter what i did, a £2 spring off fleabay worked perfectly!

[Jonjon79]

Could you attach a picture of the tiny poof please.

[tinbum]

I've not repaired any Sumatra 2500's but have repaired a few Career 707's. The 707's have an awful hammer stroke adjuster. This may be giving you trouble and not actually allowing a proper hammer strike on the valve? The last 707 I repaired had the hammer strike adjuster "overwound" by the owner causing a similar problem to the one it sounds like you have.
email me a pic or two of the hammer and stroke adjuster set-up, I may be able to point out where your problem is? PM me for my email...

I'll have a look tomorrow and try it without the adjuster in place. I know the valve stem is clear as I've measured it and can actuate it with a rod and hammer

[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.

[rovercoupe]

I might have a spring that took mine way over if you wanted to borrow it to try, will have to have a look in my parts box.

Im going off on one here but as I understand once the valve is knocked open the high pressure air flowing through the valve will hold it open so if as the pressure is going up the valve is not able to hold open and if you have re built the valve and nothing is wrong could there be a problem post valve?

A mis alligned transfer port or port seal will drop the power right off and could be enough to stop the valve operating or cause the pfft at high pressure.

Just a thought.

[rancidtom]

I might have a spring that took mine way over if you wanted to borrow it to try, will have to have a look in my parts box.

Im going off on one here but as I understand once the valve is knocked open the high pressure air flowing through the valve will hold it open so if as the pressure is going up the valve is not able to hold open and if you have re built the valve and nothing is wrong could there be a problem post valve?

A mis alligned transfer port or port seal will drop the power right off and could be enough to stop the valve operating or cause the pfft at high pressure.

Just a thought.

Mis-aligned transfer port will limit power but Tinbum says it happened overnight and has been down ever since. There may be pixies with allen keys in his gun cabinet but I doubt it!
High pressure in the air tube resists the valve opening but the flow of air doesn't stop the valve closing that's usually due to friction on the valve spindle or a bent spindle but this usually exhibits as high power (too much air getting sent up the barrel for sub12). Some exhaust valves are actually designed so that the air flow helps close them (early AA300/400/410 valve assembly is a classic example). The AA system also uses a throttle or choke screw to adjust the power by limiting flow in the transfer port.
Assuming that it is the right valve spring I would suggest that when the gun is next emptied remove the valve assembly and check that there isn't any restriction on the valve stem. It should literally close under it's own weight without a spring in.