New gun lost power

[Dornfelderliebe]

Whilst waiting for some scales to come from Abey I decided to strip down my Kral Devil which has been steadily and gradually losing power since I bought it,two Black Fridays ago.
Down from 560fps to 520 the other day with 14.5 gr superdomes.

I expected to find a damaged seal but I can see no damage to it. Nor is there any serious scoring to the cylinder. Some machining marks seem to have polished and the rear bearing section of the piston is pristine.

The only things I can think have gone wrong is the spring has shortened or the breech seal has worn ( no visible knicks).
This weekend I going to polish & deburr it.

Any suggestions chaps?

[angrybear]

Maybe a poorly tempered spring losing it's springiness.

[Neil54]

You could try scragging the spring, then adding spacers to get your desired power.

Or just leave it cocked for a few days. Then add the spacers.

[Shed tuner]

most likely just the lube burned off... that's not a "catastrophic failure" type power loss.. or that in combination with the spring just taking a set, as stated above. Just add a couple of washers and carry on...

[Brickhill]

Surely a replacement spring might be the best solution?

[Shed tuner]

Surely a replacement spring might be the best solution?

could be, but unless the spring is crap, or the gun has an excessively light piston, a spring should last a while longer.

[Travis299]

Have you tried the tissue test to check for any leaks during the shot cycle?

[Dornfelderliebe]

Thanks chaps. The spring appears to be quite soft 2.8mm wire according to my middle of Lidl vernier gauge, 33 coils long. Crossing threads I have used a nylon slip washer and spare power washer totaling 6.4 mm of preload.
It did boing badly so I've put a Guinness can piston sleeve in to make close sliding fit and gently polished the bearing end of the piston with 1500 grade wet & dry. The spring ends have come up nicely with file & 1000g.
I'm still to polish the cylinder with 1000 grade and report the piston weight when the scales arrive.
I'm quite impressed with the spring guide, metal with a stepped plastic collar/sleeve which would allow rotation of the spring rear end.
I will test for leaks upon reassembly and look up a Titan in the meantime.

[david j]

God I wish I knew half as much as these guys.

[Shed tuner]

do you not have kitchen scales (just wait for the head chef to be distracted )

[TonyL]

most likely just the lube burned off... that's not a "catastrophic failure" type power loss.. or that in combination with the spring just taking a set, as stated above. Just add a couple of washers and carry on...

Yes, this.

[TonyL]

Have you tried the tissue test to check for any leaks during the shot cycle?

And this.

[TonyL]

Thanks chaps. The spring appears to be quite soft 2.8mm wire according to my middle of Lidl vernier gauge, 33 coils long. Crossing threads I have used a nylon slip washer and spare power washer totaling 6.4 mm of preload.
It did boing badly so I've put a Guinness can piston sleeve in to make close sliding fit and gently polished the bearing end of the piston with 1500 grade wet & dry. The spring ends have come up nicely with file & 1000g.
I'm still to polish the cylinder with 1000 grade and report the piston weight when the scales arrive.
I'm quite impressed with the spring guide, metal with a stepped plastic collar/sleeve which would allow rotation of the spring rear end.
I will test for leaks upon reassembly and look up a Titan in the meantime.

You're proceeding in the right direction and will have it right soon.

As Jon says, you can use a set of kitchen scales to weight the piston. Place the piston in a sandwich bag, or similar, and you're fine. The Devils are those impressive little 25mm Gamo clones, aren't they? These rodless pistons can often be on the very light side, so adding weight here could help.

[TonyL]

P.S.....When you re-commence power testing, you might do well to try a good selection of pellets, especially those from the JSB stable, like Jumbo RS or Express. The Superdomes can often rob power, most especially in set-ups employing a very light piston and soft spring. What might be "comfortably under" with a 'Dome might not be with some of those JSBs.

If going for a slightly more stout spring, and especially if introducing some piston weight, you might find that things even out a little with a wider range of pellets?

Spring wise, I'm wondering if the TX200 Mark 3 spring would suit / be enough spring for this rifle? Failing that, what about one of TbT's HW97/95 springs would fit okay (wondering about the OD here). And also worth looking into the recently introduced Welsh Willy spring to check the specs etc.

[BC312]

If you leave a spring cocked for a length of time causes a spring to lose its strength.