853CM Low fps

[severedirkitis]

Bought a 853CM, did the 'trigger job', started shooting 10m at the local club, and noticed I wasn't getting clean holes.

Borrowed a chrono, 10 shot avg was 340 fps (rifle is rated at 490 fps).  Replaced the 2 o-rings (piston, valve) along with the foam sleeve on the piston, cleaned & re-greased the pump bore with 30wt oil, got it up to 450 fps.  Still not per spec, but much better than before.  Could definitely tell the increased effort needed to action the pump lever.

Rifle sat for a few days, double-checked the speed, now it's 420 fps.....and dropping the more I shoot.  Currently down around 400 fps.

Anyone have any ideas???

[haertig]

Maybe a sharp edge of metal or piece of debris somewhere that is nicking a seal, thus damaging it and making it not seal correctly?  Just a guess...

[stonykill]

 Did you wet down the wiper/foam ring with 30 wt nd? That keeps the pump tube lubed.

[mafatone]

If you didn't replace the valve stem, you have leak down from there. Did you use the factory seals from Daisy?

[severedirkitis]

Maybe a sharp edge of metal or piece of debris somewhere that is nicking a seal, thus damaging it and making it not seal correctly?  Just a guess...

After the loss of speed again, I examined the original seals I replaced very closely and didn't find any nicks or cuts.

Did you wet down the wiper/foam ring with 30 wt nd? That keeps the pump tube lubed.

No, I'm curious how that keeps the pump tube lubed considering it is on the backside of the o-ring on the piston?

If you didn't replace the valve stem, you have leak down from there. Did you use the factory seals from Daisy?

I did use the factory seals from Daisy: orange ring on the valve, black ring on the piston.  I did not replace the valve stem, spring, or cover (?) that also came with the kit.  Definitely my next step.


Thanks all for the feedback/advice.  Follow-up question: Exactly where and to what should I apply a new coat of oil when rebuilding this portion of the firearm?  Soak the foam ring, thin coat on the new seals, thin coat on the inside of the pump tube? Anything else?

[DavidS]

The felt wiper will coat the pump tube with a thin layer of oil each stroke, as the O-Ring passes the oil coated tube some ends up in front of the Piston O-Ring, and some gets pushed on through the system.   This keeps the valve oiled, and the small movement of oil is enough to carry away any small airborne dust that gets into the system, thus helping keep it clean.   This is also why each shot expels a very very small amount of oil out the barrel.

I hope that answers the question on how the Felt Wiper behind the piston O-Ring keeps the valve oiled.

[stonykill]

The felt wiper will coat the pump tube with a thin layer of oil each stroke, as the O-Ring passes the oil coated tube some ends up in front of the Piston O-Ring, and some gets pushed on through the system.   This keeps the valve oiled, and the small movement of oil is enough to carry away any small airborne dust that gets into the system, thus helping keep it clean.   This is also why each shot expels a very very small amount of oil out the barrel.

I hope that answers the question on how the Felt Wiper behind the piston O-Ring keeps the valve oiled.

 x2.

Yes it is behind the oring. But of course it follows the oring down each and every trip.

   Oil your wiper and I think your problem is solved.

[severedirkitis]

I hope that answers the question on how the Felt Wiper behind the piston O-Ring keeps the valve oiled.

It does,  thanks.   Should a small amount of oil be applied to the foam ring every so often?  I see that the foam ring is accessible when the pump lever is fully open.

[DavidS]

I hope that answers the question on how the Felt Wiper behind the piston O-Ring keeps the valve oiled.

It does,  thanks.   Should a small amount of oil be applied to the foam ring every so often?  I see that the foam ring is accessible when the pump lever is fully open.

Keep that foam ring (the felt oil wiper) very well oiled.

[severedirkitis]

No luck.

Replaced the valve stem & spring, actually made it worse.  It wouldn't seal at all, during a pump, you could hear air escaping from the bolt.  Put the original stem/spring back in - no leak.  But still only shooting 420 fps.

Oiled the foam ring, re-checked the o-rings for damage.... My frustration is growing.

I have noticed that when my pump action is fully open, there is a slight hiss of air (intake?) as the o-ring appears in the pump tube notch.  I shot a buddy's 853 he got from the CMP...his does not make the hiss noise, but it appears the o-ring is in the same position as mine at that full open point.  Anything to make of this?

[haertig]

It should hiss a little when the pump arm is fully extended.  As a matter of fact, you should pause in the pumping stroke at it's full open position until you hear this brief hiss.  It's got to not so good the air in first, before it can compress it.

[ "not so good"? That must be the net nanny software on this forum trying to correct me for typing "s u c k".  That's got to be some of the dumbest software around... ]

Is there any damage to that aluminum sleeve that the pump slides back and forth in?  Any dents, out of round spots, etc.?

If you're hearing air come out the bolt area when pumping (on the compression stroke) then that certainly sounds like the valve inside is not doing it's job to me.  Defective valve?  Not installed correctly?  Something pushing on it that shouldn't be?  It should only release air during the time when the hammer piece strikes it under the force or the hammer spring (the biggest spring in the air rifle that usually tries to jump out when you're reassembling the trigger housing!)

[45Bravo]

I have the same issue with my 953.

It shoots 407 FPS.
Resealed, All the way through, still shoots 410.

It holds air for ever (my nephew thought it was like a crosman and needed to be stored pumped, so he pumped it and stored it for a week pumped)

It still shoots 405-407fps extreme spread of 2-3fps on the chrono.

Well lubed, but shoots low FPS.