FWB 601 - Cylinder not fully depleting

[Doodaddy]

Earlier this year I picked up a Feinwerkbau 601SSP. Loved the gun for a while, but I've reached a point of frustration.

After firing on the upstroke during the cocking process, a chuffing sound from the barrel occurs from a release of air pressure. It hasn't always done that. Once it started, the point of impact dropped severely; obviously most notably at longer distances.

I've replaced all of the seals, the compression spring, cleaned everything including inside the cylinder, and relubed. I'm at a complete loss. It's as if the firing pin isn't depressing the valve disc long enough for all of the air to be released from the cylinder.

From what I've read, it doesn't look like I'm the only person to experience this issue with this rifle, but nothing seems to fix it.

Any ideas?

[Albert T]

sounds like the spring pressure of the hammer block is not set properly after installing the new seals.
The hammer block is not correctly opening the air release valve. On the upstroke, the pressure in the cylinder decreases (piston moves backwards) and the spring of the hammer block has enough power to open the valve, causing the rest air to escape through the barrel. Increasing the spring pressure of the hammer block should fix the problem. On the back of the receiver is a screw (probably hidden from sight by the stock) that is used to adjust the pressure of the hammer block spring. Changing the setting of the spring tension adjusts the amount of air released and thereby the pellet speed.
I had the same problem with my old Walther CM-2 air rifle.

My 2 Euro cents,
Albert T
(The Netherlands)

[Doodaddy]

sounds like the spring pressure of the hammer block is not set properly after installing the new seals.
The hammer block is not correctly opening the air release valve. On the upstroke, the pressure in the cylinder decreases (piston moves backwards) and the spring of the hammer block has enough power to open the valve, causing the rest air to escape through the barrel. Increasing the spring pressure of the hammer block should fix the problem. On the back of the receiver is a screw (probably hidden from sight by the stock) that is used to adjust the pressure of the hammer block spring. Changing the setting of the spring tension adjusts the amount of air released and thereby the pellet speed.
I had the same problem with my old Walther CM-2 air rifle.

My 2 Euro cents,
Albert T
(The Netherlands)

That's exactly what I had thought. Feinwerkbau offers three different firing pin spring screws different lengths. I have the longest one set to the deepest depth it can go. I also installed a new compression spring thinking the one that was in there may have relaxed over time. There was no change in the issue. It was at that point I became directionless and came to the boards for input.

[45ACP223]

Did you replace the white or clear seal inside the air chamber? The one that has the brass ring in it that the air release valve moves through? That is the most common part that needs replacing if you don't get a complete discharge of air.

[Doodaddy]

Changed that too.

If I fire the rifle and do not recock it immediately, I can leave it over night and the pressure is still held the next morning. For whatever that's worth.

[45ACP223]

Sorry to hear that. Every one we have worked on either had issues with low pressure during the shot, or not using all of the available air (kind of what you are experiencing with air still present after firing). If you fire the gun and reset the trigger by opening the pellet loading gate, and re-fire, does additional air come out then? Ours did.

Consistent low pressure was fixed by replacing the main piston seal (the big black, blue or red one on the end of the piston that you can see when you open the big compression cocking arm.

The 2nd issue like yours was fixed on 3 guns by replacing the valve seal. Did you look at the plunger and make sure it was smooth? May sound stupid, but did you reassemble the valve assembly in the correct order? We haven't ever seen an issue with the firing pin or firing pin springs on any of the 600's at our club.

I suggest you contact either Pilkgun or Pyramid and ask to speak with their gunsmith directly. They are very helpful and work on these guns regularly. They typically can point you in the right direction just by talking about your issues. If all else fails, send your 601 in and have them look at it.

[Doodaddy]

Sorry to hear that. Every one we have worked on either had issues with low pressure during the shot, or not using all of the available air (kind of what you are experiencing with air still present after firing). If you fire the gun and reset the trigger by opening the pellet loading gate, and re-fire, does additional air come out then? Ours did.

It sure does!

Consistent low pressure was fixed by replacing the main piston seal (the big black, blue or red one on the end of the piston that you can see when you open the big compression cocking arm.

I replaced that that one too.

The 2nd issue like yours was fixed on 3 guns by replacing the valve seal. Did you look at the plunger and make sure it was smooth? May sound stupid, but did you reassemble the valve assembly in the correct order? We haven't ever seen an issue with the firing pin or firing pin springs on any of the 600's at our club.

I'll go back and inspect it, but I didn't notice any burrs. If the plunger isn't seating/sealing well, wouldn't there be an issue regarding maintaining compression rather than not releasing?

It doesn't sound stupid. I checked that several times to make sure it wasn't that. Honestly, I was hoping it was.

I suggest you contact either Pilkgun or Pyramid and ask to speak with their gunsmith directly. They are very helpful and work on these guns regularly. They typically can point you in the right direction just by talking about your issues. If all else fails, send your 601 in and have them look at it.

I did speak to Pilks directly. They suggested it was a seal. They also quoted me nearly 30% what I paid for the rifle and a few/several weeks out for repair time. I'm glad they're busy, but that doesn't really give me incentive to send it off.

[45ACP223]

I hear ya, and feel your pain! Good luck, I hope you find out what the issue is. Let us know if you end up getting it sorted out. It would be good info. for all.

[Doodaddy]

I'm hoping to figure it out and get it posted here. I've searched around and see that many have had the issue, but with no clear resolution.

[Doodaddy]

So far no luck. I bought a chronograph (needed one anyway) and measured the velocity of the rifle at around 20 feet. I was averaging 421-427fps with H&N Baracuda Match 10.65 grains.

I'm officially at a loss.

[Rover]

sounds like the spring pressure of the hammer block is not set properly after installing the new seals.
The hammer block is not correctly opening the air release valve. On the upstroke, the pressure in the cylinder decreases (piston moves backwards) and the spring of the hammer block has enough power to open the valve, causing the rest air to escape through the barrel. Increasing the spring pressure of the hammer block should fix the problem. On the back of the receiver is a screw (probably hidden from sight by the stock) that is used to adjust the pressure of the hammer block spring. Changing the setting of the spring tension adjusts the amount of air released and thereby the pellet speed.
I had the same problem with my old Walther CM-2 air rifle.

My 2 Euro cents,
Albert T
(The Netherlands)

I go with the above.

I had the same problem with a FWB100. Increasing the spring tension a little immediately solved the problem. I solved the same problem with a friends' Pardini K58 the same way.

I had a slightly different problem with the same effect on my Walther LPM1. It "cleared its throat" with a bit of shooting. I'm sure it was a bit of fung in the valve.

[Doodaddy]

sounds like the spring pressure of the hammer block is not set properly after installing the new seals.
The hammer block is not correctly opening the air release valve. On the upstroke, the pressure in the cylinder decreases (piston moves backwards) and the spring of the hammer block has enough power to open the valve, causing the rest air to escape through the barrel. Increasing the spring pressure of the hammer block should fix the problem. On the back of the receiver is a screw (probably hidden from sight by the stock) that is used to adjust the pressure of the hammer block spring. Changing the setting of the spring tension adjusts the amount of air released and thereby the pellet speed.
I had the same problem with my old Walther CM-2 air rifle.

My 2 Euro cents,
Albert T
(The Netherlands)

I go with the above.

I had the same problem with a FWB100. Increasing the spring tension a little immediately solved the problem. I solved the same problem with a friends' Pardini K58 the same way.

I had a slightly different problem with the same effect on my Walther LPM1. It "cleared its throat" with a bit of shooting. I'm sure it was a bit of fung in the valve.

I think that would likely do the trick as well but.............I'll have to custom make something to be able to increase pressure. The FWB601SSP isn't adjustable as most would think. There are three different firing pin screws that are different lengths (1750.3441, 1750.3451, & 1750.3461) that change spring tension.

I've already replaced the firing pin spring (1750.343.3) as well as confirmed that the firing pin screw (the one that is at the back of the receiver that the spring is supported by) is the longest of the sizes available as well as screwed fully in. In order for me to increase spring tension, I will either need some form of a custom spring or a washer that will fit between the spring and the firing pin screw that has an ID big enough to go around the firing pin and an OD small enough to fit into the receiver, but have an OD large enough to properly fit against the firing pin.

I've yet to find a washer that is already that size and don't currently a way to grind one down.

[cstan]

I had the same problem. I fixed it by changing the seal on the release value. I think the seal was moving forward when the air was released blocking some of the air.

[Doodaddy]

I had the same problem. I fixed it by changing the seal on the release value. I think the seal was moving forward when the air was released blocking some of the air.

Changed that seal. Twice actually.

It's looking like the issue has been resolved by replacing the firing pin spring (again), but with a stronger spring this time.

[Ziabeam]

I had the same problem. I fixed it by changing the seal on the release value. I think the seal was moving forward when the air was released blocking some of the air.

...My experience/findings after suffering the same puny air release symptoms y'all described ...;

((((doing complete rebuild... will update if/when the anemic pressure release is completely resolved with recently received factory seals))))

[Ziabeam]

Learnt a few things;

This rifle was a very recent acquisition. The previous owner/seller, never shot it, having received it from his father who likewise never shot it in the 6+ years he had possession of it. Similarly his father indicated it had been all but forgotten lying dormant in the nether-regions of a school where it had been originally purchased/used.

It would not expel a pellet, which would get stuck at the choke, while residual air "charge" remained unused in the valve chamber, which would "vent" on the upstroke exactly as others here have described.

While waiting for the new seal kit I observed the bore, which was suspect since I noticed an amber tint in the grooves of the rifling. Looking at it with a bore scope it was NOT rust. Appeared to be a shellac from unknown petrified substance. Possibly residue from a distant owner's "home-brew" coated pellets... or long term storage/preservative attempt at coating/preserving the bore ??? Gawd forbid it is FWB's own grease gone mad !!! At any rate I ran a patch through the bore and darn near got the rod permanently stuck at the choke. Same patch/rod combo works with ease in all my other .177's.

After judicious use of a copper brush and cursory finish with JB paste the same patch/rod combo worked effortlessly.

I'm certain in my case, the ONLY reason the gun failed to fully "vent" and the pellet got stuck was the above mentioned bore restriction. Not matter how hard the hammer strikes the valve it could not overcome this issue. This effectively discredits my previous assertion that the valve seal alone was to blame. I apologize for that misinformation.

Before anyone goes and shims their hammer spring, and after checking/replacing all relevant seals, make sure your bore is not restricted.

Got the rifle resealed and back together yesterday.

After cleaning the bore, and replacing all seals here are my results, all findings made today;

10 shot string; H&N Finale Match 8.18 grain
Average velocity-------570.7
Hi ---------------------574.3
Low-------------------567.9
Extreme Spread ---------6.4
Standard Deviation-------1.9
Ad-----------------------1.4

10 yards from an improvised standing rest; (indoors)10 Shots each bull


25 yds from a Pappas one piece rest in a light breeze (outdoors) 10 Shots

[Doodaddy]

Very interesting about your bore. A squib is about a terrifying though. I ensured that my bore was nice and clean before the problem really manifested itself thinking that the lack in accuracy was related to the bore's path.

Nice choice in rest. I miss mine.