My CFX Muzzle Velocity Is Low

[mze]

Ever since I got my CFX, its muzzle velocity had been lower that other CFXs using the same pellets.

 I have just replaced the piston seal thinking that this may be the problem. If anything, the new seal is tighter than the old one leading to lower MVs.

 The gun is relatively new. Got it on August 28. 6 weeks old?

 I have removed the standard sights.

 Can anyone suggest where to look for my missing speed.

 I am averaging 680fps with 8.44 JSB Exacts at 4.53mm.

As may be seen from my sig, I have fitted a gas ram. At my altitude of 6000', I should be getting ave ±765fps. At sea level this particular ram has been tested at 890fps in a Gamo Shadow.

Let's see what the collective brains and experience can bring forth.

[shadow]

There's a seal in the loading port also and if that was removed during the tune it may have been damaged or isn't seating properly. For a .177 that is low FPS, my .22 CFX runs at 700FPS and it's stock. Ed

[mze]

shadow - 10/7/2009  7:48 PM  There's a seal in the loading port also and if that was removed during the tune it may have been damaged or isn't seating properly. Ed

Ed, nothing was done at any time in the loading port area. How do I strip this part of the CFX?

Vincent

[kiwi]

There are 2 seals on the loading port/rotating breach..
there is a O-ring around the out side that seals the breach to the compression chamber
the gun has to be striped to replace it....After the spring/guides/piston have been removed
the breach will push out of the chamber as the piston did....
Theres also a O-ring in the front of the breach that seals the transfur port the the barrel
This one can be replace with out removeing the breach....
BY...pulling out the breach handle...cock the gun "saftey on"...then the breach can be slide
 rearwards enough to gain accsess to the O-ring....the breach can be returned by SLOWLY
letting the piston retun to the uncocked postion with the cocking lever...

Pete

[ten4copy]

This should help.

http://www.airgunfix.com/blog/airguns-assembly-disassembly/gamo-cfx-disassembly-guide/

[shadow]

Yup as mentioned above thanks guys and keep a eye on it as your assembling it, those O-rings can roll out of their seat's on you. :0 Ed

[mze]

Thank you for all the helpful suggestions.

  The library has an excellent document by CDT on replacing the seals on the rotating breech.

  When I removed mine, the large o-ring was damaged in two areas. The rotary itself was damaged because the gun had been fired with the rotary not fully closed. The rotary had been slammed forward into the barrel end by the piston, making indentations in the rotary in area where the small rotary/barrel seal is positioned. This damaged the rotary requiring replacement. The piston seal was also damaged.

  I am the first owner of this CFX and have never fired the gun with the rotary not fully closed. I can only assume that the damage to the rotary was cause at the retail outlet by people messing about with the gun. The damaged o-ring and piston seal must have been caused by careless assembly. This is not difficult when the components have to be installed past the sharp cutting edges in the receiver left from stamping out metal in the manufacturing process.

  When the rotary and its two seals and the piston seal were replaced, the MV rose to expected levels.

  A word for anyone doing a lube-tune on CFXs. Remove the rotary when doing any work on the receiver which will need cleaning with any combustible solvent. If the rotary is not removed, the solvent could damage the rotary seals, and could go past the o-ring and collect under the rotary. When fired, the combustible material under the rotary could detonate causing all sorts of damage. The are several instances where this has happened because it is not generally known that solvents can penetrate the rotary area.

  Vincent

[mze]

If the bear-trap mechanism is functional, you will not get the piston back to the end of the receiver by pulling the trigger and hanging on to the cocking lever. You have to remove the stock, remove the bear-trap mechanism, replace the stock and then pull the trigger while hanging onto the cocking lever. All of this is pretty hazardous with a cocked rifle.

Replacing the small barrel/rotary seal is best done using a piece of wooden dowel to keep the rotary back for seal replacement. Cut a piece of dowel about the length of the rotary visible. Remove the "handle" from the rotary being careful not to lose the flat spring. Partially cock the gun, and while holding the cocking lever, push the rotary back wards until the dowel spacer can be put in position. Replace the small seal. Pull down on the cocking lever, remove the dowel spacer, and allow the cocking lever to return to the clipped in position.

[shadow]

Glad you found the problem and fixed her up. Ed