Thinning valve stem question

[RDB]

Had my hpa QB apart and thinned the valve stem down around the transfer port hoping for a little better flow. But lost about 50 fps. Suppose it would be like opening the throat a bit?

Anyways, could it have caused the velocity loss....change in air pulse? Or should I be looking for some nicked o rings? Passes the tissue test.



Thanks for any insight.

[MJP]

Transferport leaking perhaps? You don't need a big leak to loose a lot of velocity.
Marko

[Tucobenedicto]

Bent stem? or some debris-sharp edge stuck the stem?

[Motorhead]

Gains in throat area with nothing else done ... has a typical effect of a slight drop in dwell in spite the gains in flow area.  Not uncommon that a slight increase in hammer spring tension be required to take advantage of it.

[RDB]

Thanks guys. Back apart she comes. Pumping that ninja up from empty is going to drive me to a compressor purchase!

[nervoustrigger]

No need to evacuate the tank, just shim the hammer spring by 1/16" to 1/8" and see if velocity picks up.

With that said, unless you are running a regulator setpoint over 1200psi or so, the stock hammer spring should be plenty strong without shimming.  If you converted it to cock-on-open, it's all different.  If you want some better targeted guidance, tell us more about the modifications done to the rifle.

[RDB]

Thanks Jason, pretty much used your build as a guide. Opened up barrel and probe to .165. drilled and tapped the tank block for a spring stand for the valve spring like Bob's build. Disco transfer port seal. .032 Belleville added to the washer stack.

Put it together last week and ran it over the chrony a few shots. Averaged 870 with cphp. Popping crab apples at 40 yrds no problem. Hey I can live with that. My 177 is a fly shooter at 25 so I was happy with this 22 Cal.

Pulled it apart yesterday to do a cock on open mod and thinned the stem down while it was apart. Put it back together and couldn't get the fps back to 870 .  830 was all I could get before the spring spacer would hit the back of the hammer. Back to cock on close and only 810. Only change was to the valve stem.  Just slid a 1/16th washer over the spring guide and am now back at 860. I can live with that for now. But I think I may have thrown some efficiency out the window from what I had before.  Report is slightly louder and longer than before.  I'll source a new stem and see if she come back up. Heck..... maybe the reg just settled down after a few hundred pellets and I totally missed it.