More PCP QB tuning

[wvkeith]

Installed a piece of silicone tubing on the valve stem as Rich from Mich suggested. This made a big difference as far as noise reduction and decrease in peak velocity as well. Before installing the tubing, the first shot with a 150 bar fill was in the high 800's with a straight downward drop from there. After I added the tubing, I filled the gun to 170 bar as someone suggested and shot a string. The good news is that I'm starting to get somewhat of a curve and a lot more shots per fill. I don't know if I got the tubing a little too long, or if 170 bar is too much for the hammer spring. Shooting .25 cal 30.7gr kodiaks.



Before, I was only getting about 10 shots, now I could get 50 or more. I'd like to see it reach peak velocity sooner, and have a higher velocity.

[DanoInTx]

I don't know your particular gun, but now it seems like you are experiencing a bit of valve lock on the higher fill.  I also don't know how the tubing debouncer is supposed to work, but it sounds like the idea is to limit how far the valve opens and to help it close faster...sounds cool.  It looks like you are onto the right track now that you are seeing a bell curve of sorts, but you're right, it's too steep going up and too steep coming down, your POI must be all over the place.  My suggestion would be....if you think you have too much tubing on the valve stem, remove some and try another shot string.  If it flattens out to your liking, all done, if not you might try either increasing the hammer spring weight, or the actual weight of the hammer.  Been a long time since I was inside a QB, but I'm sure you can shim up the hammer spring with a few washers.  Again, fill, chrony, graph with EVERY change, so you know what is doing what.  No point in making 2-3-4-5 changes then trying to figure out what made it better/worse.  Max shots per fill is great, but what is going to really matter is a low deviation from shot to shot.  Be a bit careful when shimming up the hammer spring, I don't know what valve is in your gun, but the standard QB78 valve stems will bend and flatten out if you pound on them too hard....don't over do it:)

hth,

[wvkeith]

I snipped off a little of the tubing and drilled out the end plug to give the tubing more room to slide. The tubing may have been rubbing the inside of the plug and not letting the valve open enough. Will chrony today at 170 bar fill and see if there's any difference. If that doesn't do it, I'll try a 160 bar fill (170 is the max for this tube).

[wvkeith]

Opened up the hole in the inside plug and lubed the silicone tubing. Fill pressure was 170 bar. Shooting .25 cal 30.7gr Kodiaks. Getting 10 pretty good shots, which is what I wanted. I have a 72ci paintball tank with coily remote that I can use for every day plinking. I wanted the 10 hard shots for hunting. Even though the gun has plenty of power, it's still somewhat quieter than when I started. I guess this is attributable to the tubing debouncer.

1.  832
2.  835
3.  850
4.  870
5.  869
6.  876----->52 fpe!
7.  874
8.  851
9.  849
10. 827




I know it's not the best curve, but it's much better than what I started with:




Now if I can get a RJ barrel band, I can start some accuracy testing. Hard to believe this little gun started life as a 7 fpe plinker!

[DanoInTx]

Looks pretty good, but I think you could still squeeze it a bit closer.  Try a couple washers under the hammer spring, or if possible add some weight to the hammer.  You might see those numbers get a little bit closer.  Again, don't over do it, or you might end up with a hammered valve.  I would think with a 50+/-fps spread you'd see some vertical stringing at any decent distance.  Close, but there is some fine tuning to be done still.  How does it sound now btw?  Is it still quiet?

[wvkeith]

I agree that it needs some tweaking to get it just right. It's a lot quieter than before I added the tubing to the stem. I think the tubing is preventing hammer bounce and the wasted air that was blown out the muzzle.

[DanoInTx]

I think if you can flatten it out even more you may even end up with more usuable shots.  Right now you are definately seeing a good bell curve.  Nice work!

All you QB conversion guys here and on the Yellow are making me want to build another QB.  Think I am going to run mine on remote HPA though, and maybe start with an AR2078.  I want to shoot .177's at 500fps or less, should be a sweet mini-sniping rig:)  Wonder if I could detune a conversion tube enough to shoot that slow?