Trying to tune QB78 PCP

[wvkeith]

Posted this on another forum but thought I would try here as well..

Got my QB78 with Jack Haley pcp tube going and shot a 10 shot string. It starts out strong and drops straight down from there. As far as I know, the valve is stock with a slightly stronger valve spring. The hammer spring is longer than stock and there seems to be a bit of pre-load. It has the ice maker tubing in place of the breech o ring and it has the extended bolt probe mod. The starting pressure was 150 bar and ending pressure was 90 bar.

Shooting 30.7gr. kodiaks

1.   861
2.   861
3.   853
4.   845
5.   836
6.   813
7.   800
8.   780
9.   764
10. 751




I plan to do the  debounce using silicone tubing. Any ideas of how to get a better curve are appreciated.

[DanoInTx]

less hammer spring might help.  Don't know anything about how your gun is constructed or how much air it should hold, but almost looks as if the top of your bell curve is shots 1 and 2, can you put slightly more air into it, or is 150bar tops?  If you are going to do the tubing on the valve debounce trick you might not want to bother tuning for a bell curve yet as that will most likely completely change with the debounce mod...kinda like tuning your car to run on unleaded then filling it full of diesel.  Just my $0.02

[jon]

Ditto on the fill pressure, looks like you are right smack dab in the middle of the pea of your shot string, put more air in it.  I usually go 10 bar at a time til I find where I want to be.  Good luck!

[DanoInTx]

Looked like that to me also, still seems a bit steep....like maybe too much "thwap" from the hammer.  Wonder if anyone makes a hammer spring preload adjuster like they do for the Crosman 22XX guns?  Would be a much simpler way to tune for a good string, then just lock it down when it's where you want it.