Questions on QB79 power mods (Staying on Co2)

[Gertrude]

Hello All,
I am putting together a QB79 in .22 cal for a friend who wants to get his son interested in shooting. He does not want to do HPA at this time, but he may in the future.
 The gun currently has the adapter block and 9oz Co2 bottle. It was originally shooting 14.3gr CPHP's at 520 fps.

 I would like to get it pushing the CPHP's as fast as possible on the Co2.

 I have modded the valve by removing the piercing pin, filter/screen, put cutouts in the sides of the front half of the valve, opened up the hole on the end, replaced the valve spring with a slightly lighter and longer spring, turned down the O.D. of the poppet, opened the I.D. of the valve, opened the throat up to about .207

The temps here are about 103 to 105 degrees.
I have a Co2 bottle pressure tester and it is showing about 1120 psi.
 The gun is now pushing the CPHP's about 585 fps.

Now I would like to change the TP Seal to Icemaker tubing, but I see that the factory TP Seal is "Stepped" with the wider side to the valve, and the narrower side to the breech.

 So my first question is, when you guys are switching the TP Seal to icemaker tubing, Are you drilling the bottom side of the breech to fit the icemaker tubing ?
Or
Are you just cutting the icemaker tubing to get "sandwiched" and seal against the bottom of the breech ?

Also, I've noticed that the icemaker tubing is slightly larger (O.D.) than the hole in the airtube (at the valves exhaust port).
 So are you guys opening that hole up to allow the Icemaker tubing to fit and press against the surface of the valve ?

Also, will the factory Hammer Spring be enough to handle the 1100+ psi I am seeing at these hot temps ?
 Do I need to put some washers on the Hammer Spring guide to get a little more hammer strike ?
 Or,
Do I need to change out the factory spring ? If So, can you tell me what spring to get ?

Any advise and/or pics of how you did yours would be greatly appreciated.
Thanks

[nervoustrigger]

No need to modify the receiver.  Cut the tubing to 0.100" +/- 0.005" and it will form a crush seal.  Cutting it square is the key...it is somewhat forgiving but it needs to be pretty close to square.  What works for me is to overhang the tubing off the blunt end of a snug-fitting drill bit.  That keeps the tubing from crushing when slicing it with a sharp box cutter blade or razor blade.

I'm thrown off though by your statement that the tubing is a slightly larger OD than the hole in the air tube.  The icemaker tubing I use is 0.170" ID and 0.25" OD.  The hole in the air tube is somewhat larger than 0.25"...unless the factory has changed it.  So what I've always done is build up the diameter of the tubing with some tape so it's a snug fit to the air tube.  That not only helps keep it in place when you go to attach the receiver, but if the valve's exhaust port is not perfectly concentric to the hole in the air tube, it keeps the tubing from shifting and creating an opportunity for blowby. 

Regardless it sounds like something is different for you, and frankly I think it's perfectly reasonable to enlarge the hole in the air tube so it's a snug fit to your tubing.  That would also give you an opportunity to address any lack of concentricity if necessary.

Regarding the factory hammer spring, if memory serves it is about right for 1100psi.  Try putting a couple of washers on the spring guide to shim it for more preload and see if the velocity comes up.  If you're up to it, doing the RVA mod would really make tuning a breeze, and readies it for the inevitable change to HPA.

[larspawn]

There’s def more power available. IIRC my Mike Melick gas hog shoots the 14.4 at 760 or more.

On my tunes I did not match the transfer port but did smooth them out. Just “mashed” the tube in there.

The biggest gains were tossing the debris screen and drilling out the valve and second best gain was extending the probe to seat pellet further in barrel. A section of wire from utility staking flags is a really good option. Third best mod for me was smoothing the leade/feed area which is notorious for tearing up probe o-rings which decreases power.

In fact I think my first tune I followed Bob’s guide which I think is still in the GTA library. Good read even if you don’t do all of it.

[larspawn]

Read this one...

https://www.charliedatuna.com/airgun_docs/Charlies%20QB%20Quik-Tune.doc

[Ribbonstone]

Been awhile between QB co2 posts,,,but likely a lot of archived information.
(EDIT: thanks larspawn....you did the leg work.)
Really have to kind to draw a line between how much speed/power and how much custom work/mods you're willing to do.

Don't really know what the maximum anyones ever gotten with one,or how much custom work it took to get there.  It's a whole lot more than 585fps, although co2 guns do tend to prefer heavy weight pellets for making the most energy.

For most of use, the no mod tune would center around getting the "thick"gas (co2 being a lot heavier than air) to flow to the pellet now rather than later.

Just waching the streiker harder does increase speed somehwat,but a lot of the co2 is getting to the party too late to add to the fun.  Making the path to the pellet less restrictive helps a lot more than just whacking the issue valve harder.

But your basic, simple-quick tune would be a"polly port"...once cut from icemaker tubing..has always worked for me(once you get the length just right), some of the other suggestions either didn't seal for me or were too high (which lest the barrled action kind of titter-totter on the high spot).   LIkely the original transfer port is now tapered...it was soft, and became that shape when the barrled action squished it down.

Normal instalation usually leaves a good bit of slack in your icemaker (assuming your 'fridge has one)...enough to disconnect, cut off a few inches, reconnect, and no onw will ever know.

I've not found it to be too fat.....don't know,in all the years I've played with QB's,I still haven't used up the  9" of tubing "slack"I scrounged.

Removing the "extra"stuff that is in the valve (atleast in the older ones, the valve was the same as a QB78)...tossing the fiber filter and cutting off the piercing pin helps. A QB79 has no use for the pircing pin, and the fiber filter will sooner or later just shed fibers into the valve seal.

Test here, he may be pleased with the speed. If not, add a washer to the spring guide,and try again. Might try two and test yet again.  MIght be enough to make him happy.

From here, increasing gas flow gets more tricky. The main constrictions are the smallest hole between the pellet and the valve stem, the shape of the valve stem,weight of the valve stem, diameter of the valve stem, and the material of the valve stem,smoothness of the passages ways from the valve to the pellet,etc.

[Gertrude]

thanks for the replies guys,

well I just looked and apparently I bought the wrong size tubing. I got 5/6 OD X 3/16 ID.
I will have to go get some 1/4 OD and see how that fits, before opening the hole in the airtube.

[Gipper]

Another option for the transfer port is to use the rubber flat oring from a 2240. Fits and seals perfectly. This was suggested to me by Tim Mac from Mac1 airguns.

Gipper

[YEMX]

.

[Ribbonstone]

Actually, your use is a pretty good one for co2.  Two 9oz bottles (one to shootfrom andthe other to go get refilled before the first one runs out) pretty much makes life easy.
Power is not what co2 does best, but well adjusted it can be efficent and if kept at a reasonably stable temperture, can consider it regulated.
 Applaud the idea...and still keep several co2 rifles running(including a QB)...it's just that I've never really been too happy with co2 in a "hot rod"...tanker or bulk fill.

Generally, once I get to something like 13-15 foot pounds, I stop (although that goes up to about 18-20 foot pounds for a .25).  Does seem that the harder you push one, the more sensitive to temperture change (and a good bit of that is self-cooling from the large amounts of co2 being converted from liquid and ejected at each shot).

Can read along with some of the HPA conversion  threads....it's pretty much the same situation with co2.  Got a constant supply of pressure, just don't really have quite the same plentium issue with co2 QB79's on air have when powering up....so most of the good mods are about improving the flow of gas.

[Hobbyman2007]

Do the new QB’s  still use a flow though probe . If so you might want to open that up to match the rest of the ports . Come to think of it the last one I had none of the porting was lined up . They all had to be reworked . I think I might have managed to get 700 FPS out of it . A friend has it now and is still shooting it after close to 5 years.