Answers to Common Industry Brand QB78 - QB79 Questions

Mike Melick Tuned QB78

Q:  I've been shooting my QB in hot weather, 100F+, and noticed the power level dropped considerably...is this "valve lock"?

A:  Anything over about 90 degrees will slow the rifle down. If a very light spring is installed in the valve you may be able to use the rifle up to about 95 degrees, or you can install a much heavier spring for the hammer. These have a tendency to smash the end of the valve stem and bend stems but if you need it......... The Mcmaster-Carr part number for the conical spring is #1692K36 and should help hot weather shooting. It may cause a valve leak when first piercing the cylinders, so pop the cylinder then open the bolt right away so the pressure can close the valve. The washer that sits under the spring can be removed with this spring as it's just a tad bigger than the ID of the tube and holds the pin just about perfectly in the middle without the washer. It's also lighter than the OE spring and so can give more FPS in the right conditions but will also use more gas per shot.

Q:  Can I free float my QB barrel, should I add a second set screw to the breach to hold it tight?

A:  I've worked on many QB's now and have found that just removing the barrel band will help some rifles but not others, why? I don't know. Really no need for a second screw in the receiver, just use a hardened 10/32 set screw and snug it down good. The newer OE set screws are harder than the old ones but will still round out with the cheap allen wrench supplied with the rifle. Buy a good one and they seem to fit better and don't round out as easily. I have a .22 that didn't shoot very well at first, asked around and was told to remove the barrel band and try it, helped a whole bunch. On some rifles a small washer placed on the hold down stud will help settle a rifle down, probably due to the inletting of the stock not fitting quite right. On highly modified rifles the chill effect on the gas tube will make the barrel band and the barrel move quite a bit, so on those types remove the barrel band for sure.

Q:   What do you use for the breech seal on the QB guns?

A:  I've been using the 1/4 inch OD hard poly tube, the ID is .170. It's the same stuff they use for ice makers in the fridge, nothing fancy. Cut it between 1/10 and 1/8 inch, too long and it will restrict the flow, to short and it will leak. At 9 cents a foot you can make a few screw-up's and still not be out anything.

Q:  I have a QB78 that leaks down fairly fast-- at least fast enough for the tube to get cold and sweat. I do get a few shots, maybe a 3rd of normal, but they aren't as powerful as before. I don't hear CO2 escaping, but perhaps my hearing is not that good. Also, I don't feel any CO2 coming out of the barrel or the breech. I'm wondering which seal this would be?

A:  Your leak could be a loose valve. The body of the valve will sometimes become loose and then the o-ring on the cap of the valve will not seal. A piece of 3/4 inch by 1/8 inch steel strap or a big screw driver will tighten it. If you need to remove the valve because it is leaking, clean the seal on the valve stem and then check it with a magnifying glass, any dirt or shavings left in it can be gently scraped off and it will sometimes reseal.  Alan at AB Airguns goes great work on resealing the stems.

Q:  What is the thread type on the end of the QB main tube?

A:  Looking in my record book(this may not be correct) 22mmX1mm.

Q:  I read a procedure that entails drilling two holes on either side of the piercing pin hole. What does this do for my QB? Will this give me more consistent shots? Also, do you recommend getting rid of the filter in the valve? I did this, and now I find that my stem seals in both of my QBs are embedded with debris, and are now leaking slowly. Looks like I'll have to send them off to AB Airguns to have them fixed?

A:  Well, the first one is where did the dirt come from? Shame, you didn't clean your cylinders before putting them in the gun. Don't feel bad, I've done the same thing and had the same result. If you get the stems resealed at Alan's make sure everything is spotless and try to keep it that way. His seals are harder and can leak easier than the OE ones. I have cleaned the OE's with a good cleaner and a straight pin. Just gently scrape the gunk off and out. His seals will up the fps and shot count for you so.....As for drilling the two holes in the end of the valve, this allows more gas to flow into the valve and will help the velocity. The piercing pin will stay centered even without the hole in the end of the aluminum body, so I just make a slot at 90 degrees from the tightening groove to improve flow.

The next step is to reduce the diameter of the head of the valve. Do this slowly and don't get it hot; if it feels warm to the touch cool it as you work it down, don't get the area around the seal to thin it can break out. Next, if you have a Dremel tool remove material in the brass cap from around the area where the head of the valve sits. Open it up and the gas will flow better with less restriction. As to removing material in the port passage in the brass cap, I remove a little but not much, it's easy to take too much and break through. Side to side is better than fore and aft. I have noticed that the machining of the port will almost always leave material on the left side of the port when looking into the cap from the side inside the valve, or to put it another way, when looking into the exhaust port the material to be removed is on the right side of the cap.

I have a collection of what not to do to the brass caps in my reject box, one slip of the hand and it's junk. Have fun, be careful.

Q:   I have a QB79 in .177 caliber. Right now it's hitting around 690 in 70 degree weather. I have done the mods listed on Charlie's website. I even opened up the bolt probe to allow more 02 to flow, opened the barrel port to the largest size. What else can be done to get my little friend over 700. BTW the pellets I use are the Gamo hunters which are 8.3 grains. I have had the best of luck with these, and they group the best in my gun.

A:  You may have a restriction at the bolt probe; time to center drill, bob and install an extended one. Look for the little flags in peoples yards from lawn spraying companyies and wire locaters. Ask if you can have one when they are done with them. The wire is just a hair over 1/16th inch in diameter and will work good for a probe. A good 2 part epoxy glue holds them in place just fine. I've gained 50 fps on some rifles by doing this. Also check your valve stem seal, if it looks ragged sent it to Alan at AB Airguns in Kansas and he will replace it with a new material that will help the fps. Just make sure you don't get any dirt in the gas tube as these seals will not seal with dirt in them. I have gotten an increase of about 40 fps on some rifles with these seals and use them in all of mine that I sell.

Stock care on the QB-78

The new style dark colored stocks are a great improvement over the old style orange colored ones but can stand a little help. When you take a new rifle out of the box it will be covered with a light coating of preservative from the factory. To remove it is quite easily, pull the action out of the stock ( see page 6 of your owner’s manual). Remove the butt cap by taking the 2 screws out and set it aside.

Take a rag with some WD-40 on it and start rubbing the stock. You can spray the stock with WD if your rifle came with the warning sticker on the right side of the shoulder stock. Soak the sticker well with the WD and let it set for about 1 hour; comes right of most of the time or use a hair dryer to warm the glue.

After you have removed the preservative you will start to see the color in the wood and the grain much better.  Remove as much of the WD as possible with a dry rag, and then put a drop of liquid car wax (the cleaner type works good as it smoothes the varnish some) about the size of a quarter on one side of  the stock and start smearing it all over the stock with your hands.

Cover all of the stock but the rear cap area and the inletting for the action. Keep smearing it around until it just starts to get tacky, then set it aside and let it dry, wipe it off once dry with a clean rag and do it again. If the varnish coating is still rough and the stock doesn't want to shine, you can take some Soft Scrub and rub it on the stock gently with your hands and it will help smooth it out. Don't get to hard with the rubbing cause it will cut through the varnish.

When you are all done it should look like something that should come on a much more expensive rifle.

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