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I do not claim to be an expert in the tuning of the QB-78's and probably never will but I have worked on a number of them after doing a lot of research. The information was wide spread and while some of it was of little value, a lot of it was invaluable. I'd like to thank Craig Pitts (Co2unes) for his help in the past.

The information here is a compilation of information found throughout the forums posted by many and is nothing new for the most part, but rather a guide put together with the things that do work and in a way for all to understand. There is no new magic involved, and for the mechanically inclined, not too complex. This is nothing more than what I do depending what I want to accomplish.

This Tune-up Guide is not only a plain simple tune guide but includes in great detail sections on minor and major modifications to hot rod your QB-78. It has diagrams and pictures as well as detailed instructions on what to do and where as well as the tools needed to do the job. When and if you do these mods, the responsibility is your and I am not responsible for any damage or results that are good or bad. There are several alternatives, tune variables and possibilities, and what you do with them is up to you. One of the most important things to keep in mind is that maximum velocity isn't always the best approach.

As with any other TuneUp articles I publish, read this from beginning to end before starting the project, maybe more that once. After you read it thoroughly, make a determination just how much you want to do and prepare for it.

In the first part we are going through the basic tune but as you read through this article and review the mods, you may want to incorporate some or all of them along the way. How far you go in modifications is up to you. Before starting this project, be sure you have all of the necessary tools, seals and parts needed to accomplish the task at hand.

The o-rings needed:

2 each #113 o-rings NOTE: DO NOT USE BUNA/NITRILE HERE. THEY WILL FAIL)
3 each #012 Buna (the barrel o-rings)
1 each #006 Buna for (.22cal bolt seal) or for the .177 1x3 mm
1 each 1/4 OD poly tubing (for the breech seal). The length should be between .80 and .100 long and the ends should be cut flat. This can be cut from refrigerator ice maker tubing

The tools needed:

For the more advanced, you'll need to evaluate what you can or want to do based on your mechanical abilities and the tools you have available.
Again, please be patient and take your time, especially with any of the mods. The time, effort and care you put into this project, whether it's several hours or a couple of days or even a couple of weeks will directly affect the end performance results. OK…if you're ready, let's get started.


First, to make it a lot easier, you should make a valve removal tool. Actually it's a spanner tool. It's quite simple, takes only a few minutes and by looking at the picture you can see how to do it with no real explanation. Just use an old 7/16 3/8 inch drive socket that will fit the inside of the tube and after you make it, remove any rough edges on the outer edges. Picture courtesy of Rich:

Removing the stock:

Before we can do anything, we gotta get the stock out of the way. There's always something huh? The guns safety (#3300) must be removed in order to take off the stock. This is achieved by placing the safety in the full safe position and punching the safety out a small wooden dowel rod or drift punch from the opposite side.
Now remove the locating screw nut on the bottom of the gun (#3200) and lift the receiver, or action out of the stock.

Removing the trigger casing assembly:

Remove the two screws (#2200 and 2300) that hold the trigger assembly and passes through the hammer spring area from the bottom. Take notice how and what the rear screw passes through for reassembly later.

Now let's do a little work on the trigger to improve it:

Open the trigger assembly by removing the two case plate screws (#1814) and remove the casing plate or cover. As you can see, it is a very simple trigger and easy to work on. Wash it out thoroughly with a solvent such as mineral spirits and remove any paint or loose pieces of metal from the seer and other parts. Let it dry, then using 400 grit wet/dry sand paper, then 600 grit, polish all of the trigger and mechanical contact areas, especially (#1805, 1804) to a smooth finish as well as any other area that you can see where there has been any rubbing contact metal to metal . The trigger spring (#1809) may be replaced with a lighter spring to make a lighter trigger pull, but care must be taken here so as not to be so light to make it dangerous. Some have taken a coil or two off of the spring, but I suggest replacing it with one of about the same length but a slightly smaller wire diameter. This can be serviced after the gun is reassemble with the stock off if need be. Apply a Moly oil combination such as Gene Sunday's Mystery Oil thoroughly to the trigger assembly after it's reassembled and allow it to drip dry as much as possible. Pat dry any excess lube and reinstall the Casing Plate or cover.

Looking into the small hole in the cover you can see the trigger seer travel. This can be adjusted by adjusting the top screw at the rear of the trigger. (#1807) Adjust it so there is about 3/32 to 1/8 inch travel before release when looking in the hole. This can be fine tuned after the trigger is reinstalled later when testing.

Separating the barrel from the breech:

Remove the barrel cap. Loosen the screw in the side of the barrel support bracket and slide the barrel bracket off of the barrel and onto the tube. Remove the barrel from the receiver by unscrewing the Allen set screw (#2800) on top of the breech between the bolt openings and where the barrel meets the receiver. Remove the barrel by pulling the barrel out of the receiver. Remove the barrel band from the Tube. Remove the barrel o-rings from the barrel and discard any damaged ones. Now would be a good time to clean the barrel using GooGone and patches with a pull though as well as checking the crown. You did get your needed parts…right???

Also, check the muzzle crown. The surface of the muzzle needs to be uniform around the whole muzzle. Now take a Q-tip and slide it into the muzzle and slowly back it out. If no fibers catch on the muzzle, it should be fine.


There are a couple of approaches to re-crowning the barrel but for most people with limited tools, this will work. Start out by taking a pellet and inserting it into the muzzle head first pushing it just inside the barrel. I took a brass Philips head screw that was large enough to cover the surface of the muzzle (you do not want it to go into the muzzle, but cover the outer rim) then chucked it up into a variable speed drill. You'll want to do this slowly. Using a valve grinding compound and then a super fine machining compound (valve grinding and finishing compounds are available at any parts store) slowly refinish the muzzle crown keeping the drill as straight up and down as possible. Then, take a small piece of 600 paper, form it into a cone and work it a little in the edge of the muzzle in a twisting back and forth fashion. When you think it looks good, again, take a Q-tip and slide it in the slowly back out. If no fibers catch on the muzzle, it should be fine. Now remove the pellet pushing it out from the breech end with a small long dowel or being careful, a cleaning rod.

Now for the cleaning of the barrel. Again, there are a lot of ways and suggestions. What I wound up with was very simple and inexpensive as well as safe. I dripped some GooGone on a soft nylon bore brush (do not use a brass bore brush) and worked it back and forth a couple of times in the barrel. Then I ran patches with GooGone on them through it. I wound up doing this three or four time before it was clean, and I thought it was clean before I started. I was really surprised. Run a few extra patches clean dry patches through it to remove all of the excess GooGone.

Removing the bolt assembly:

Remove the long Locating Screw (#0300). Beneath the bolt where you would load a pellet is a screw. Unless the barrel is removed, you will not see this screw. Remove this screw and now the breech and the tube can be separated. Note the breech seal that sets between the barrel and the tube. Take notice of the location of the hammer pin (#0900) and the cocking pin (#1000) are located and remove them by lifting them out and set aside.

Remove the bolt assembly by removing breech plug (# 1900). It might have fallen out fell off when you removed the rear trigger screw. Looking inside the rear of the bolt you will see an Allen screw that must be removed. Removing this screw will allow the bolt action handle to be removed so the bolt itself can be slid out of the action. After you remove the Allen set screw, the bolt handle will pull out of the bolt. Now you can slide the bolt assembly out of the action.

We might as well get this part over with. File smooth and deburr any metal where the bolt handle extends out of the gun along the length of the slide track the bolt slides on as well as the hammer and cocking pin slots and polish smooth using 200-400 paper. Using a small cylinder hone or a dowel and piece of 200-400 paper rolled into a tube, clean out the inside of the bolt chamber.

Thoroughly clean all of the parts then install the new bolt probe seal and apply a thin coat of 100% silicone dielectric grease to the bolt probe seal and a thin coat of moly to the bolt assembly and reinstall the bolt, bolt knob and Breech Bolt set screw. Set this aside for now.

Removing the valve assembly:

Looking down into the tube you can see a slotted area that looks like a plus sign about 2/3rds of the way down. Be sure the Valve Set screw (# 0500) is still in place. Using a wide blade screwdriver, attempt to loosen the valve about one turn counter clockwise. You may not be able to loosen it without making a special spanner tool (see pic above) out of a 7/16, 3/8 inch drive socket. Even then it sometimes can't be loosened with out causing damage. If you are unable to loosen it, continue on.

Remove the Valve Set screw (#0500). Be sure the locating screw and the valve set screw are both removed. Now remove the valve assembly by using a piece of dowel rod a little smaller that the tube diameter and inserting it into the front of the chamber and with the rear of the tube placed on a piece of wood, tap out the valve assembly out through the rear of the chamber. This will require some effort initially, especially if you were unable to loosen the valve, but once it breaks loose and starts moving it will it will come out easily. It can be driven out from the rear also, but be sure the inside plug (#0200) is in place in the tube (not secured with the locating screw) so as not to damage the valve stem.

Let's do some work on the breech:

Mod available…see modification section for additional information: Now deburr the breech port (Just above #1500). Deburr the inside of the breech (#1200) paying special attention to where it is drilled for the transfer port and barrel set screw. Slightly bevel the forward breech ID where the barrel slides in and polish this area. Polish this ID of the breech using 220-320 emery cloth removing the sharp surface edge.

NOTE: Special emphasis needs to be placed on the inside of the tube where the Valve Set screw threaded hole is and also the transfer port inner edges. Although a little difficult to reach and clean up, unless you do, you can almost bet on cutting the seals when installing the barrel. If the above is done properly, you will never cut another set of o-rings (#2700) while installing the barrel.

On the bottom of the breech you will see where it is beveled to fit the tube. (Just above #1500) Lightly file the sharp raised to create two flat edges about 1/32 inches wide on each side of the port. This provides the seal with a flat surface contact area and chances are you will never blow another breech seal.

As for the breach porting and bolt probe extension, they go hand-in-hand (see mods), but in many people's opinion, really doesn't seem to enhance performance enough to justify the extra work. It only adds about 5-10 fps at most and while at the same time, reduces usable shots by using up or dispensing available Co2 much more rapidly.

Matching of the breech port to the barrel port is important for smooth uninterrupted flow of Co2 from the breech to the barrel. (See figure 1 B-2)

Now about this valve thingy:

The valve is a two piece assembly that is screwed together. To separate the valve assembly, unscrew the two halves. Unless you were able to loosen the valve in the earlier procedure, it is going to be tight because they are LocTited together, or at least mine were on both QB-78's. Care must be taken not to damage the surfaces of the valve. I used a piece of rubber hose and held it in a vise and used a flat file edge (until I made the spanner) as a wide screw driver on the end, although any sturdy, flat wide tool/blade can be used. Once it breaks loose it will unscrew easily. Note: when opening the valve, take notice how it is assembled on the inside. Thoroughly clean and lube all of the parts with a fine coat if 100% Dialectic Grease. Install the new seal and reassemble, but do not tighten it completely. Leave the assembly about ½ turn loose. If you tighten it, you will more that likely damage the seal when you install the valve.

Mod available…see modification section for additional information: Below, if you have the tools and are accomplished enough to perform the task, is a modification for the valve that can be done before assembling and lubing the valve. This mod is to improve the flow of the propellant for more efficiency. Thoroughly clean all of the parts that are on the inside. Lube the valve body (#0400) using Air Tool Oil, or preferably 100% pure dielectric silicone oil and reassemble.

Take part #'s 0407/0408 and modify to the specifications in the diagram Figure-3 C-1 and C-2below.

The Tube:

Next, finish honing and cleaning out the tube assembly. To do this, I use a 3/8 inch dowel 16 inches long with a slot cut in one end about 4 inches long. I then place a folded in half four inch wide piece of 200 grit wet/dry sand paper and place the other end in a drill and polish it making sure the paper is wet and rinses off the paper as you do it, and then do the same thing with 400 grit paper. Then take a clean cloth and dry thoroughly inside and out. Lube the inside of the chamber with preferably 100% silicone oil or a thin film of 100% dielectric silicone grease or if nothing else, a good Air Tool oil.

Well, I guess we are ready to put this mess back together again.

Be sure the valve has not been tightened up. It should be about ½ turn loose. Insert the valve assembly into the front of the chamber tube. Line up the threaded screw holes on the valve with the two holes in the tube on the same side of the tube (the bottom) with the single small slot. The valve port hole in the valve should be in line with the two holes on the side (the top) with the long slots. Carefully push the valve assembly into the chamber slowly until the valve port hole reaches the first small hole in the tube. Using a small screw driver or pointed punch, line the holes up, and when it's centered in place, reinstall the valve set screw (#0500). Now, using a wide blade screw driver go down through the front of the tube and tighten the valve. Tighten it pretty snug.

Now place the inside plug (#0200) into the rear of the tube and install it with the threaded holes side inserted first. This plug is threaded on both sides but different sizes. The larger of the two holes should face the bottom of the tube, the side with the small slot. Slide it in until you can see the holes in the tube just ahead of the slots. Install the Locating Screw (#0300) into the threaded hole next to the Valve Set screw on the bottom of the tube and tighten. Install the locating screw (# 0300)

Install the new breech seal/gasket (#1500) in the seat in the tube. Note: Do Not Lube This Seal.

Now you can reinstall the hammer spring assembly (#s 0600-0700-08000) in through the rear of the chamber. Install the hammer pin (with the round head and goes in the front hole) and the cocking pin (with the square head and goes in the rear hole).

Take the time at this point to be sure that everything seems to be in its right position.

Place the breech over the tube aligning the hole for the breech hold down screw. Be sure the two pins (cocking and hammer) are in their slots in the bolt and be sure the breech seal is in place. When all is aligned, install the hold down screw (and washer if it has one) (#1600-1700). Some don't. Do not tighten this screw yet.

At this point there is a mod using a ¼ thick spacer between the cocking block (#0800) and the tube plug (#2000) to increase power by allowing a transfer of more Co2. You can do that now.

Now install the trigger. Do not cock the gun until the trigger screws are in place and snugged up. Install the front screw first and just loosely tighten it, then after installing the Tube Plug (#2000) (see above) and Breech Plug (#1900) and aligning the holes, install the long rear trigger screw and snug it up. Snug up the front trigger screw. Now cock it and slide the bolt back and forth a few times to be sure it travels freely back and forth. If all feels well, tighten the screw in the breech and the front and rear trigger screws. Now you can cock and "shoot" several times to be sure all is working properly.

At this point you can fine tune the trigger by adjusting the top screw using a tiny screw driver and cocking and pulling the trigger. Be careful not to set it so close as to make it dangerous.

Install the barrel seals on the barrel without any lube. After the seals are installed on the barrel, lube the outer seal surfaces with a fine coat of 100% dielectric silicone grease. Just prior to installing the barrel, put a fine coat on the breech where the barrel slides in. Install the barrel clamp over the barrel. When installing the barrel into the breech, do not twist the barrel (twisting it as you install it could very well damage the seals). Just slide it in being careful not to damage the seals as they pass over the holes. If need be, you can depress the seals at those points with a small screw driver to help get past the edges. Slide the barrel clamp down over the tube and tighten the screw slightly. Just snug it up because it will break if over tightened. It does not need to be tight.

Finally we're going to put the stock back on. Slip the action back into the stock and install the washer and nut and tighten. Install the safety by depressing the spring pin that you can see in the hole with a small screw driver and while depressed, slide the safety through the hole from the bolt side.

The bolt may still be a little difficult for around 500 shots but it will loosen up as it breaks in and after about 500 or so shots and becomes much easy to work.

The only real maintenance to these guns is a few drops of silicone lube (I use a small rag with silicone shock oil) applied inside the air chamber (where the co2 powerlets go) every 1000 shots or so. I also sometimes spray my pellets with a light coat of silicone spray as co2 has a drying effect unlike o2. You may want to put a drop of lube on your finger tip and touch it to the tip of the co2 cartridges when installing them.

Figure 1


Figure 2

Major Modifications for the QB-78:

As for the breach porting and bolt probe extension, they go hand-in-hand, but in many people's opinion, really doesn't seem to enhance performance enough to justify the extra work. It supposedly only adds about 5-10 fps at most and while at the same time, reduces usable shots by using up or dispensing available Co2 much more rapidly. That being said, if you do decide to do the breech mod, then you should also do the bolt mod.

Bolt Mods: you can do one or the other…if you do the barrel to breech mod you should do the major.

Minor Bolt Mod: With the bolt out you will see a small port in the front of the bolt probe as well as the hole looking at the end of the probe. You can, if you like, carefully enlarge the oval hole in the bolt probe a little and even slightly enlarge the drilled passage threw the probe. This will up the velocity without changing the number of shots per fill.

Major Bolt Mod: The alternative, major surgery on the bolt. Remove the bolt o-ring. You will want to cut off the existing bolt probe in front of the bolt sealing o-ring as shown in Figure 3, D. You can cut it off just a little further ahead so as to have a little more drilling room in the front of the bolt, but the end result that you want is the total length. Smooth out the rough edge where you cut it off. Now using a 1/16 inch drill bit, drill into the front of what's left of the existing probe a short distance. When all is set, you can use a 1/16 inch drill bit cut off to the proper length (see the diagram for the total length) making sure the end that pushes the pellet is flat and true. What is important here with the probe is that the pellet when pushed into place by the probe is past (in front of) the port by at least an eighth of an inch and the seal itself is behind the port. Use Loc-Tite to secure the probe in place. Using a 200 or so grit paper remove any sharp edges on the probe so as not to damage pellets and keeps them straight as it pushes the pellet home. Your new probe is complete.

Figure 3

Barrel to Breech Mod:

Looking at the barrel port you will see that it is a tapered and an elongated hole. The object here is to use a Dremel Tool with a tapered grinding tip (some people use a drill bit) and make the taper a wee bit larger. When doing this, care should be taken to not go beyond the edge of the metal to a point where you get into the o-ring groove. After the tapering is done, you then want to use you Dremel Tool with a metal cutting tool to elongate that hole just a little more on each end…not by much.

Close matching of the breech port to the barrel port is important for smooth flow of Co2 from the breech to the barrel. (See figure 1, item B-2) Note: do not cut the breech port to match with out serious evaluation and measurements as you might have a sealing problem with the breech seal to the tube.

Valve modifications:

There are several things that can be done to the valve to improve efficiency and flow. The ultimate objective is to move as much measured propellant as fast and as smoothly with the least amount of restriction as possible. How much modification you want to make is up to you. While doing these mods on the valve (and any other put of the tune for that matter), take your time and work with care.

Carefully review figure 3 and compare A-1 and A-2 and identify with them the differences in the port of the stock valve and the modified valve. Also do the same with the valve stem, C-1 and C-2. 

Let's work on the valve stem first. Most people don't have a lathe, but these procedures if done carefully can be done with a drill press or even a drill that is secured or if you have another pair of hands. While working on the valve stem and tapering it, it is important that you do not get it to hot as it will damage the valve stem seal shown on figure-1 (seal surface) and the Co2 will leak out of the gun.

We are going to taper the brass on the valve stem (#0408). Draw or cut a fine line around the brass about ¼ of the way down from the seal end of the valve stem. This will be your upper limit although it doesn't need to be precise. Using which ever tool you have available, chuck the valve stem into it. Using a fine cutting file, carefully while it is turning, taper the brass from the upper point where you drew the line to the end until the end has a diameter of approximately 5/16 inch. If you have a micrometer and want real precision, use the numbers on the picture. As you are doing this be sure to keep the brass from getting hot by stopping and applying a wet rag to it. After you reach that point, take a piece of 400 grit wet/dry sand paper and smooth it up real nice and where your starting line was, bevel it with the paper and take off a little of the surface up to the top where the seal is being careful not to come in contact with the seal itself.

After you have finished with the valve stem, take the piercing pin (#0407) and chuck it up. We want to cut the spring seat down to a point that it matches the diameter of the tapered end of the stem valve. After turning it and filing it down, smooth it out with the 400 paper also. Looking at the piercing end where the sharp point is. Taking care not to damage the point itself, take off the sharp edge right behind it where it goes from a taper to the shaft just enough to round it a little for smoother flow.

Next we're going to do a little work on the Exhaust Valve body (#0410). Looking at Figure-1 A-1 and A-2 you can see that it has been tapered inside. Tapering this must be done with care. Remember that the breech seal sits on this port. You don't want to open up the port or hole at the top (start just below the edge) and as you taper it inside, you need to be very careful not to go too far out to the edge at the bottom of the taper. At the bottom of the taper I would stay between 1/32 and 1/16 of an inch from the seal seat wall. If that seal seat is damaged, it is junk. To do this part of the mod it's best to use a Dremel Tool with the tool #N (followed by a degree mark) 194. After you have it tapered, polish it out using a thin long nail and a piece of emery paper or 400 grit paper. When you are finished clean it thoroughly.

It's time for a little hole punchin' in the main body, the aluminum shiny silver piece. Ya gotta be careful here. Look at the front of the body where the large screwdriver slots are. In the center is the hole where the piercing pin protrudes. Mark the flat surfaces evenly in each quadrant just a little in from center between the inside and outside of the valve. Looking at the picture below will give you a general idea. Drill these holes using either a 3/32nds or 7/64ths inch drill bit. Be sure when drilling these hoes that the drill bit does not go toward the outside of the valve but angle it ever so slightly toward the inside of the valve. If it happens to goes through the outside wall you can just throw the Valve Body away.

Now look at the washer (#0404). We want to modify this washer a little also to allow the volume to flow more freely. Below on the left is the washer stock and on the right is the washer modified. It's just a matter of putting notches in the washer on either three or four sides. After you grind or file them out, smooth out the rough edges by laying a piece of 400 grit paper on a flat surface and polish it. I'm not very good at this graphics thing, but you get the idea.

When assembling the valve, there are some parts that you may or may not reinstall. You do not need to install the filter (#0402), and the screen (#0403) With that in mind, be sure to clean everything thoroughly when assembling  the gun so there are no contaminates inside and especially keep the Tube clean inside and when installing cartridges, always wipe them off first. If you are going to bulk fill, install a fine mesh screen filter in the line somewhere. I put mine in the inlet side of the shutoff valve. You may also want to find a slightly heavier spring for the valve but it's usually not necessary. Ok…lube the valve as described in the tune section and reassemble the valve with the new o-ring and remember not to tighten it up yet.

After this modded valve started leaking real bad, Rich had to remove some damaged parts and spring. Notice the destroyed filter screen.

QB-78 Valve Pic courtesy of Rich:

That's it for the valve body. You can bet it will breathe a whole lot better now. I'll bet you're glad to be out of here huh? So…how ya doin'….are ya still with me…or did you get lost back there somewhere????
 

So now what will this entire project do for you? This should improve the velocity considerably depending on what all you elected to do. If all was done and done well you can expect a velocity in the area of somewhere around 680 or so up to 725 fps in .22 caliber on a day with temps in the 75-85 degree range with a fully charged with liquid Co2 chamber.

Ok…now that you have all of this completed, you can go back to the first section for the assembly instructions.