QB78 'blind' tuning

[Toddspeed]

There is so much information on this forum, I thought it would be easy to build a QB to do just what I wanted it to. So I bought a 78s, (my first air gun by the way) used it stock for awhile, then tore it down and started following "the $200 pcp" thread as a guide.

Porting for more flow seems pretty simple, drill out the flow through bolt a little along with the transfer port through the bottom of the barrel, get a maurader TP seal, open up the sides of the valve and remove the filter. Check, I'm all set for more power. Then it happens... I tripped and fell down the rabbit hole of actually tuning the valve.

I've read about HDD, TSS, SSD, RVA I'm thoroughly confused.

Obviously, the idea is to avoid hammer bounce and wasting gas, but what's the best way to do it for a QB78 .22 still on 2 powerlets?

According to the "$200 pcp" thread, I should cut about 1/8 inch off the hammer spring. But before I do I want to draw on the considerable knowledge base available here. Keep in mind that while a Ninja SHP is in my future, for now I'm using the 2 powerlets, also, I don't have a chronograph, so I realize I'm not going to set any records as far as efficiency, but I'd like to get the most efficient 'blind tune' I can.

Since the "$200 pcp" thread is a few years old, is their a better way to tune a qb?

If I cut the hammer spring will that create a problem with piercing the first CO2?

A little help please?

[Nvreloader]

Todd
Here is some reading/info for down the road trip for you,
https://www.gatewaytoairguns.org/GTA/index.php?topic=85896

Save the info, as you will referring back to it several times,
I made a master list of the posts/topics over the lasts several months,
that I have been researching for the same info, on several good site dealing with type of info for 78/79 modding, etc,
if you want a copy of this list, I can pm you the info, if you want it?

3 of the best 78/79 Gurus for these models are, Bob, Nerverous Trigger, Ribbonstone, plus several others,
they are very sharp, so pay attention etc, I am just following along, attempting to get close,
you can do a search for QB 78 or 79 and you will finds lots of info to read thru,
be sure to have a cup of Java handy, as there is a lot of info to read thru etc. 

I am just starting my mods on the 79, using both fuels, Co2/Hpa,
but the Little Gremlin under the bench keeps throwing monkey into the plans etc,
pretty soon, I'll throw him some wrenches.

Hang in here a couple of days, and I am sure you get some info,
to help you slide down the "Dark Side rabbit hole.   LOL 

HTH,

Tia,
Don

[Toddspeed]

Thanks Nvreloader, I've been reading lots of stuff from those three, I actually was hope one of them might chime in and push me in the right direction. i would love to see the info you've collected.

[Nvreloader]

Todd

PM sent.

Don

[Nvreloader]

Todd

Back to part of your post on HDD, TSS, SSD, RVA and being confused,

Join the crowd, as I am not completely sure of each of these items/abbreviations,
the only I slightly know about is the RVA - rear velocity adjuster.

I don't know if they are the same or work the same etc.

I am also hoping that one of the enablers will clarify each and how they work etc.

Tia,
Don
 

[SpiralGroove]

There is so much information on this forum, I thought it would be easy to build a QB to do just what I wanted it to. So I bought a 78s, (my first air gun by the way) used it stock for awhile, then tore it down and started following "the $200 pcp" thread as a guide.

Porting for more flow seems pretty simple, drill out the flow through bolt a little along with the transfer port through the bottom of the barrel, get a maurader TP seal, open up the sides of the valve and remove the filter. Check, I'm all set for more power. Then it happens... I tripped and fell down the rabbit hole of actually tuning the valve.

I've read about HDD, TSS, SSD, RVA I'm thoroughly confused.

A little help please?

Hey Todd,
Welcome to the GTA .

Bob's post on the $200 PCP assumed you were going to power the QB79 with a HPA tank - not CO2.
All the mods he did were to optimize air flow in the gun and increase the air (Plenum) capacity for each shot.  Since the HPA bottle includes a regulator, the gun is modded for greater consistency too.

I added a Rear Velocity Adjuster (RVA), which pretty easily, allows one to adjust the gun's velocity from 0 to 900+ fps.  I never bothered any other efficiency mods (SSG) as my guns already got 70+ shots per fill.  I wish I could post my pictures, but can't due to the photobucket fiasco.

To add HPA to the QB78, other mods must be made to add a tank block, replacing the co2 powerletts.
Please see Nervous Trigger's and Ribbonstone's posts on these mods.

I hope this helps .

Kirk 
     

[nervoustrigger]

Like many CO2 models, the QB's are "over-sprung" and waste CO2.  Presumably that is to prevent valve lock at high temperatures...CO2 is about 850psi at 70F but a whopping 1900psi at 120F which is not unthinkable if left in the hot sun.  There's just no getting around the temperature sensitivity of CO2 so if the goal is to make efficient use of it, the practical thing is to tune for modest temperatures.  As such, I would heed Bob's advice on clipping a couple of coils as a good starting point but a chronograph really takes the guesswork out of it.  What you would do is keep shortening the spring by 1/2 coil increments until the velocity just begins to decline.  Lacking a chronograph, you can improvise by doing this until your point of impact just begins to noticeably fall at 25 yards.
 
Later if you convert to the Ninja SHP regulator (~1100psi), just shim the spring with some washers until the velocity plateaus.  It won't take much.
 
In terms of tuning aids like RVA, SSG, HDD, TSS, etc...there's tons of information on the forums but I realize it can be a little daunting for someone just getting into the hobby.  For the QB platform, I would suggest you start with an RVA (rear velocity adjuster) which is nothing more than a screw that goes through the end cap and presses against the back of the spring guide.  By moving the screw in and out, you change the preload on the hammer spring.  More preload = higher velocity (to a point) and greater CO2 usage (fewer shots).  Less preload = lower velocity and more shots.
 
Once you get the hang of it, you may or may not want to advance to something more sophisticated and versatile like an SSG (spring stopping guide).
 
Quick point about doing an RVA on the QB platform.  You need to remove the rear cocking pin--the one in the spring guide, not the one in the hammer.  Doing this converts it from cock-on-close to cock-on-open and allows the RVA to function as needed.  BTW, you don't need to shorten the hammer spring if you are doing an RVA.  That's because removing the rear cocking pin allows the spring guide to slide back roughly 3/4".  In other words, it reduces hammer spring preload by 3/4" so for starters, add a 1/2" spacer on the hammer spring to put you in the same ballpark as Bob's recommendation of removing two coils.

[Toddspeed]

To add HPA to the QB78, other mods must be made to add a tank block, replacing the co2 powerletts.
Please see Nervous Trigger's and Ribbonstone's posts on these mods.

I hope this helps .

Kirk 
   
[/quote]

First, thanks for the reply. Looks like an RVA is a good starting place for me.

But here's another question:

I see that every time using a Qb78 with HPA is talked about, a tank block is recommended. I was planning on adding a foster fitting to my end cap, and using a short line to connect to a ninja bottle mounted under the fore stock. I figure it would allow me to use a shut off, plus I could always go back to powerlets if I want to have the original look. Is there a problem with this idea?

[Toddspeed]

[  What you would do is keep shortening the spring by 1/2 coil increments until the velocity just begins to decline.  Lacking a chronograph, you can improvise by doing this until your point of impact just begins to noticeably fall at 25 yards.
 
Thanks nervoustrigger, I would not have thought about improvising like that in lieu of a chrono.

[SpiralGroove]

To add HPA to the QB78, other mods must be made to add a tank block, replacing the co2 powerletts.
Please see Nervous Trigger's and Ribbonstone's posts on these mods.
I hope this helps .
Kirk 
   
First, thanks for the reply. Looks like an RVA is a good starting place for me.

But here's another question:
I see that every time using a Qb78 with HPA is talked about, a tank block is recommended. I was planning on adding a foster fitting to my end cap, and using a short line to connect to a ninja bottle mounted under the fore stock. I figure it would allow me to use a shut off, plus I could always go back to powerlets if I want to have the original look. Is there a problem with this idea?

Todd,
There s/b a few posts on the GTA where members mounted their HPA bottle under the forestock using fittings capable of handling the pressure.  With most HPA, mods, you wouldn't be able to go back to OEM unless you do considerable work. 
Fortunately, once you go to HPA, you really shouldn't have a desire to go back to co2 as it only provides inferior performance.  If you have kids and want a simple OEM QB, (IMO) buy another as they can be had for $100.   

[Toddspeed]

I had this in mind, a one way foster fitting threaded into the back of the piercing block / seal of the cap.
Then drill a hole without disturbing the piercing pin, so the high pressure air can enter the cylinder. If you put some powerlet's in it the one way will close and keep the CO2 in.

I got the image off the web, so if it's anyone's out there, feel free to take credit.

I'm not sure I will ever go back to CO2, just have a thing about making permanent changes (at least visible ones ) Maybe I have commitment issues.

[nervoustrigger]

Go ahead and get hitched to HPA.  She's an honorable, beautiful young lady.  If you later feel the need to file for divorce, it wasn't her fault.  She was always honest and gave you her best. 

CO2 will lie to you and embarrass you in front of your friends. 

[Nvreloader]

Jason

Priceless,
You little match maker........

Tia,
Don

[Toddspeed]

A true poet.

[SpiralGroove]

Go ahead and get hitched to HPA, she's an honorable, beautiful young lady.  If you later feel the need to file for divorce, it wasn't her fault, she was always honest and gave you her best. 

Yes, very Dylanesque .

Five HPA Marriage essentials:
- RVA
- Rear Cocking Bolt
- Maxi Valve
- Trigger Tune
- Rigorous barrel cleaning

[WesBob]

I was like you Todd, and had commitment issues, and probably used the same picture to do mine. I did just like you said drilled and tapped a hole from the front for 1/8npt and then drilled a smaller hole from the tube side, missing the piercing needle. Worked great. I've never gone back to co2 though, so was probably extra work not needed . Anyway, doesn't hurt, and gives you an extra option. However, once you start modding your valve for extra power, the front peircing pin will probably go, and you'd have to stick it back in to use co2. Not a big deal.

[Toddspeed]

I was like you Todd, and had commitment issues, and probably used the same picture to do mine. I did just like you said drilled and tapped a hole from the front for 1/8npt and then drilled a smaller hole from the tube side, missing the piercing needle. Worked great. I've never gone back to co2 though, so was probably extra work not needed . Anyway, doesn't hurt, and gives you an extra option. However, once you start modding your valve for extra power, the front peircing pin will probably go, and you'd have to stick it back in to use co2. Not a big deal.

Wes, I've just been reading the thread about your QB with the 88 gram expansion tank. Very interesting. What was your motivation for this instead of using the Ninja SHP? Also I've already modded my valve, but I kept the piercing pin in. I made a guide for it so it will stay centered and avoid interfering with valve operation. I'm going to cut about 1/8 inch off the hammer spring, would you recommend also putting one o ring in between the hammer and the valve to help with efficiency? Also, how many pumps does it take you to fill your 1 88 gram tank + cylinder?

[WesBob]

Hi Todd, well the main reason I used the 88g cartridge was I already had a couple from building my crosman 1322 pumper pcp. I had used them as an auxiliary tank on the pump gun attached to the side of the tube and was able to get 70 some shots if I remember correctly, just using the pump on the gun to pump up the gun and tank. Took to many pumps though,  lol, so I switched to a 12g quick change adapter which gave me the option of 9-10 good 560fps shots on air or 30-40 shots @500fps on co2.
Anyway, that is a really long answer! I had them laying there when I bought my Qb78 so I stuck both of them on the gun coming back along the barrel on both sides and was able to get a lot of shots! It felt to heavy for hunting though so I did the reverse under the gun mod with one of them. I do recommend the oring mod, especially if you get a regulated tank as I just got at least 1.65 fpe/ci and over 1.7fpe/ci if my volume calculations are right, compared to the 1.2 - 1.3 I was getting before. However with the oring bstaley mod it has seemed to limit me for some reason to around 18-23 consistent shots, depending on the pellet, and rva setting, 21-25fpe. Once I can afford it, I'm thinking I'll do the same as you've been thinking and buy a regulated 13ci bottle

[WesBob]

Forgot about the number of pumps. I'm not actually sure, it's around 10ci volume including the gun tube so isn't bad at all. I think to top off after the 20 shots is only 20-30 pumps. I'll be degassing today to put the stock back on so I'll count and let you know from empty and topping back up.

[WesBob]

I degassed the gun and refilled it, took around 136 pumps (was leaking to start with, took a few pumps to get it to seal). Shot a 26 shot string and used around 530psi, which was less efficient, because I was using light pellets (14.3gr) and got an average efficiency somewhere around 1.42fpe/ci. Average velocity was 836fps. Extreme spread counting the last drop off shots was 62fps, taking off the last two it was 49fps and if I take the one abnormally fast shot it is 33fps,lol. Anyway, not too bad spread for 24 shots, which is 3 mags for me (I have a repeater system on my breech). Anyway probably useless info unless you plan to go unregulated, but maybe will help you decide regulated is better, lol!