My tuned qb78 (got it to shoot with my son)

[bitterman]

Before anyone asks I have a PAL.

My Diana Model 48 in .177 is to much for a 7 year old so a while ago I picked up a qb78D. It was as with many people recieve it not working, the problem was the valve and also the breech seal.


The qb78D has the following mods:
-HDD
-Probed the bolt
-Valve port ported
-Washer cut for more flow
-piecing pin cut back
-4 holes in the end of the valve body for more flow
-Filter removed from valve body
-Tapered valve for more flow
-Polly breach seal
-Ported and debur'd barrel port
-Full hammer polish and debur'd
-Lightened Hammer
-New crown
-Extra set screw in the bottom of the barrel band

All was done previously and doing 724 FPS @ about 70-72F

It had slowed down so figured time for some experimentation and improvements.

I took the qb78D apart and noticed the hammer spring was not doing much. Most likely why the last time my son used it in hot weather it was very under powered.. Was also way to easy to cock compared to new.

I took it apart and ended up with stretching the spring slightly and adding 3 washers as shims.

End result early this morning was...
-Gun was about 72-74 (Was inside @ 73F due to air conditioning)
-Loaded 2 cartridges
-Fired a few Crosman Priemiers 722-726 FPS (about 16.65 ft/lbs)... so same as before
-Fired a few 18.1 JSB's and got 672-680 (about 18.37 ft/lbs)....


So I left the qb outside was about 82F when started but now upto about 84

.22 Crosman Premiers 14.3 gr 775 771 (19 ft/lbs) (2 shots back to back)
.22 JSB 18.1gr 720 (20.8 ft/lbs ) then left the gun for a couple minutes and did 4 more shots 720 718 716 (fast right after the other) let it sit about 2 minutes took another and it was 721

Now I need to order a new hammer spring from Eric and ensure when we put the qb78 away my son does not leave it closed like he did last time he shoot. I expect it being compress for months in the gun cabinet weakened the hammer spring and thus the slow down. Also had a split rubber breech seal from the valve to the barrel contributing to the problem.

I get about 30 shoots from 2 12g co2. Without the HDD and lighter striker co2 use was crazy high and also it was lound!!!! The combination of the HDD and lighter striker helps to conserve co2.

Bruce

[AirGunEric]

Have any pictures of your HDD setup?

[bitterman]

This is a picture. Basically the pin in the picture and U shaped piece replace the front pin. The unit wedges in the slots and creates friction to stop the hammer bounce.
QB78HDD-2.jpg

[AirGunEric]

Ah ok- I thought maybe you had designed you own HDD- that is the Woodward unit.

[bitterman]

You can do similar things just with a lighter striker, but hte problem is you then need to increase the hammer spring and depending on the balance you choose, could shorten the life of the valve stem.

Bruce