QB78 Tuning the two viewpoints I seem to have seen

QB78 Tuning the two viewpoints I seem to have seen

Joined: August 16th, 2006, 10:39 pm

December 24th, 2010, 9:05 pm #1

I see two camps on the tuning of a QB one favors more spring tension on hammer and valve to get it open and closed quicker. They also suggest restricting port size...which did seem to flatten my curve.

The other camp favors light spring tension both hammer and valve(which I would like easy to cock), lightened hammers, easy flow, and full size ports.

I had an extra valve shell from my Mike Melick valve that could not be made into a pedstal valve because it had been modified so I opened up the end more and made a small washer for the ledge and came up with a heavy and a light conical spring to try.

I will try heavy valve spring with heavy hammer spring , and then light valve spring with light stock QB hammer spring. I will use the stock hammer 56 gram for the heavy setup and the light 50 gram with the light setup to begin

What you guys think???

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Joined: September 28th, 2006, 7:53 pm

December 24th, 2010, 11:26 pm #2

Tom looks like you have pretty much covered all the standard and some not to common mods on your QB. I tried most of those mods 2 or 3 years ago then I decided to try this, but you have to use your lathe. I made the valve back in different lengths. If I remember the factory valve back is 1.900'' long. I made them from 1 3/4'' all the way up to 3'' in length. What that does is just change the volume of air in the valve. Just one more thing to consider when tuning the QB. I got alot of differing results but it was alot of work. My mistake was I didn't keep very good records of the results and now can't remember alot of the results I got. I do remember though that volume did make a difference. Good luck. Benson
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Joined: November 17th, 2006, 3:51 am

December 25th, 2010, 12:02 am #3

I see two camps on the tuning of a QB one favors more spring tension on hammer and valve to get it open and closed quicker. They also suggest restricting port size...which did seem to flatten my curve.

The other camp favors light spring tension both hammer and valve(which I would like easy to cock), lightened hammers, easy flow, and full size ports.

I had an extra valve shell from my Mike Melick valve that could not be made into a pedstal valve because it had been modified so I opened up the end more and made a small washer for the ledge and came up with a heavy and a light conical spring to try.

I will try heavy valve spring with heavy hammer spring , and then light valve spring with light stock QB hammer spring. I will use the stock hammer 56 gram for the heavy setup and the light 50 gram with the light setup to begin

What you guys think???

Lighten the striker!

This is yesterdays string for the .177QB PCP using a 43gr. striker and a .07-.08 transfer port.

10.6gr. Kodiaks
150BAR fill

30fps: 827-857-827 = 26 shots
20fps: 837-857-837 = 22 shots
10fps: 847-857-847 = 15 shots


This is todays tests. SAME spring tension, same transfer port, the ONLY change was a reduction in striker weight to 32grams (an 11 gr. drop). I just got out the dremel with big heavy cut-off wheel and trimmed the sides of the striker at the head and tail, leaving the top and bottom alone.

Same (from the same tin) 10.6gr. Kodiaks
Fill dropped to 140Bar:

30fps: 804-834-806fps = 34 shots
20fps: 814-835-816fps = 26 shots
10fps: 824-835-826fps = 20shots.

Even this reduced speed (call it 16 foot pounds) is more than I really need from a .177 PCP, but the drop in striker weight really seem to increase the shot ****/consistency .

From farting around with various other .22 PCPas, believe reducing the striker weight is the best/easiest direction to take to extend shot count at a minimal cost in velocity/energy.
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Joined: August 16th, 2006, 10:39 pm

December 25th, 2010, 6:42 pm #4

...I have a delrin valve just took it out of
tube the stem protrudes( on hammer side) .166 further than the brass
headed stock valve. Its the way Jack Cuts the valve seat and shape of the delrin head

Should I trim this down to match the length of the stock valve stem?

I will still have to reduce the OD of the modified valve body rear half to fit this tube its tight inside...maybe by .003
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Joined: November 17th, 2006, 3:51 am

December 26th, 2010, 1:18 am #5

...is to control the amount of valve stem that is sticking up to be hit by the striker. That's basically how Sumatras control power; striker spring stays the same but the "power wheel" blocks the striker hit to the valve in varying degrees.


I tried using it with the QB by making several length valve stems but found my results to be harder to control than the other power mod methods. In the end, just made the Delrin valve stem to end up with the same length protruding from the valve head as a stock valve stem. Others may have had more patience and gotten better results.

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Joined: August 16th, 2006, 10:39 pm

December 26th, 2010, 7:16 am #6

Which valve spring would you try first? Heavy or light with the light hammer, and lightest hammer spring?

I will remove the back section as well on the hammer in the morning....well I mean once the wife leaves the house....
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Joined: November 17th, 2006, 3:51 am

December 26th, 2010, 2:40 pm #7

..start to forget the big stuff.

May want to look at the valve stem return spring (yeah...would have to open it all up again to get to it for each trial, which makes playing with the return spring my least favorite/ most time consuming way of fiddling with a QB). That does much to change the closing cycle of the valve, which tends to conserve air.
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Joined: September 28th, 2006, 7:53 pm

December 26th, 2010, 9:34 pm #8

Robert there is a fast easy way of changing the valve spring on the QB. If you grind a 7/16'' socket to fit the slot in the front of the valve and have a long extension that is all you need. Make sure gun is empty, no air, co2 or pellet. Take off end-cap or fill-cap or remove bottle if regulated. Place socket and extension down into tube and insert into slot in valve. Crack loose and remove valve back completely. Point muzzle to floor and valve back and valve spring will come sliding out. Can even remove valve stem by pointing gun toward floor and bang on recoil pad a few times and stem will come sliding out. Now put any spring or valve stem or valve back into the tube you want. The valve stem will drop back into the little hole in the brass part still retained in the tube. A Little up and down movement is all that is required to seat the stem. Now lay gun on its side get the valve spring put it into valve back and carefully slide it back in using socket and extension and when you hit the brass part turn the extension with your fingers to thread the valve back together. Get ratchet and tighten up valve back again and your done. Much faster to do then to explain. HTH. Benson
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Joined: April 28th, 2008, 6:45 am

December 27th, 2010, 4:56 am #9

A piece of 3/4" flat bar works as well. nt
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Joined: November 17th, 2006, 3:51 am

December 27th, 2010, 5:11 am #10

Robert there is a fast easy way of changing the valve spring on the QB. If you grind a 7/16'' socket to fit the slot in the front of the valve and have a long extension that is all you need. Make sure gun is empty, no air, co2 or pellet. Take off end-cap or fill-cap or remove bottle if regulated. Place socket and extension down into tube and insert into slot in valve. Crack loose and remove valve back completely. Point muzzle to floor and valve back and valve spring will come sliding out. Can even remove valve stem by pointing gun toward floor and bang on recoil pad a few times and stem will come sliding out. Now put any spring or valve stem or valve back into the tube you want. The valve stem will drop back into the little hole in the brass part still retained in the tube. A Little up and down movement is all that is required to seat the stem. Now lay gun on its side get the valve spring put it into valve back and carefully slide it back in using socket and extension and when you hit the brass part turn the extension with your fingers to thread the valve back together. Get ratchet and tighten up valve back again and your done. Much faster to do then to explain. HTH. Benson
That short tube makes it easy. Less fun on a QB78, but found it possible to do.

Have never tried on a long tubed PCP QB. Guess you could still do it, but it does seem that manipulating the valve stem and return spring gets difficult. Guess it depends on what type of valve and return spring set up you have, but it gets to be a bit like building a model ship in a bottle whe you're working down a long PCP air tube.
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