I received a very nice Christmas present in the mail from mangerover on the CAF.... in the form of a used QB78D in .22 cal.... It had a valve in it which I have to return (nice work Gipetto) to Curtis, but came with a spare stock QB valve, which is what I wanted to start with anyways.... I wanted to use a piece of 3/32" music wire for the valve stem, and a smaller OD poppet.... to increase the airflow without making the valve any harder to open.... so this is the gun that will get those parts.... Here is the back half of the valve and both poppets, old and new....

The hole for the valve stem has been bushed down for the 3/32" stem by using a short piece of K&S brass tube that is 1/8" OD x 0.014" wall,, available at any hobby shop.... The original stem was 3 mm, so I had to drill the back of the valve body out to 1/8" and the tube was such a tight fit I had to press it in place with my vice.... I stopped when the front of the tube was flush with the back of the throat inside the valve, and then just filed the back of the tube to length.... Once the ends were deburred, the music wire stem was a perfect sliding fit, easily as good as any PCP valve I have seen.... I made the poppet out of PEEK, with a 5/16" OD, just a simple cylinder with a spring seat turned on the front to fit the stock QB valve spring, which is also 5/16" OD.... The stock poppet is 3/8" OD, and since the valve is only 7/16" ID, there was only 1/32" around the poppet for airflow.... While that is enough to do the job, the gap around the new smaller poppet is 1/16", so there is twice the area around the poppet, for basically no restriction to the flow.... The only thing I did to the valve to improve flow was to drill the throat of the valve out slightly to 7/32" (0.219"), is was 5 mm (0.197") stock.... With the smaller stem, I now have the equivalent throat area to a hole of 0.198", which would be like the stock throat with NO stem in place.... The QB valve has a raised seat with a 1/4" diameter, so drilling the throat out that small amount did not affect the seat.... and the 5/16" OD poppet sits nicely on it.... I smoothed the stock, angled 3/16" exhaust port into the new larger throat, rounding the top front transition between the two slightly, using a small spherical burr in my Dremel.... The only other change to the back of the valve was to mill a 1/4" recess for the transfer port to sit in.... I will be using a tubular Teflon TP in slight compression, that seals directly on the valve and barrel.... so the barrel O-rings will be redundant.... The receiver will have the transfer port hole drilled out to 1/4" for the Teflon TP to pass through....
The front of a stock QB valve is quite restricted, with only a single 5mm hole in it.... While maybe large enough, I had a modded front end left over from a previous QB project and decided to use that.... The two front ends are shown below....

I had milled two 1/4" slots, one on either side of the front end, right back to the threads, and installed a pedestal to hold the front of the spring using the original hole in the front.... The air now passes along the outside of the spring instead of through the middle of it, on the way to the poppet and seat.... This QB (because it had a Gipetto valve) was missing the steel backing block for the valve, so I have to make a new one.... fortunately I have an M5 x 0.8mm and an M6 x 1mm tap....
Bob