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Thread: Shimmed Pump Cup Reduced Speed

  1. #1
    Spilled Milk
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    Shimmed Pump Cup Reduced Speed

    Since my 1322 ws already at the legal limit, I picked up a 760 for $20 and decided to try the "shimmed pump cup mod." Shimmed the cup and supported the raise with a small section of plastic coffee can lid. (7/16" shim, 1/2" support.) Never knowing what to expect with these mods, I tested it out against the "stock" chrony tests and found decreases in excess of 80 - 100 fps! Not a problem as this is all a great learning experience, but being so I can't help but wonder why the decrease? Can anyone shed some light on the fps reduction, when so many claim this mod to be extremely effective?

    Thanks.

  2. #2
    Moderator rsterne's Avatar
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    My guess your shim was distorting the cup in some way and it was no longer sealing properly in the cylinder.... Otherwise, no idea....

    Bob
    Dominion Marksman Silver Shield - 5890 x 6000 in 1976, and downhill ever since!
    Airsonal:
    .177 Diana 34, 1750 CO2 Carbine, .177 Uber-Pumper, .22 Uber-Carbine, .25 Discovery, 22XX PCP 8-shot Carbine, 2260 PCP Rifle (50 FPE), Hayabusa PCP, .22 B-26, DAQ .308 Exile

  3. #3
    Spilled Milk
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    Thanks rsterne. That's most likely the case, the plastic shims were pretty "shabby" anyhow. I guess it was just a good mod gone bad. A good leasson learned though. Pump cup shim is alot of work for what might be reduced speed. Now I think I'm gonna see what I can do with some 760 parts. Maybe put the barrel on a Vigilante....

    Thanks again

  4. #4
    Spilled Milk
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    So after removing the plug shims and re-assembling the 760. I see that I must have caused something to go wrong with the plastic transfer hole from the valve to the barrel. I'm guessing I may have scratched the pump cup itself as well. In any case there are permanent leaks somewhere. I'm not going to repair it, but how does someone go about fixing a problem with the transfer port/barrel alignment in a 760 anyways?

    Thanks

  5. #5
    Administrator AirGunEric's Avatar
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    The alignment is only controlled by where the barrel or valve is positioning- rotate one or both.
    I'd say I care- but I'd probably be lying...


  6. #6
    Spilled Milk
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    I guess it is something else then. I took great care to make sure they were aligned with each other when I re-assembled it. I figured it was not tight enough against the transfer port of the valve. Maybe I just totally messed up the pumping mechanism all together. Like you said though, wrong platform for mods. Worked great out of the box putting out RWS Hyper Velocity at well over 530 fps. As far as construction goes, way too flimsy.

  7. #7
    Spilled Milk
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    Read a thread on the Crosman Airgun Forum saying that a 1322 Valve has a "Length of brass body = 2.625"

    The diameter is exactly the same as a 760 valve. I did not measure the 760 valve (like an idiot) when I had it out and I dod not take the valve out of the 1322. What is the difference? Is the 1322 valve longer? Can they be swapped out?

    Thanks

  8. #8
    Administrator AirGunEric's Avatar
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    The 1322 valve has a threaded hole to mount it to the tube, the 760 valve does not have the hole, otherwise the newest valves are the same.
    I'd say I care- but I'd probably be lying...


  9. #9
    Spilled Milk
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    So I took a look down the back of my 1322 and saw that there was no bleed hole in the 1322 valve. I guess that's the real difference between one meant for a .177 with a long barrel and a .22 with a shorter barrel. I'm guessing the 2240 valve has bleed hole either?

  10. #10
    Administrator AirGunEric's Avatar
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    The 1322 just doesn't get to 500fps in stock form- it is not "detuned" for Canada.

    The 2240 does not have a bleed hole for the same reason.
    I'd say I care- but I'd probably be lying...


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