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Old 06-10-2014, 06:29 PM   #1 (permalink)
Law
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Tuning a pump question

I've got a trilogy competition with a pump kit on it, and I'm trying to get the pump strike lighter. With the stock wgp springs and stock reg at (300ish psi) it shoots great, but the stroke is stiff. I've have a pile of spyder and cocker springs so I thought why not give it a try, and after some playing found something that was lighter, and seems to shoot at about the velocity with the reg around 220psi. But now when it shoots, the pump handle kicks back unless I hold it forward, and the velocity is really different if I forget to hold it. So, I'm guess I'm wondering if it's possible to fix that? is it a lower or higher reg pressure thing, or should I keep searching spring combos? The tough part is that the stock reg has to get taken apart to adjust which is time consuming, so I'm just sort of guessing at what to set it too as I can never find a good hour all together. Any help would be appreciated! Thanks!
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Old 06-10-2014, 06:34 PM   #2 (permalink)
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Just hold the pump handle. All lightly sprung snipers will pop the handle back when shot. If its a big problem throw a second return spring into the pump handle.
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Old 06-10-2014, 07:05 PM   #3 (permalink)
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You dropped the pressure and with lighter valve springs probably have a lot larger volume of gas is pushing through giving a harder "blowback" effect on the bolt. Return spring will help but somewhat defeats what you're trying to do. Getting the pressure back up a bit and playing with springs might help a bit.

But... are you *sure* the velocity is that much different or are you guessing? The sound signature will definitely change but I've built several that the kickback has little or no effect on velocity... but they're a lot quieter when not holding the pump handle.
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Old 06-10-2014, 07:27 PM   #4 (permalink)
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Some of that is not even valve springs. Pump stroke is around 7/8"-1", but the hammer travel is only about 7/16" on the back half, so you may have nothing to keep a slick pump action from sliding open in the first half of travel. The timing varies with the pump and bolt weights, so some pumps are worse than others about no-hold velocity changes. When reffing large games, it's a good idea to make people hold their pump handles forward. A light action with a long barrel will usually show a difference.

I generally don't like my pump handle flopping around if I point my barrel up in one hand. If the handle and bolt weight isn't too heavy, I'll fit the spring to just support the weight vertically. I'm no pistol player, but I can't keep both hands on my marker all of the time either.
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Old 06-10-2014, 08:49 PM   #5 (permalink)
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You should be holding the handle forward anyways. You're robbing yourself of velocity otherwise.
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Old 06-10-2014, 09:39 PM   #6 (permalink)
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I thought the whole reason for pump arm return springs was so that you didn't have to hold the pump handle forward.
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Old 06-10-2014, 10:11 PM   #7 (permalink)
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Originally Posted by Spider! View Post
I'll fit the spring to just support the weight vertically. I'm no pistol player, but I can't keep both hands on my marker all of the time either.
This. Too much return spring makes for an unnecessarily stiff pump stroke. Maggot is being too polite unless he doesn't have SLPS springs for the trilogy.
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Old 06-10-2014, 10:24 PM   #8 (permalink)
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Thanks guys! It's got the hitman mod on it which I started liking, so I was hoping to not have to hold the handle all the time.

I'm not sure how much the velocity is changing honestly as I was just eyeballing the shot, but it looked about 30fps ish holding the handle if I had to guess which is to say that it was very noticeable at the 100' I was shooting at.

So sounds like raising the pressure a bit would be a good start, and possibly back to a stiffer valve spring, but try and keep a lighter hammer? It's got a weak return spring in the kit already, which seems about right.
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Old 06-10-2014, 10:54 PM   #9 (permalink)
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Won't higher pressure/stiffer valve spring generally require more hammer spring tension, causing the pump stroke to be stiffer towards the end? I'd rather have a lighter hammer spring to make the pump stroke more even.

Shoot the gun over a chrono to see if you're really losing velocity. I don't lose velocity with mine and mine is light enough that it will sometimes cock itself if I'm not holding the pump handle.
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