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Old 09-19-2010, 08:17 AM   #1 (permalink)
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Raising velocity

So I rescued a vm68 from an unpaid storage unit. Took it apart, cleaned it up (it was stuck shut with 10+ year old paint). Put it together and it shoots great, but at around 200. I have the valve on the biggest hole and I'm using HPA, there's no co2 used around here.
So my question is, what's the best way to raise the velocity so I can use this beast of a gun? Should I try to find a heavier main spring, lighter valve spring or both? Should I just do the automag spring mod I've seen? Or should I bore out a larger valve hole, transfer hole and bolt?
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Old 09-19-2010, 08:30 AM   #2 (permalink)
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The easiest thing to do would be to get CO2, unfortunately.

Other than that, try lightening the valve spring or shim the mainspring.
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Old 09-19-2010, 11:24 AM   #3 (permalink)
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Also make sure the old paint is entirely gone, and the bolt, valve and all other parts slide smoothly--friction in the hammer/bolt will reduce the velocity by reducing the time the valve is held open.

Using a large drill bit to "cup" the bolt face and enlarge the channel from the bolt face to the air passage might help some also.

Make sure your HPA tank is actually putting out 850PSI versus a low-pressure setting.

Make sure there's no paint in the passage from the valve to the bolt (there's a screw in the top of the body which can be removed). That hole can be enlarged if need be but I don't think it's the primary problem.

Find a CO2 supplier. Heck, for less than the cost of a decent HPA tank you can buy a CO2 fill setup, 2 CO2 tanks, and rent a (full) 50lb bottle from a welding supply store.
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Old 09-19-2010, 12:14 PM   #4 (permalink)
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I concur with Greywolf and Jordan....These were designed with Co2 as a power source and attempting to run them with HPA, while possible is going to be problematic at best or near impossible.

Enlarging the passages, maxing pressure out of the HPA reg. and the longest possible dwell time for the valve to be open is the direction you want to head.
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Old 09-19-2010, 12:29 PM   #5 (permalink)
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VMs have such a huge draw of air that the reg lag from the HPA will starve the valve. Later VMs might work better with there internal EC system.

But it should still work under ideal conditions.... which might be hard to find with a gun sitting in a box for 10 years..

The main reason for drops in FPS are that the orings have started to swell, and cause friction. There are 5 damn orings! A quick test is to remove the bolt/hammer orings, and see if it works differently. Likewise, the detent can be pinching, adding friction.

Like said above, dried paint, and especially shell fragments can really slow things down. Lots of warm/soapy water, then LIGHTLY lubed. Check to make sure the bolt is not upside down, or between "clicks" (adds friction, and hard to spot). Make sure the valve screws and port screw (behind feed) are not too long pushing against the valve/bolt.

Those are the standard things. Next would be a weakened mainspring, but long gone are the days when every proshop would carry spares. Its very common to ship up the main spring. I think dimes fit in the upper tube? or washers or whatever. You could also drill a hole through the rear upper plug, and thread in a bolt that you could use to adjust the spring tension... while firing. Much easier then bothering with shims.

Same goes for the automag-type kit. You would need to shim that as well, and it still might not solve the problem. Drilling out the valve can increase FPS. It was the standard mod to drill out the smallest hole... that nobody ever used. But the transfer port above the valve is only so large, so drilling out only has a small impact. It also diverts from the recock jets, potentially making harder to recock, though that shouldnt be an issue with HPA... being a stable airsource.
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Old 09-19-2010, 04:57 PM   #6 (permalink)
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OK, so I suppose I should have mentioned that it has some sort of aftermarket delrin bolt with no o rings, but the passage in it is pretty small so I should probably enlarge that. I do have the stock one but it's not in very good shape. I suppose before anything else I'll take the springs out and head to the hardware store that has tons of springs near me and see what I can find. I'd be happy if I could get 250 out of it.
I do have a couple 9 and 7oz that look nice under the barrel but I think EMR is the only field I go to that has co2. Otherwise I have to get it filled at an army/navy near me that's like $4 a fill. I'll bring them to castle with me and get them filled and see how it shoots then.
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Old 09-20-2010, 05:40 PM   #7 (permalink)
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Hit me up at Castle. I'll bring a few extra VM parts.

PM me if you are interested in anything beyond o-rings and repair bits.
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Old 09-24-2010, 10:19 AM   #8 (permalink)
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VMs can run at 600 psi -1500 psi. They operate best between 900-1200 psi.
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Old 09-24-2010, 11:51 AM   #9 (permalink)
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Well I went to the hardware store and found a lighter valve spring. It seems to shoot better but I have no chrono here to check. I'm headed out to castle this weekend and I'm going to try it over the chrono with my big air tank that puts out 750 psi, my little tank that puts out 850 and co2 and see what happens. I may just end up buying a ninja reg for my big tank so I can increase the output on it. I have a guerrilla one that I won at a game and it's given me other problems anyway.
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Old 09-26-2010, 11:09 PM   #10 (permalink)
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Where were you?
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