You could tighen the screws that attach the trigger frame to the mainbody, for starters. If that doesn't make a difference, keep reading.
First, make sure you disconnect your gas source. You don't want to get your fingers mashed while you do this.
Remove the trigger frame from your marker using the metric wrenches that should've come with your marker.
Once you've taken it off, look up into the opening where the trigger frame fits in- does the striker have any signs of wear in or around the notch where it should be catching? If it does, then there is probably a small gap that's preventing it from hooking on during firing.
If not, there is an even bigger gap. Check your sear (the seesaw-type metal piece in your trigger frame that springs up and down) and see if the spring under it is ok or not. If the spring isn't pushing the back of that sear up and into your marker, then you will have to get a new spring to make sure that sear goes up and catches the sear.
If the sear is in the up position (push it down and see if it does bounce back up) and the striker is ok, then might be that the recharge time for the valve is so fast that it doesn't give the striker a chance to catch onto the sear (highly improbable) once you pull the strigger so it goes crazy.
Either that or your the parts of the sear and striker (one or the other, or perhaps both) are so worn from use that they no longer catch onto its counterpart.
Ask your friend why he sold the Victor II to you, and if there were any problems with it...either way it's wear and tear, so he'd be the one responsible for it. I'm looking at the diagram on Kingman's website as we speak and I can't see anything else that would be the cause.
Also, which trigger frame did you get? Not all trigger frames fit as well as they should...
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Back from a two year LDS mission in the Bay Area.
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