Canadian Airgun Forum

My old Crosman Mk 1 .22cal is shooting at maybe 50% power. I have disassembled it twice now and resealed it with new orings, ensured the exhaust hole lines up, the hammer does not hang up in the hammer sleeve, etc. It still shoots really weakly. I made sure the valve body lined up perfectly with the barrel port on assembly. Any ideas what's going on?? I ordered a new valve spring, just in case, but I am grasping at straws!
This particular gun never had the the power adjusting screw at the front of the barrel hosing, BUT: it has been a a year or two since I last shot this gun, but I am almost certain it used to have two power levels available, one half cocked and one fully cocked. Maybe I am dreaming, but it only has the one level now. It shoots about the way it used to shoot when fully cocked. It is terribly weak compared to my 2240
Thanks!

It definitely should have 2 power levels.
The only thing I can think of is there is something binding or blocking the hammer from being cocked all the way.
Using the right hammer spring? It's the one with the pin end, which holds the cocking knob in the hammer itself .

Oh check to see the trigger is assembled the right way. That is where the 2 Sears are.

Here's a decent guide
https://www.pyramydair.com/blog/2020/02 ... al-part-2/

Try adjusting your trigger pull weight. If you go too light you will lose the high power stage. Back off the adjustment screw and try your high power stage again. If you can't get it back then possibly there is a damaged sear contact point or you have some debris in there.

The best way to adjust the Mk1 trigger is to cock to high power and then turn the screw inwards until the sear drops to low power stage. Then back out the screw a little (maybe 1/8 turn) and try your high power to see if it's available. If not, back out the screw a little more until you have high power again.

Thanks for those replies, they give me hope that I am not going crazy! I thought there were two stages on the cocking knob! I'll have a look and see if the sear has been installed incorrectly. Also, I have a new valve spring on order, but I gather that I should be possibly looking at a new hammer spring as well? Is there a Canadian source for those?
I do have the trigger pull adjusted fairly lightly, I'll tweak it back out again and see what happens. And thanks for the info on the best method of determining pull; much better than what I was doing.
I'll report back when I get to the bottom of this.
JD

Once you get the full power working again, check your velocity. With Crosman 14.3gr pellets you should be getting around 400-415 fps at room temperature. If so, your springs are fine.

You can coax a bit more power by installing a lighter valve return spring and a heavier hammer spring. Remove the spring in the valve and find one that is weaker but fits on the end of the stem. You can bend the last coil to get it to stay on the stem. Trim to length. Check CanTire, HomeDepot, or any other local store that might have small compression springs. To bump up the hammer spring I have installed a smaller (and thicker wired) spring INSIDE the original factory hammer spring. It think I found some at Home Depot for about a dollar or two. Changing these two springs can get you up to 440-450fps. Your shot count will drop to about 22 per powerlet and the pistol will have a loud bark. You can always switch things back at this stage.

Contouring the stem can get you another bit more speed (maybe 10-15fps). Hogging out the valve might get you a tiny bit more but you need a longer barrel to get the big gains.

Many years ago I put together a review on the Crosman Mk1 that can still be accessed. CAF forum member Leonardj contributed a lot of information, as did member KEN. I polled dozens of Mk1 owners from around Canada and the USA to get the stats about various Mk1 versions. You might enjoy the read.

https://forum.vintageairgunsgallery.com ... an-mark-i/

I am sure it has been tried before, but will placing a 1/16 or 1/8 spacer inside the hammer, between hammer spring and hammer, function as a stiffer spring? In other words, preload the hammer spring.
I still can't figure out why I can/t get the two positions on the cocking shaft. I have it apart again, and have been looking at the schematics and great photos on the disassembly link. It sure looks like I am doing everything right, but the cocking shaft only registers the one position, and will not go fully to the front to engage the high power position.

I doubt you will get much from a thin shim on the hammer spring. The double spring trick works great. Try Home Depot for a heavy tension 1/4" OD spring that will slip inside the factory hammer spring. Use both together for more power.

post61389.html?hilit=crosman%20mark%201#p61389

Did you try unscrewing the trigger adjustment spring to bring back your full power setting?

Crisis averted, I have the old Mk 1 shooting the way it used to, with two power levels! I disassembled it one more time, and checked the availability of the two hammer positions before I bolted everything back in place. Perhaps by doing that, I aligned things properly. Anyway, when I got it all back together it was all good. I then took it apart one more time, and inserted a small diameter high tension compression spring inside the hammer spring. The jury is still out as to whether or not I gained anything, but I think I might have a <bit> more fps.
Thanks for all the help, this forum is a great thing to have available!
JD

If you don't gain as much fps as desired then you can change out the valve return spring for something lighter. This will allow the valve to stay open a fraction of a second longer for more flow and power. A 3/8"OD spring works well for the valve and you can just cut to length and bend the last coil (if needed) to get it to stay on the valve stem. A stronger hammer spring in conjunction with a mild valve spring will increase your valve duration. Get ready for a bark and a little barrel lift during firing. It's a bit like shooting a rimfire pistol only more fun because you can take it to a gravel pit and smash cans.

Here is what you want for the valve.... $2.21 at HD. Trim to length.
https://www.homedepot.ca/product/paulin ... 1000127115

This one works well for the hammer spring insert... also $2.21 at HD. Trim to length.
https://www.homedepot.ca/product/paulin ... 1000127099

Please post how you are doing with this pistol. It's a great design!