Sumatra. 22 carbine valve and hammer mods.

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Froggy.

Member
May 28, 2015
21
0
Limpopo, South Africa
Seems I'm close to getting my carbine perfect for my application... Pigeon pest control.
I was already using a softer hammer spring and was relatively happy with the results. I even tried a longer barrel but ditched that idea.
After the fire valve malfunctioned after about 10000 shots I decided to mod both the valve and hammer.
My biggest gripe with the softer hammer spring and standard hammer was the slow lock time when firing, making the rifle hold sensitive.
Pic of factory valve stem and deformed poppet seat. Notice the valve spring is the same spring as the factory hammer spring! It took 22lbs to open the valve!!.html]
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I made a new poppet 11mm diameter... Factory unit was 12mm...figuring it would lesson force needed for opening... Replaced valve spring for a 4 lbs unit..html]
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Assembled the rifle and couldn't believe the velocity gain with the softer hammer spring. Before valve mod the rifle shot 18gr jsb's at 930fps max, now 1070fps...25gr 940fps and 32 at 850fps. It was a air hogg power monster just like when it was factory stock standard, only with a much much lighter trigger.
To bring the velocity down and to get a faster lock time I made a new hammer. Factory hammer weighed 70grams... New hammer 43grams..html]
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Now I'm getting there!! Feels like shooting my cz200, can't believe the difference a lightweight hammer makes.
Still pushing 18gr at 980fps at 150bar power wheel fully open, but it's shooting a bell curve now, so just some minor adjustments needed.
I'll post some strings later, I've only 145bar left in my dive cylinder.
 
Some basic disassembly.
Remove the screws marked with red circles. The marked in squares should be removed and refastened with locktite.... They always seem to work themselves loose.
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Carefully wiggle the stock and the receiver cover plate should just lift off.
When re assembling just make sure the pin holding the cocking lever is centered because the cover holds it in position. .html]
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The roll pin keeping the spring guide in place is 3mm diameter. I use a 2.5 mm pin ground down to fit inside the roll pin as a punch. Tapping with a plastic screwdriver handle is adequate. I replaced the roll pin with a solid 3mm pin that's ground flush both sides of the receiver... It makes removing the hammer easy because it just slides out when you press on the back of the spring guide to release preload,and the receiver cover secures that it won't work itself out.
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Pull out the spring guide and spring, the hammer should just slide out. From factory there will be a ton of grease so you might have to shake the rifle while pulling the trigger to get the hammer out..html]
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Some hammer spring measurements. Right is factory spring. .html]
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Upvote 0
Nice. 

I love the Sumatra / Career 707 under-lever actions. I have more expensive guns but every so often I love using one of my Career rifles at the pistol range with iron sights. 

They have always been too loud to use at home where I live. I spent some time searching to see if I could find any valve mods to make them more efficient with air. I think the sheer volume of air released for each shot (even on low power) makes them louder than they need to be.

Did your valve mod have any effect on the overall noise level?

I'm not sure I fully understand why these guns are louder than others with equivalent power.
 
Upvote 0
Outstanding. I need to do some modifications to my Sumatras. I have the lighter springs and I know I'm going to put it in. Not sure I'm able to machine a new hammer though. I've never had a problem with my Sumatra being too loud. I have LDCs on them and at the power level I shoot them they are very quiet. Even shooting heavy pellets at full power is not too loud with a proper LDC.
 
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