Adjusting the KJW MK-I Velocity




One of the great selling points of the Kuan Ju Works (KJW) MK.1 replicas (pistols and carbine) are their near-legendary velocities right out of the box using “green” gas (also known as “Taiwanese” or “Top” gas) and .20 gram bb’s.  However, with the pistols shooting around 430 fps, stock, they are not practical for most fields where velocity restrictions for side arms, close-quarters combat, and night operations are in the 300 – 350 fps range.  In addition, the carbine is becoming an increasingly popular, inexpensive alternative to spring-action sniper rifles.  Consequently, many carbine owners are itching to breech the 500, 550, and, even, 600 fps marks.  While the availability of aftermarket parts and upgrades for the KJW MK.1 is slim, there is a very simple and inexpensive way to adjust the replica’s velocity.


                The KJW MK.1 is a gas powered, non-blowback AirSoft replica.  As such, it does not rely directly on springs or pistons to propel the bb.  Instead, it relies on gas stored in a reservoir in the magazine.  When the trigger is pulled, the hammer (part number 35) is released and strikes the gas valve (part number 57) on the back of the magazine to release a burst of gas.  The distance that the valve travels with each hammer strike determines how much gas is released.  The more gas released, the greater the velocity and vice versa.


                The distance that the valve travels with each hammer-strike is determined by two factors; the force of the hammer and the resistance of the valve spring (part number 60).  The harder the hammer strikes, the farther the valve travels.  This is regulated by the resistance of the valve spring.  A valve spring with more resistance will reduce the distance of the valve’s travel while a weaker valve spring will allow for greater travel.  The velocity can easily be adjusted by changing either or both of these factors.  It is far more practical, however, to adjust the power of the hammer than the resistance of the valve spring.


                The power of the hammer is determined by the resistance of the mainspring (also known as the “hammer spring”, part number 45).  As the trigger is pulled, the hammer strut (part number 36) pushes down on the mainspring plunger (part number 44) compressing the mainspring.  At a certain point in the trigger pull, the disconnector (part number 34) releases the hammer and allows the mainspring to decompress and push the hammer strut, forcing the hammer to strike the valve. 


Adjusting the resistance of the mainspring is as simple as replacing the spring and/or adding spacers to the equation.  (Spacers reduce the amount of room for the mainspring and force it to compress more.  The more compressed it becomes the greater its resistance, exerting more force when it decompresses.)



Adjusting the Mainspring:


                To adjust the mainspring, first remove the mainspring housing (part number 41).  To do so, pull the housing latch (part number 47) on the back of the mainspring housing out from the grip of the replica.  (The stock must be removed from the MK.1 carbine to access the housing latch and remove the mainspring housing.)  Then, pull the mainspring housing out of the assembled replica. 


Once the mainspring housing is free, use a small screwdriver, hex wrench, or punch to push the bolt stop retaining pin (part number 43) out of the housing.  Be very careful since the mainspring is under tension and may “shoot” the housing latch and mainspring detent (part number 46) out of the housing.  (Try pointing the housing into a plastic bag when pulling the retaining pin out.)


                Downgrading velocity will require replacing the stock mainspring with one or more replacement springs and, possibly, spacers.  Experiment with different configurations to achieve the desired results.  If one spring alone is not powerful enough, the addition of spacers and even a narrower (smaller outside diameter) spring nested inside of it will increase the resistance.  Springs nested inside of a larger spring combines the resistance of all of the springs achieving a greater, net resistance than each of the springs alone.


                Upgrading velocity is as simple as adding spacers to the mainspring.  The greater the space occupied in the mainspring housing by spacers, the less room the mainspring has to compress.  The result will be greater compression on the mainspring and greater resistance placed on the hammer strut.  Too many spacers, however, and the mainspring will not have enough room to compress and will not allow for a full trigger pull.  Using several washers instead of one large spacer (e.g.: an AirSoft bb) will allow for greater control and customization.  Alternatively, the stock mainspring can be replaced with a stiffer spring or spring combination or supplimented with another spring nested inside of it.


                When adding spacers, make sure to insert the mainspring plunger into the housing first, followed by the mainspring.  Then add the desired spacers.  The mainspring detent then goes in after the spacers.


                Afterward, reassemble the mainspring housing, reassemble the replica, and test fire the gun.  When downgrading velocity, it will immediately be evident whether there is enough resistance in the mainspring to fire or not.  If it will fire, chronograph it and determine whether it needs more tuning.  It may take several combinations and experiments to find the right mix of springs and spacers.



Finding Springs and Spacers:


                Springs and spacers can easily be found at local hardware stores (Ace Hardware has a good selection) and hobby shops.

                The stock mainspring is ~1 9/32 inches in length (~33 mm), ~7/32 of an inch in outside diameter (~6mm), and it has approximately 12 coils per inch.  Most any spring matching the length and not exceeding the outside diameter of the stock mainspring may work.  Springs may be slightly longer than the stock mainspring so long as they allow for enough compression.  The number of coils per inch does not matter overly much.  However, too long of a spring or too many coils per inch will prevent the mainspring from compressing far enough within the mainspring housing to allow for a complete trigger pull.


                Nylon washers with an outside diameter of no more than 1/4" of an inch (~6 mm) work very well as spacers.  Different brands of nylon washers will have different depths (thickness), though most are ~2 mm deep.  Other objects, including metal washers, may also be used as spacers.  An AirSoft bb is a quick and very cheap way to add a 6mm spacer to the mainspring.


                Shop around and pick out a variety of springs and a quantity of spacers.  Different spring and spacer combinations will achieve different results.  It will be necessary to experiment.





·         Refer to the parts diagram at,, for illustrations of the parts and their numbers as referenced in the article.  The full manual, including disassembly instructions, may be found at,


·         Downgrading the velocity may result in greater gas efficiency, allowing a magazine to fire more shots per refill than “stock”.  Conversely, upgrading the velocity uses more gas and will result in a decrease in the number of shots per refill.


·         Other gas non-blowback and blowback replicas with similar actions may also be adjusted this way.  However, the specifications of the mainspring and performance may vary.


If you have a KJW MK-I that you would like to have Chad tune up for you, you may contact him at:




When attempting to upgrade the velocity with more mainspring resistance, the gas valve in the magazine will suffer more abuse.  It may not stand up to the extra force of the hammer for very long. 



Chadwick A. Moore

April 22, 2004