K frame light primer strikes in double action

Having a minor issue with my mdl 64 PPC wheel gun, when shooting double action I get light strikes on WLP primers. They look deep enough but just not enough to go bang. Works every time in single. The hammer spring tensioning screw is bottomed out as tight as it goes. The gun is a very modified Keefer special, tuned of course for DA. 
Is there a particular new hammer spring I should get - seems like most places are marketing lighter ones, I need the opposite, a full power one. Wolff, factory S&W, Wilson combat? What should I be looking for? I don't mind a little bit heavier trigger. It's a fun gun.

Switch the Federal primers (if you can get them of course)

Most PPC revolvers require Federal primers to ignite reliably.

On old trick is to slide a fired primer over the end of the mainspring tension screw to get a little bit more.

I had to stick with Federal primers as well after getting my new production Python slicked up.

Use a set screw to increase tension on the mainspring.

You can try a shim under that tension screw and it might work since you are close.  The spent primer cup that Jim mentioned would probably work best so it is centered and retained.  I think the hammer travels a little farther back in Single Action which gives you enough extra inertia to fire those Winchesters.  The best solution is the Federal primer suggestion though.  That was instilled in me 30 years ago and has never failed.  Even when Winchester changed to these new copper colored primers (they claimed to be more sensitive) Federals always work and Winchester would give light hits.

Good luck.

John

Andrew,

I recently tried Wolff and Wilson reduced power springs on 15-3.  With the screw bottomed out Wolff is 7lbs total trigger pull in DA.  Wilson was 8.5lbs trigger pull in DA but "felt" smoother.  So I stuck Wilson and tuned to 7lbs.  guess what?  No go.  Even at 8.5lbs Wilson is not lighting up primers reliably but feels much smoother albeit heavier. Wolff at 7lbs IS lighting them reliably but does not "feel" as buttery.  Weird.  I am using Freedom Munition for testing factory ammo. 

Previously had a tuned 10-5 with stock spring shaved down (I shaved it myself).  It was very reliable at a 7.25 in DA. 

Let me know if you want a stock spring, I got a few, but running full stock will defeat the purpose of a tuned pistol.  

As others mention in SA the hammer is further back and picks up more inertia. 

Yevgeny

If you know how to remove the mainspring and don't have another one, then you can do this.

Remove the mainspring. It should have a slight bow. Lay it on it's side on a piece of paper. With a pen/pencil scribe the arc that the spring makes on the paper. Keep the piece of paper.

Then take the mainspring and place it on a solid background with the arc or hump up. Gently press downward on the arc with your fingers. Go very gently. What you are doing is taking some of the bow out of the spring. Check the spring against your outline on the paper after a downward press. It will not take much bend or bow reduction or straightening to increase the tension enough to ignite primers other than Federal. Go slowly and gently comparing to the line drawing as you go. Probably a 1/16 to an 1/8"  of bend reduction will be enough. You likely will notice a slight increase in DA trigger pull.

If you have another mainspring, then modify that one until it works, keeping the original mainspring as it is, if you are happy with it when you have Federal primers. Since your gun is a Keefer gun then it likely is an older gun. If so, then the stirrup that the mainspring hooks on is attached to the hammer. With those guns, you can remove the mainspring and re-install the mainspring without taking the side plate off. The newer guns the stirrup is not attached to the hammer, and it is very difficult to remove and replace the mainspring with the side plate on, as that stirrup easily fails out of the track  it fits into on the hammer.

Thank you all for the responses! Unfortunately I am committed to WSP primers, have many and much loaded ammunition, so I will follow Allgoodhits advice and attempt to add some tension to the current spring until I get reliable ignition.