Hi, I also replaced a wick and wire in my 1948 312 last week. Have better smoke and choo choo is ok normal.
Now your problem? Is your 312 somke in boiler?
is the piston metal or sintered iron? Did you install the top gasket correctley?
did it cover the sound chamber. Did you put the two screws that hold the smoke unit in place. One screw goes into the bottom of the sound chamber. If it's not there you will be blowing air thru the screw hole.
How my ohms does your smoke uint have. I unwound mine and wound up with 31.5 ohms. I was going for 35ohms from 70 or so ohms
the old wire had 53 ohms and the wick was like a ball of wax.
any questions ask me or let me know when and how you fixed your problem.
jake