Its quite common on old 14Bs for the flapper arm to fall down from heat and gravity, causing the flapper to not cover the WG hole all the way. Its also possible that the preload on the WG is not set properly.
I'm going to describe the preload thing for what is probably the first time on this site, someone might as well add it to the faq.
The internal WG flapper should have some preload on it to hold it shut. This decreases lag and boost threshold. Not enough preload, more lag. Too much preload, more boost creep. Even new turbos right out of the box commonly have an improper preload setting, so it pays to double check. You can check and adjust with the turbo on or off the car.
When preload is set properly, you have to pull the actuator rod a bit to get it over the pin on the flapper arm. For a new turbo, remove the actuator rod to test it. If it slides on easily, its too loose. If you really have to pull like hell, probably too tight. MY preference is to pull slightly on the rod and look through the hole. I like the pin to be to the right by about 1/4 of the width of the hole. Maybe half if I plan to run high boost. That gives you an idea of how much you want to be stretching it over. Again, its just to ensure that there is always some of that spring pressure holding it tightly closed.
To adjust the preload you have several options. The most common are bending the bracket, or adding washers between the actuator and comp housing. I like to just bend the braket, its faster and more precise. Use a large pair of channel locks, they work best. But a BFH used strategically can get the job done as well. Move the actuator away from the turbine housing to increase preload, or towards it for less preload.
Back to the flapper not covering the hole problem... You really should remove the turbine housing to check this (can be done without removing the rest of the turbo
). If the flapper is not aligned with the hole, odds are it has slipped in the direction of the flapper arm (towards the front of the car, gravity pulls it that way). Bust out the BFH again and beat it back into place carefully. I can usually tell if its bad just by looking at how far the flapper arm is from the turbine housing, but its hard to convey that over the internet, so I recomend pulling the housing to check.
Also, massive cracks where once side is higher or lower than the other can prevent the WG flapper from sealing. There are 3-4 "normal" cracks that all small mitsu turbos develop that wont have a significant impact on spool, so dont panic just because there are cracks. Just be wary of the ones that have really shifted the seat around preventing the flapper from closing.