I determined that the spring rate on the regulator spring was way too high, so I stopped by the hardware store again and they pointed me to the springs in about 20 seconds. Got the lightest-gauge 1/4 inch spring they had, and it's working fine now. I have it set conservatively, boost spikes to 14 psi in second and drops to 13 with some fluctuation, spikes to 13-14 in higher gears and drops to a steady 12. I'm going to leave it there for a while, crank it up to 16 psi after I get a few other issues resolved.

Here's a picture of the final result. And yes, the markings are essential to the function of the MBC


Total money invested including tap, drill bit, other tools: less than 20 dollars

Total money in parts: less than 11 dollars