Every once in a while I get a complaint of a Techco supercharged car not nearly making the boost numbers you’re supposed to see. It can be several different things but every once in a while the linkage for the bypass valve slips which prevents the bypass valve from closing entirely. Here is an email I wrote to someone on how to check it.
Check supercharger bypass valve for correct operation. You’ll need to remove the front intercooler cover (part that says TECHCO) and then remove the intercooler cores. You’ll see the bypass valve blade in there. Now turn the bypass valve from the outside of the housing by pushing forward or pulling back the bypass valve linkage. When the linkage is all the way toward the front of the car, the bypass valve blade should be open. When the linkage is all the way toward the back of the car, the bypass valve blade should be completely closed. If the valve blade doesn’t operate EXACTLY like I said, the bypass valve linkage slipped on the bypass valve blade shaft. Readjust and retighten set screw (it needs to be fairly tight so it doesn’t slip again).