All good advice, but you really need to read the FAST directions yourself and look over the shop's work. You never know what could be going wrong.
I ditched the FAST hoses. Too hard to work with and had a couple leaks. I have a hose manufacturer close by. I had them crimp and make all the lines I wanted, rated up to 200 or 300 psi, can't remember. I put this on my 85 blazer, using a 91 tank and sending unit (91 has fuel injection) The stock TBI sending unit will hold a stock Delco pump from a TPI camaro. (50-60psi) TBI pumps are only like 20 psi, something low like that.
The first pump I used was a Walbro. Terrible pump, really loud, and kinda ended up doing what you are describing. It would run fine first half of a drive, sometimes stumble and want to die. Other times it would stumble right away. Most of the time I could drive it a solid 2 hours before it acted up again. Good first 100 miles, terrible next 2 hundred, and in the trash after 350 miles. Replaced with an AC Delco, factory Camaro TPI unit. Quiet and trouble free for the last 2000 miles. When the old pump acted up, the computer would sometimes throw an ERROR code. O2 Sensor. After much frustration trying to hunt down a problem where there wasn't one, I finally figured out it was the pump.
Where is your pump (now two pumps, which isn't necessary) located? If you are running an inline pump (external) it HAS TO be mounted below the gas tank and as close to it as possible. The problem with the cab mounted tanks is the supply line is really high. This could cause problems like FAST mentioned. If you can do an in-tank pump, that would be best. What did you do about the return from the regulator?
Also, make sure the vacuum hose going to the regulator is coming from the correct port on the throttle body. (read instructions) I set my pressure to 45, with the motor running, but not connected to vacuum. (instructions say this) Once the vacuum line is put on, the pressure drops a couple psi. So your 43 to 20 sounds really weird. Probably a supply problem (aka pump, obstruction, collapse hose, voltage drop, bad ground)
*deep breath*
ok, who knows if any of this helps. Keep us updated. The FAST guys are helpful but only as good as blind help can be. You really should read over the directions and call FAST yourself with the problem. Even if you can't work on it yourself, you can write down what they say and be more knowledgeable when dealing with the installer.
