This is a long-time coming and I'm a bit late in posting my findings, but hopefully this will be of some value if there is anyone else is fighting the dreaded "lean spike to rich bog" issue at low RPM's on tip-in. Below is my eventual root cause fix, in addition to new RPM-specific accel pump adjustment from the newest T198 software rev. I should note that although I found a lot of helpful FiTech info on here and the Chevelles forum, I hadn't seen this specific fix mentioned (nor in FiTech's documentation). FiTech's only suggestions were "turn up the accelerator pump 30%" and to plumb a fuel line inside the car so I could watch for a pressure drop on tip-in.... (would've been nice if, ya know, there was a way to read this pressure to the computer) My Setup: Naturally aspirated SBC, GoEFI 600 TBI, Tanks Inc in-tank pump with Walboro 255 LPH pump connected to the orange harness wire. The Fix: Adjusting PWM threshold RPM Under the PWM fuel pump control in Pro-tuning settings on the handheld, I had to turn down the threshold RPM for the internal relay to turn on the pump 100%. Default setting was 2500 RPM, I had to change it to 1500. In my understanding, this setting should've been covered redundantly in the fact that FiTech does recommend disabling the PWM in my scenario, by setting the low/mid/high flow PWM values to 100%. Honorable Mention: RPM-specific accel pump adjustment I mention this because it did help band-aid the lean spike only at lower RPM's, and the default values were set much lower at low RPM's than higher rev's. Again I did not really see this documented anywhere but this gives a lot more flexibility than just kicking up the general accel pump and fast accel. Right now mine are set too high, now that I found the PWM issue I'm dialing them back. That being said, we'll see where the sweet spot is for my car at low RPM's, and how that compares to the default values (which were about 40 instead of the ~240 they are now). Getting this setting in the handheld requires a firmware update to the T198 or newer software, which you normally have to request from FiTech, or can access from the Pro-Cal software.