Boost backfire rough idle


Another backfire thread..... Ive been reading this forum all summer trying to learn and figure this ol girl out. Little history. This motor has about eight hundred miles on it, and the car sat for 3years. So.... I installed a new gas tank and aeromotive 340 with a hotwire and fuel filter. Sent my injectors to caspers for a cleaning(siemens55's)Started driving it a bit to put a tune on it and at 15psi it went bang and blew the ceramic out of the #4 spark plug.(autolites) then found 3 other plugs with loose ceramic so bought some ac's and gapped at .28. Car backfires at 12psi and the idle is rough. Checked compression and has 130 +-3psi in all cylinders. Installed new Taylor wires, new coil pack, new module. Backfires at 12psi. New injector harness. Changed fuel filter again. Thought I may have lifted the head and did a retorque(rjc head gasket), no change. Installed cometics and removed every valve to check the seats, everything looked good but #6 looked richer than the others, no change, new IAC, no change. Changed out the 110t with a new 218/222 and .005 under roll master with TA's adjustable push rods, no change. Had gns rebuild the cam sensor, and tried 2 differnt caps, no change. New crank sensor gapped right(credit card width both sides), no change. Checked all the ic plumbing and used several cans of brakleen and propane checking for vaccum leaks, found none. Have 14" of vaccum at idle, seemed a touch low. All my scanmaster numbers look good, showing 810mv @wot before it backfires, took a little fuel out at idle with the translator to get my blm's to 122. Checked translator settings and bought a new lt1maf, no change. Ran 2 other coils on the car and 1 other module no change. Fp's at 43 line off, rises with boost. New batt power lead and ground from caspers, re-ran grounds with new terminals on clean bare metal(back of head) grounded the coil/module base to the fire wall, grounded the alternator to the frame and body. Neg battery grounded to the passenger fender and front of passenger head. New fuseable links installed, new crank sensor plug and wiring, swapped in multiple tps sensors, no change. Alt voltage seems a little low at 12.9 and have caspers volt booster installed. Checked ecm connection for corrosion, looked good. Re-seated e-proms. NO CHANGE...... ESC is original, could I have a bad chip or bad ecm? Bad injector driver? Put a brand new injector in the #6 hole case I had one hanging open, no change. Car hold fuel pressure for a couple hrs when off. Got a new chip from Eric on the way. And bobs gonna look over my ecm. Super frustrated...... What am I missing here? Burned most the summer and alota green backs on the Reagan rocket... My god I've read a lot of posts related..... So please excuse this gigantic one.... Any input is appreciated.... The one thing I have definetly learned is, there are some super smart Buick guys on here that are very willing to help guys out. Sometimes with out of the box thinking! Great community to be part of....
Have you verified your timing ?
Cam advanced or retarded ?
Crank bolt tight ?

Can't see your mods..what heads , plugs
Alky ?
Pump gas?
Race fuel ?

Installed the cam dot to dot at zero. #1@tdc
Pump gas(91) but put 10 gal. Of 110 in it to see if it helped.
No kr's or deg.of timing being pulled on the scanmaster.
No alky.
Base timing has not been verified
Crank bolt has all 225flbs on it
When you say 12 psi, at what rpm is the backfire?
What kind of timing is in your chip?
Your injectors are way too small for your combo on gasoline without ALKY

At or near 4k rpm is where the backfire happens.
Running an extender chip w/18deg of timing for 91 octane. Just varifired idle timing at 28deg. These injector are not huge, but supported 20 psi on the old motor with same heads and turbo. Scanmaster is showing over 800mv at wot with 0deg kr.
Changed the cam about a month ago to be sure I didn't have a lobe going away. One that came out, 110t looked fine. Checked all my rocker movement again recently to be sure, all was good.
#6 was rich when the heads were off and I replaced that injector with a brand new one and no change. Hope Bob has some good news for me when my ecm gets to him
This alternator hasn't ever been a real stud, but there's always 12.9-13.1 at idle and lite throttle. I'm gonna go make a test hit now and get a full throttle voltage reading.
It sure sounds consistent, I am leaning towards electrical noise or connections at the ECM. A few members including myself made the mistake of using non resistor plugs with the stock ECM and had stumble problems at WOT but no backfire, no knock. Turns out the noise from the non resistor plugs caused the ECM to reset its self.
We all had power logger files that verified the same responses from all the sensors. Do you have a power logger?

My ECM at the time was modded for the 55 Siemens like yours, I thought it was the problem until the other members joined the thread and we found the problem.

14 inches of vacuum is probably ok with the size cam you are running.
No power logger, just ye-ol scanmaster.
Non resister plugs....interesting. Cheap thing to try in the morning. Really do appreciate everybody's input and inquiries.... Summer already staring to fade here...
That is with the volt booster. I get the backfire with or without the volt booster in the equation. Gotta go to the autoparts store in the morning, I'll pull the alt and have it tested.
Most I have seen have an amperage load test. I had suspected my alternator was part of the problem and bought the voltage booster but after the problem was found I never installed it. It would be great if the alternator could be tested at the same RPM as the back fire occurred?
Well hopefully its a dude at the parts store....maybe I can see if he can crank up the rpms for the test. My area's plagued with female autoparts employees...... Hang my head every time.....seems 1 in ten's easy on the eyes