Throttle Body Backfire

Fresh TBI swap onto a internally stock, Vortec-headed 350.

Truck runs great, pulls great, sounds great, except for a hesitation/TB back fire when going from idle to WOT. Also, sometimes from part throttle to WOT (like from turning a corner, then flooring it). Sometimes it's a single "pop", then it takes off. Other times, its a series of "pops" and I have to get off the pedal before it will recover. I've been playing with the base timing (yes...bypass was unplugged each time). Started at 0° and up to about 6°. As you might expect, it gets worse with more advance. It gets better at 0°, but still does it. No stored trouble codes, other than code 42 that I never cleared after unplugging the bypass.

I've got it locked down at 0° for now. Haven't spent much time troubleshooting it. Just got the truck running Friday, trans dialed in Saturday, and started playing with this issue today. I'm out of town all week for work. Just looking for ideas and things to check when I get home next weekend.

Folks seem to apply the term "Vortec head" to different types of heads. If you are using the 8 bolt intake style heads they are much less restrictive than swirl port TBI heads. A calibration for those heads generally needs greater acceleration enrichment and greater fuel as rpm increases. You may be able to cheat some by increasing fuel pressure but the best approach is correcting the calibration.

They are the Summit "Vortec" clones (SUM-151124). I'm running a Summit 226018 intake manifold and a Transdapt 2202 adapter.

I had read about needing to adjust the tables to be able to get the most out of these heads. I had emailed OldSchoolEFI a while back about having him do some tuning for me (I'm in Vancouver, WA), but I'm not quite ready for that yet; I need to build a shortblock for it first. My plan is to rebuild an old L31 roller block I have in the garage, but time and resources are in short supply right now. In the meantime I am cheating a bit by running an 18 psi spring in the FPR (I think it was from Brian at TBIChips.com).

You think the heads could be my issue?

Both the timing and fuel needs for those heads are different from the stock TBI engine. Do you have the tools to burn chips? Are you interested in learning to burn your own?

Dave at Old School EFI has experience tuning Vortec head equipped TBI engines. Maybe it would be worth the cost to get a starter tune for this relatively mild engine? You could easily invest as much time trying to make the engine run well enough with a stock chip as it would take to pay back the cost of a starter tune.

Backfiring out the intake is it being lean when you stab it it doesn't have enough fuel for the sudden blade opening and big gulp of air it gets

I'd agree with delcowizzid on that, I've just experience a similar problem with a Vortec headed 350 and adding more MAP AE stopped the lean backfire.
It would it would nearly cut out if I stood on it from a standstill and it would backfire back up the intake if a stood on it at 60MPH.
I could also momentarily flood the motor if I added too much initial TP AE.
TP AE only is initial and very short, MAP AE seems to take over where TP AE ends and then at WOT PE should take over adding additional fuel.

L98 short block with addition of vortec heads, spreadbore edel, TBI adapter w OE TBI and 74#/hr injectors. Started on stock tbi swirl head peanut cam tune. Did a plug and play '747 swap in place of oe '299. Only change to bin was to address larger injectors. Sent bro home and within an hour he's texting me that it's backfiring. Google searched vortec spark map. Cut and paste spark map into bin. Bros immediate response during the datalog capture drive...I can feel the difference. Cleaned up the VE and have not revisited spark.

Reached out to Dave via email. Perhaps a basic starter chip would work for the time being.

WOW! What a difference! GIGANTIC thanks to Dave for setting me up with an awesome starter chip. For a 6000# rig with a worn out short block, this thing moves out! It's like a whole new truck!

No more backfire at WOT. Still an intermittent backfire under certain conditions from off idle, to part throttle - like letting off the throttle to make a turn, then giving it part throttle after you've completed the turn. However, it's manageable, and only did it twice to me. For an "off the shelf" chip, I'm truly impressed. Definitely going to need some fine tuning, but I've got a few other projects I'm working on, so that will have to wait for a bit.

Dave - FYI; I did finally manage to download TunerPro. Thanks for the help. Looking forward to working with you on all three of my rigs.