Finally figured out my rich open loop idle problem. - Hope this helps anyone else who tries the 454 TBI swap.

I noticed that after the TB swap, cold start-up was WAY rich. Didn't make sense how just changing TB's would make it so rich (no, I wasn't using the 454 injectors). Checked all the vaccuum lines, gasket, PCV valve, MAP sensor, TPS voltage, minimum idle air, etc... nothing.

Then it occured to me: bigger bores = lower vaccuum.

I started reducing IAC counts vs. coolant temp (close up that vaccum leak) and sure enough, everything got a lot smoother after that. - I still have some tweaking to do as it starts to get colder here, but it's MUCH better now.

Hope this helps.

I can't say I agree with that assessment. Vacuum with closed throttles is not like what you think.

You changed throttle bodies and it includes a fuel pressure regulator. Did you compare fuel pressure? I'm sure, all else being the same, that you'll find the problem there.

Yes, this shifts the airflow from the IAC to the throttle blades. In turn pulling the fuel past the blades at a higher velocity. Couple of tips: during a warm idle set the idle screw so that the IAC counts are low, no more then 10 counts. This produces the largest possible blade opening (less idle timing can also help in this area, it's all a trade off).

Then observe where the cone of fuel strikes, it should be on the bores just above the throttle blades. Obviously do this with the engine up to temperature and running at idle.

May need to adjust the injector pod height to get the strike point correct. The reason these items help is that by having the fuel strike the bores just above the blades allows the airflow past the blades to vaporize the fuel. And, the fuel flow is even around the blade, not pouring off the rear of the blades.

This action can actually be seen, once the fuel gets close to the blades it just disappears.

RBob.

Also, junkyard TB's often have problems with the injectors (o-rings), and gaskets between the injector pod and base. Be sure those are not pouring fuel in.

My co-worker went through several junkyard sets of TB's and injectors getting one that worked right.

Reducing the IAC on a TBI has always yeilded best results. You might also look at getting a spacer to raise the injectors up giving them the ability to spray on the walls instead of bouncing off the throttle blade.

How high is too high for the pod?

Could excessive IAC opening cause a surge at WOT (especially with a large bore TB on a single plane)?

I'd say when the fuel is bouncing off the wall and onto the throttle blade you're too high .
Get it right at the edge and you're setup for success.
I don't think excessive IAC opening at WOT would cause a surge. I never had that issue. For a simple test, just close the IAC with the paperclip/backup mode trick, unplug it, go for a spin at WOT and see if the surge is gone. If it is then the IAC might have something to do with it, if not, nope . Oh, and if you wanted to disable the IAC "better" change the AE - pw added for IAC opening constant to 0. That way the ecm isn't adding in fuel when it shouldn't be making for a possible low throttle opening rich issue.

I noticed the lower vaccuum after the 3rd time I had swapped this thing on. I've been chasing this for a while. Both times previous I had a dead laptop, so I had to put the stocker back on. This time I got a chance to hook up the laptop & see what was going on. That's when I noticed the vaccuum was lower at idle.

- Adj. FPR set the same as the other TB, - was also completely overhauled (o-rings, gaskets) when I swapped in my injectors.

- Spray pattern looked good, right on the walls. Checked that when I was trying fix the problem with minimum idle airflow.

So that's what made me think that just the bigger bores alone could drop the vaccuum. - Everything else was the same as the old TB. So my train of thought was bigger bores meant bigger vaccuum leak. So by closing the IAC a few steps, this would even the airflow back out.

I'm not saying this is universal for everyone, but it worked for me.

What fitting on the TBI is your MAP sensor getting it's signal from?

Same port as the stocker; on the back, between the fuel lines.

man my fuel hits way up the side walls in the throttle bore since i raised my fuel pressure to 30 psi. also i noticed that if you rev it by hand under the hood with the air cleaner off, the fuel jets down faster than the engine pulls it in.
drilling 3/32 holes in the throttle blades will have a better effect

somehow i dont think putting my open loop at 18 is gonna work, maybe at idle, but it aint gonna wanna go.