u/OkSignature8062

Fresh 350 SBC in my 1978 Firebird formula with a TH350 auto. Running a brand new Edelbrock AVS2 650 carb and HEI distributor. Looking for advice on where to go next diagnosing a high idle issue because I feel like I’ve narrowed a lot of things down already.

Here’s the full timeline/details:

Motor fired up and ran pretty good initially during first starts. Was working on getting coolant burped, timing dialed in, and carb adjusted. At one point I had timing too advanced and the engine started acting crazy:
- idle shot up to 2000-3000+
- occasional backfire/fireball out of open headers
- RPM would keep climbing

Since then I’ve done a TON of testing and adjustments:

What I’ve already checked/done:
- Retarded timing multiple small adjustments clockwise and the engine behavior improved a lot
- Starts cleaner now
- No more violent runaway RPM
- No more major fireballs/backfires
- Idle now starts around 1200-1500 and slowly climbs/stabilizes around 2200-2400 instead of endlessly racing
- Mechanical advance in HEI seems free and springs back normally
- Vacuum advance disconnected/capped
- Brake booster line capped
- PCV disconnected/capped for testing
- All carb vacuum ports capped for testing
- Sprayed carb cleaner around carb base/intake/vacuum connections with basically no major change
- Mixture screws reset to baseline (1 turn out each)
- Choke adjusted 2 notches leaner
- Fast idle cam does not appear to be hanging up or moving weird
- Pushing throttle linkage closed by hand does NOT lower RPM
- Primary blades visually look barely cracked at idle
- Secondary blades appear closed too
- Covering the carb opening with my hand DOES immediately drop RPM / almost kill engine

The confusing part:
At this point the engine no longer acts dangerous or violently runaway. It just seems to settle into a very high idle that I can’t get down enough to properly check/set initial timing because it’s idling around 2200-2400.

Throttle blip after warming up does not lower idle either.

So my questions are:

1. Where would you go next from here?
2. Could this still be carb throttle blade/transfer slot/secondary stop related even if everything visually looks closed?
3. Is this still potentially a vacuum leak or does the hand-over-carb test mostly rule that out?
4. Would you pull the carb at this point or continue diagnosing on the car?

Trying not to randomly crank adjustments anymore and want to diagnose this correctly before I make things worse.

Hey guys, looking for some help here. This is my first time doing all of this so I’m trying to learn as I go.

I’ve got a fresh small block Chevy 350 (ATK crate motor) in my 1978 Firebird, running an Edelbrock 650 cfm carb and TH350 automatic.

Here’s what’s been happening:

First start about a week ago went solid

Since then I’ve started it a few times to:

Burp the cooling system

Start dialing in timing

It’s been running pretty good overall

Cold starts around 1200–1500 RPM which I assumed was normal with choke

Today things got weird:

Started it up and it ran fine for a few minutes

While checking timing, it started getting hot pretty quickly (almost 220°F), so I shut it off

Let it cool down and topped off coolant

After that:

Restart was sluggish

Then RPMs started climbing on their own

I gave it a small throttle blip to bring RPM down

Instead, it kept climbing, backfired, and died

Next restart:

Immediately idling around 2000–2500 RPM

Blipping throttle does nothing (RPM won’t drop)

I tried covering the carb with my hand:

RPM dropped instantly to ~1000

Other notes:

Idle speed screw is backed out as much as possible

Primary throttle blades are barely open (small visible gap)

Secondary blades look fully closed

PCV and hoses are basically new

Timing was being set around 12° initial and ~28° at 3000 RPM

My questions:

Does this sound like a vacuum leak even though it was fine earlier today?

Could the carb suddenly need adjustment like this?

Is the choke or fast idle cam possibly hanging up?

Could heat from getting close to 220°F have caused something to stick or shift?

The fact that covering the carb drops RPM makes me think vacuum leak, but I’m confused how it would just randomly show up mid-day with nothing changed.

Any help or things to check would be huge. Trying not to keep restarting it until I know what direction to go.