Stumped with backfire from carburator

[huero]

Hello everyone,
I just got my rebuilt SBC 400 installed in my 56. And everything seems fine with the exception of a backfire out through the carb on start up.
Idles nice, engine revs nice but on start up (cold or warm) the engine backfires out of the carb. No fire just fuel. Ignition can be set anywhere from 12-18 btdc and the engine reacts the same.
I have checked fuel pressure (6 psi), fuel level (just below the site plugs. 2 working distributors (1 brand new AC Delco HEI w/12V feeding it), idle screws 1 turn out from seated (they responded well turning in and out) gave the best vacuum at 19-20 psi vacuum somewhat steady. New holley 600 Vacuum secondary carb, checked for vacuum leaks (non present that I could find. Although, I'm open to tips and proven methods other than just sight and sound).
Checked and double checked spark plug firing order and verified timing mark at true TDC.
In Park idle is about 950 rpms, in drive about 750.
I have not driven the car, I want to fix the backfire issue first. When I pulled the plugs they are reading really rich.
I don't know what to check next. Any advice will be greatly appreciated.
Here are some videos that might be of help.

https://www.youtube.com/watch?v=9_X2UZ4gzf4

https://www.youtube.com/watch?v=h23HvG37rHo

https://www.youtube.com/watch?v=A7oMU2Ns2gw

[MadBill]

The one decent backfire in the first clip, it sounded like you let go of the key too soon. Can you do another?

[huero]

The one decent backfire in the first clip, it sounded like you let go of the key too soon. Can you do another?

I can try tomorrow. I got to get ready work work tonight.

[mag2555]

Your first back fire could have blown out the power valve which is now making for the rich condition.
The back fire in the first place sounds to me like the motor is lean when it's cold!

The proper choke adjustment is for it to close all the way on the first pump of the gas pedal , and then once the motor starts the choke needs to open anywhere from 1/16" to 1/8" to allow the motor to keep running.

You current idle screw adjustment of only 1 turn out is in response to how rich it is now due to a bad power valve, or your size Cam your running is calling for different idle bleeds in the Carb, which in a 600 are not the replaceable screw in type.
If you can get the motor running again and to the point where the choke comes off, then if at idle you see fuel dribbling out of the boosters you've got a blown power valve.

[huero]

Thank you for the response Mag.
The carb has no choke.
Correct me if I’m wrong. But it’s my understanding that if the power valve blows then the idle screws would not be responsive. I know little about the science behind carburetors but I will check the power valve.
Thank you for your time.

[stealth]

Hydraulic cam?

Lifters not set correctly..........

[user-17438]

Was tdc properly indicated?what is your base timing without your vacuum advance? Do you have too light of a spring in the mechanical advance? What is the cranking timing?

[huero]

Hydraulic cam?

Hydraulic Roller
I 465./E 485 lift

[huero]

Was tdc properly indicated?what is your base timing without your vacuum advance? Do you have too light of a spring in the mechanical advance? What is the cranking timing?

TDC was verified and set at 10-12-14-16 with vacuum advance plugged
the specs on the hei distributor are as stated

The ACDelco 93440806 distributor has a preset timing curve. The timing should be set at 32 degrees total at 3,000 RPM with the vacuum advance disconnected. 10 degrees inital timing at idle should provide this total. At 1,600 RPM the mechanical advance is 12 degrees. The vacuum advance is set to 20 degrees at 7.5 in. of vacuum.

[Kev7s]

Pull distributor and have phasing and timing curve checked.

[huero]

Pull distributor and have phasing and timing curve checked.

This is the second and brand new distributor put in.

[Kev7s]

Pull distributor and have phasing and timing curve checked.

This is the second and brand new distributor put in.

Sorry, not to mean but that does not mean much nowadays.

[huero]

My first distributor was a custom piece built for a similar engine I had. I thought I was having a ignition problem so I purchased the ac Delco unit as a back up

[CharlieB53]

I have seen an intake valve 'leak' slightly stone cold yet once fired and running seal up and run just fine.

It can be a PITA but a cold cyl leak down test may identify a leaking intake valve.

Another method is maybe pull one plug wire and cold start, repeat with the next cyl. Try to identify if it is one particular cyl that is causing your backfire.

[rfoll]

You might want to check for crossfire. Contamination in the cap or 7&8 wires too close together. I had an issue with a cap difficult to install in the car and the carbon cap center would draw a mark on the rotor.