Idles great, sounds good….but…

[Justasgood]

Not sure how to upload video.

Revving engine to 2500-3000 is fine. But sustained or harder acceleration causes backfire through carb.

Need some guidance to correct. New build(very reputable local race engine builder). Using Progression distributor and Holley Street demon carb(750cfm).

Initial timing is set at 18 with all in at 36.

Engine Setup (440 Big Block bored .060)
• Displacement: 440 CID
• Compression Ratio: ~10.5:1
• Camshaft: .530 lift
• Duration: .230 / .236
• Type: Performance hydraulic roller.
Forged 440 source pistons with Manley connecting rods.
• Estimated Power: 500+ HP
• Builder: Reputable race engine builder

⸻

️ Air & Fuel
• Carburetor: Demon 750
• Intake Manifold: Edelbrock Torker 440 (single plane)
• Fuel System: Electric fuel pump (adjustable pressure)

⸻

Ignition
• Distributor: Progression ignition distributor
MSD coil and wires
• Wiring Upgrade:
• 1-wire high-output alternator
• 30A relay supplying full 12V to ignition


⸻

Exhaust
• Headers: TTI long tube headers


Any thoughts on direction to resolve?

 

[BLK 68 R/T]

What is your fuel pressure set to?

 

[AR67GTX]

I’ve had issues with backfiring through the carb when I leaned the jets out in a Holley too much. Richened it with larger jets and it stopped. Could be.

 

[Justasgood]

What is your fuel pressure set to?

6lb

 

[Justasgood]

I’ve had issues with backfiring through the carb when I leaned the jets out in a Holley too much. Richened it with larger jets and it stopped. Could be.

How hard is it to re-jet the Demon Carb?

 

[toolmanmike]

Not sure how to upload video.

Revving engine to 2500-3000 is fine. But sustained or harder acceleration causes backfire through carb.

Need some guidance to correct. New build(very reputable local race engine builder). Using Progression distributor and Holley Street demon carb(750cfm).

Initial timing is set at 18 with all in at 36.

Engine Setup (440 Big Block bored .060)
• Displacement: 440 CID
• Compression Ratio: ~10.5:1
• Camshaft: .530 lift
• Duration: .230 / .236
• Type: Performance hydraulic roller.
Forged 440 source pistons with Manley connecting rods.
• Estimated Power: 500+ HP
• Builder: Reputable race engine builder

⸻

️ Air & Fuel
• Carburetor: Demon 750
• Intake Manifold: Edelbrock Torker 440 (single plane)
• Fuel System: Electric fuel pump (adjustable pressure)

⸻

Ignition
• Distributor: Progression ignition distributor
MSD coil and wires
• Wiring Upgrade:
• 1-wire high-output alternator
• 30A relay supplying full 12V to ignition


⸻

Exhaust
• Headers: TTI long tube headers


Any thoughts on direction to resolve?

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Videos need to go through You Tube and a link posted here.

 

[Justasgood]

Videos need to go through You Tube and a link posted here.

First wash and tuning

 

[BLK 68 R/T]

6lb

Try bumping it up to 7.5lbs and see if any change.

 

[Don Frelier]

I would consider installing an air fuel ratio gauge.
My guess is it's too lean, but I'm guessing a gauge would give you real data.

 

[Just Send It]

A backfire through the carburetor usually means that the timing is too retarded, the timing curve is too slow, or the carb is too lean.

Step 1 is to confirm your timing.

With that large of a cam, the engine may want more initial timing than 18*. To find what it wants, set your RPM to an even number (I usually do 1000 rpms). Then advance the distributor 1 or 2 degrees and see if the rpms increase. If the RPMs increase, back the idle down to 1000 again using the carb screw. Then add another degree or 2 until the RPMs stop climbing. When they stop climbing, back the timing off by 1 degree and stop the car to let it heat soak for 30 minutes. After the 30 minutes, try to start the car. If the engine kicks back against the starter, slightly turn (retard) the distributor until the engine turns over and starts.

I'm not familiar with that distributor, so I'm not sure how total timing is set. But if it takes 20 initial, you'll need to figure out a way to lower the total advance down to 34 or 36 degrees and control thr curve so it works correctly.

From there, check your vacuum and make sure you have the correct power valve for your vacuum reading.

If it still backfires, look at the carb squirter sizing, accelerator pump cam profile, and then jetting.

 

[pnora]

For that much cam and 10.5 compression it sure sounds weak.

 

[BSB67]

Revving engine to 2500-3000 is fine.

In neutral/park, or driving down the road?

But sustained or harder acceleration causes backfire through carb.

In neutral/park or driving down the road?
What start rpm? And what rpm for the backfire.

Did you set-up the ignition? What does the map/program look like?

 

[Justasgood]

For that much cam and 10.5 compression it sure sounds weak.

Interesting. How should it sound? Sure feel it in my chest. May be between 10 and 10.5. My instructions were to keep it at or under 10.5:1.

 

[Justasgood]

In neutral/park, or driving down the road?


In neutral/park or driving down the road?
What start rpm? And what rpm for the backfire.

Did you set-up the ignition? What does the map/program look like?

Driving down the road. Set up by the engine builder.

 

[Justasgood]

For that much cam and 10.5 compression it sure sounds weak.

This is the cam:

 

[padam]

Great looking car.
I can tell what’s NOT the problem. Those wheels are the best choice.

 

[Justasgood]

For that much cam and 10.5 compression it sure sounds weak.

 

[Justasgood]

Great looking car.
I can tell what’s NOT the problem. Those wheels are the best choice.

 

[BSB67]

Driving down the road. Set up by the engine builder.

View attachment 2015364

Did you verify the initial timing set-up? If not, that would be the first thing to check to make sure ignition and motor timing are in-sync.

 

[PRHeads]

Did you verify the initial set-up. If not, that would be the first thing to check to make sure ignition and motor timing are in-sync.

I was just getting ready to suggest the same thing.
That the timing, as checked with a timing light, agrees with the timing map.