Secondary Backfire

[ToddMcF2002]

Hi 440 with a Quick Fuel 750 (vacuum secondary) that is backfiring out of my secondary if I stab the throttle fast. Always visible flames, and I had one large fireball that pretty entertaining. If I rev up slow no problems. I think my secondaries are going lean. I have make a couple changes at once that is worth noting:
- New MSD 6AL
- I recurved the MSD billet distributor with light silver, light blue, black bushing.
- Timing set at 20 initial (a bit high for a 440 with a mild/stock cam???) (need to check all in advance)
- switched to a cast iron stock intake
- new MSD cap, rotor, Autolite 85's and MSD wires. Checked firing order multiple times (check order again! Check for timing light activity on all new wires)
- exhaust smells, burns eyes. I thought it was rich but I'm hearing that indicates lean? I can't learn much from the old plugs because I fouled them all last week with fuel diagnosing the bad MSD
- no notable play in accelerator pump
- need to check for any vacuum leaks (need to check with brake cleaner)
- I have NOT adjusted the secondary open rate (Quick Fuel has an adjustment screw)
- poorly seating intake valve? need to check compression
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I've bolded what I plan to look into. Any ideas on anything else I should look at?

 

[snakeoil24]

Reason 1. lean condition.
2. Bent Valve.

 

[dvw]

Slow the secondary down. Sounds like its going lean due to the sudden influx of air with no additional fuel.
Doug

 

[ToddMcF2002]

Thanks for the inputs folks. Before messing with the secondary I was going to tweak timing. Logically a fireball up the carb means the intake is still open indicating excessive advance possibly?

 

[RemCharger]

Old rule of thumb is backfire- too lean or too much advance

Thanks for the inputs folks. Before messing with the secondary I was going to tweak timing. Logically a fireball up the carb means the intake is still open indicating excessive advance possibly?

 

[ToddMcF2002]

Fixed. Turns out my distributor moved when I locked it down last time and it was running 8 degrees BTDC. Put it at 15 with all in timing at 32 by 2800 and its very happy.