LE9 305 ESC backfires through exhaust on deceleration

[TDoc]

I have a ‘86 k5 Blazer with a LE9 305 with Electronic Spark Control. It has always backfired 1-3 times through the exhaust on deceleration. It runs great aside from the backfiring.

It has new spark plugs, cap, rotor, wires. How do you check and set timing on an ESC engine?

I recently put dual exhaust on it with no change on backfiring. Same as it was with stock exhaust. Still running stock manifolds.

I live at 6900 ft elevation and thought it may be running rich but plugs look good to me, maybe a bit lean?

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[TDoc]

Factory quadrajet, intake and ESC. Air pump still operational but no catalytic converter.

 

[fast 99]

Is the air pump diverter valve functioning?

exhaust leaks?

Most common cause is O2 entering exhaust causing unburnt fuel to ignite.

 

[PrairieDrifter]

Exhaust leak, or retarded timing. I am no help with the esc systems.

The plugs look pretty dang good though, maybe a hair lean.

 

[DoubleDingo]

My 81 does it too, no ESC, no EGR, and the only way to get it to not backfire is let off the gas easy. If I lift my foot quickly it backfires big time. It's neat to see in the dark, as the exhaust exits on the driver's side, so I get to see fireworks at night in my mirror...lol...

 

[TDoc]

Is the air pump diverter valve functioning?

exhaust leaks?

Most common cause is O2 entering exhaust causing unburnt fuel to ignite.

No exhaust leaks since exhaust was redone. Where is the diverter valve? On the air tube to the manifold?

I was going to take the belt off the air pump and see if that changed anything. I’ll try retarding timing a bit before I pull air pump belt. But I don’t think I need the air pump anyway since no cat.

 

[fast 99]

Yes, as a test could disable air pump for a short drive. Did you check exhaust with a load? Really doesn't take much of a leak to cause a backfire.