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?

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[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.

 

[TDoc]

I took the belt off the air pump and no more backfiring! I don’t have a catalytic converter so no need for the air pump?

I’ve left the EGR and all hoses connected. I don’t see any air tubes into the exhaust manifolds. Ok to keep driving like this?

Engine runs fine and thankfully no more exhaust backfiring!

 

[fast 99]

No air injection tubes?

 

[DoubleDingo]

Photos of the engine would help

 

[TDoc]

I don’t have any good pics of the exhaust manifolds but here’s what the passenger side looks like. Just the single big tube where the plug is in the link. https://www.summitracing.com/parts/rnb-674-245/make/chevrolet/model/k5-blazer/year/1986

 

[TDoc]

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

You must be registered for see images attach

 

[hey mister]

I don’t have any good pics of the exhaust manifolds but here’s what the passenger side looks like. Just the single big tube where the plug is in the link. https://www.summitracing.com/parts/rnb-674-245/make/chevrolet/model/k5-blazer/year/1986

That large tube is for the warm air intake that goes into the carb. That silver part is called the stove and the pipe going from the stove to the breather snorkle is called the stove pipe.
There is no tube from the air pump going to the stove.

 

[SirRobyn0]

You still get benefits from the air pump. Replacing the diverter valve so it functions correctly would be what I would do.
And honestly I don't know how you can stand that vintage of engine (80's ESC 305) without a cat. It's probably because your air pump functions, and that does do some exhaust clean up on it's own.

I'm just speaking from personal experience you do what you want, but my 84' 305 ESC truck sucks to drive in traffic without both the cats (I have aftermarket duals so on cat on each side) and the air pump. I have driven it with no cat and a working air pump, no air pump and working cats, both not working, and both fully functional. These engines (this generation of the 305) was literally build for all this emissions stuff rather than the other way around like with most engines.

If your determined to remove it, at the very least don't throw your air pump, or it's hose or any thing related away. Run it for a while. If you decide it's never going back on sell it or give it to someone that can use it rather than throwing it away as some components are no longer make.

 

[Turbo4whl]

These engines (this generation of the 305) was literally build for all this emissions stuff rather than the other way around like with most engines.

Poor little 305 gets a bad wrap by many of the members here on this forum. Why, well it has 45 less cubic inches than it's bigger brother, the 350.

What is this engine? It's an air pump. Bigger is better right? Well maybe not if you are trying for fuel economy and cleaner emissions. The historic Small Block Chevy, born in the 1950's at 265 CI, quickly up to 283 CI and powered big heavy vehicles.

So maybe now I have you thinking... what happens when you decel? Throttle closes, engine vacuum jumps up. What is the engine again? Big air pump. So this high vacuum draws air back from the exhaust.. Backfire!

GM knew this, added a decel valve. Your 305 had one. Is it there? Is it working?