After fire on deceleration

[zac1981c20]

My 81 c20 has been popping out the exhaust like a popgun anything over 45 when I let off the throttle but recently I’ve noticed it around 35-40. The truck does have a leak at the manifold to down pipe, the truck runs and drives decent enough to daily me to work. I’ve noticed theses connectors down by the transmission I’m not sure what they are or if they have anything to do with it any help or advice is appreciated! Thank you in advance

 

[MarineOne]

I'm guessing your reverse lights don't work Based on connectors ?

My questions for more detail would be
1- does it do it both when engine hot and cold? Better or worse hot vs cold?
2-does it do it only in gear or does it do it if you pop it into neutral aswell?
3-what is timing set at?
4- what are plugs/what's gap on them

Somany things could cause this issue
-Over fueling
-under fueling
-dead hole
-bad distributor
-leaking plug wires
-carbon build up
-timing is off
-mechanical advance is stuck
-burned valve
-wiped cam
-rocker adjustment
-stuck lifter
-bent pushrod
-carb floats too high
-leaking top seal(Rochester carb)
-manifold leak (venturi affect popping exhaust gas )
-fouled spark plugs / broken or burned plug

 

[Ricko1966]

First fix your exhaust leak could be as simple as going rich from closing the throttle and yhen getting an air source at the exhaust leak. But you know you have an exhaust leak,so start be fixing the things you know are obviously wrong.

 

[fast 99]

Exhaust leaks can easily cause exhaust backfire under decel. When the throttle is closed the engine momentarily goes rich. Heat inside the pipes added to the oxygen content from the leak sets the fuel mix off. That is the reason for the anti-backfire [decel] valve on air pump equipped vehicles.

 

[DoubleDingo]

Hmmm, my truck does the same thing, and don't have any exhaust leaks. It is a warm air choke truck, and it only does it with an electric choke carburetor on it. The original warm air choke carburetor converted to electric doesn't cause it to backfire, but any carb designed as an electric choke carb does. If I let off the gas slowly it won't backfire, but a sudden release of the pedal and it will backfire.

 

[fast 99]

Hmmm, my truck does the same thing, and don't have any exhaust leaks. It is a warm air choke truck, and it only does it with an electric choke carburetor on it. The original warm air choke carburetor converted to electric doesn't cause it to backfire, but any carb designed as an electric choke carb does. If I let off the gas slowly it won't backfire, but a sudden release of the pedal and it will backfire.

Are you sure there isn't a leak? SAFELY with a helper lightly load the engine [power brake] check for leaks. If a leak is present, it will be more obvious cold. I use a cheap Harbor Freight stethoscope hose only.

There needs to be O2 in the exhaust for a backfire.

 

[DoubleDingo]

Are you sure there isn't a leak? SAFELY with a helper lightly load the engine [power brake] check for leaks. If a leak is present, it will be more obvious cold. I use a cheap Harbor Freight stethoscope hose only.

There needs to be O2 in the exhaust for a backfire.

Pretty sure there's no leaks. It doesn't tick, doesn't smell of fumes when running except out the end of the tailpipe. And the backfire only happens with carbs designed to be electric choke from the factory. I'm not saying there isn't a leak in the exhaust, there could be, and if there is it's small, but that doesn't explain why the backfiring only happens with certain carbs.

 

[fast 99]

Richer fuel mixture, no 2 carbs will be the same this long after production.

 

[DoubleDingo]

That could be. Ironically it has done it with two electric choke carbs. I just need to find the time to do a complete rebuild on the original carb '81 warm air choke carb.

Another irony, the dashpot, or whatever it's called that pushes out when the throttle is released doesn't push out on these electric choke carbs, but does on the original warm air choke carb. To me, that means the vacuum ports are different from the warm air choke to the electric choke carbs.

The first electric choke carb I put on was an original from an '85 c20, I rebuilt it years ago as a backup for when my '81 carb needed to be shelved for a rebuild. I couldn't pass emissions with that one, so, late last year I bought a rebuilt electric choke carb from National and it acts the same as the '85 electric choke carb. It passed emissions, but it will backfire if the throttle is released quickly.

 

[fast 99]

You have a vacuum decel valve? Those were installed to keep HC's [fuel] down during decel. Holds the throttle plates open slightly to burn off remaining fuel. That would explain the carbs but you should still check for leaks.


edit, and yes vacuum ports can be different.

 

[DoubleDingo]

I know each vehicle at this point is basically molested far beyond what it was like when it rolled off the dealership lot. Mine has all the factory equipment, including the smog pump. Is that pump, by design, supposed to pump air into the exhaust? If so, even if my exhaust was leak-free, wouldn't it still have air in the exhaust, thus causing backfires if the decel valve wasn't working properly?

 

[Ricko1966]

The diverted valve prevents air from being pumped in on decel for exactly that reason.

 

[DoubleDingo]

Where is the diverter valve?

 

[Ricko1966]

It should be in this order,pump,diverter valve,hose,check valve,injection pipes.

 

[Ricko1966]

Found a pic.