back firing

[warrenrhen]

I drinstalled headers and taken most o the vacuum/fuel lines off. IT starts right up at the touch of the key ith not even choke plates in the carb, ove my 79 spider home a couple months ago with most of the emissions intact. I have since installed single plane manifold with used 32/36 weber, headers, removed all vacuum lines except vacuum advance to the distributor. Removed all the excessive fuel lines etc. The car starts right up at the touch of the key even w/o choke plates in the carb. Idle very smooth at about 800 rpm and accelerates well. but when I let off the throttle it back fries with a few pops and then a very loud bang. I suspect excess fuel but with the non lopey idleing and the conservative jets I installed that shouldn't be the problem either. Any help??

[18Fiatsandcounting]

Is it backfiring through the exhaust or back through the carburetor?

Popping or a loud bang in the exhaust is usually unburnt fuel being ignited in the exhaust. Since the car was set up to run with all the vacuum tubing connected and now it's disconnected, the air/fuel ratio is probably goofy on deceleration.

Temporarily increase the idle to 1200 rpm or so, and see if that helps. If it does, the engine is going way lean on deceleration and the fuel is still burning when it enters the exhaust pipe (and thus the bang).

-Bryan

[warrenrhen]

I'm a little confused. If it's running lean there shouldn't be extra fuel to burn in the exhaust pipe, right? could it be late timing not burning sll the fuel or weak spark like a coil? I'll try setting up the dle tomorrow and see if that works. Thanks and yes it is backing firing in the exhaust.

[18Fiatsandcounting]

If it's running lean there shouldn't be extra fuel to burn in the exhaust pipe, right?

Lean mixtures burn more slowly, so a lean mixture is still burning when the exhaust valve opens and the gases start traveling down the exhaust pipe.

Many older (carbureted) cars do this "pop-pop-pop" thing in the exhaust when the throttle is suddenly closed, especially at high rpms. When the throttle is suddenly closed, the only gas being let into the engine is from the idle circuit, but this goes way lean since the engine speed is way above idle, and thus the pop-pop-pop sound. Some people actually like this sound, especially when it's more of a gurgling like from a muscle car.

Big bangs in the exhaust are a very different matter, as they can cause damage to the exhaust valves, headers, mufflers, etc. And you're right, these bangs can be from significant amounts of unburnt fuel in the exhaust, such as with a really rich mixture.

Have you taken a look at your spark plugs lately?

-Bryan

[warrenrhen]

Thanks for the explanation Bryan. I have not yet looked at the plugs nor have I checked the timing because I gave my timing light away some time ago because I hadn't used it in decades. Now that it's running next week is a good timing to start checking all the basics.