backfire on a 12A?
 

My 83 has been backfiring as of late. Gunshots out the exhaust and what not. Is this normal for the 12A or does it mean something is wrong? Any ideas?

you maybe run lean...check fuel filler or your pump is dying....how is the power?

i think they have an anti-backfire valve? it's only on deceleration right? backfire is from excessive unburnt fuel vapor build up in the exhaust so no it's not lean. if anything it's too rich.

Let me go over what I did before this, that might help. A few weeks ago I ran some seafoam thru the carb to clean it up a bit. It smoked out my whole street, pissed off my neighbors and so forth. Its kinda been on and off since then since I'm still fighting with this beehive cooler since the oil is still trickling out (the banjo sealing washer is all I need now). The power is definately there though. I have a retired mechanic saying that maybe one of the butterflys is stuck, but he's not sure and hasn't even seen it yet. Hope this sheds some light.

is the car all stock?

To my knowledge, yes its all stock.

make sure your choke is not stuck.

What conditions cause the backfiring? If it's during hard acceleration, it's probably leading ignition. My money is on the leading J109 being on its way out.

Check for exhaust leaks.

that was my next though...........

I'm fairly certain the choke isn't stuck since it idles at 700 really nice. It only does it when ikm at about 5k and I hit the clutch.

J109? I'm not familiar with it.

j109 refers to the ignitors mounted to the dizzy. it's the number that's stamped on them on all fb's. the sa's were j105.

When you say igniters do you mean spark plugs or am I gonna need a new dizzy?

Any way to test if that's it? How much will it cost to replace?

Also how hard is it to replace? What else am I gonna have to redo when I replace it? Etc Etc.

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look through the 1st gen archive...I believe there are 2 articles on testing igniters. Or just use the search button....use "advance" tab, then select 1st gen archive

If it's backfiring on decel, the most common cause with a fully-stock motor is a failing anti-afterburn valve.

On the SA's it's part of the ACV; not sure if that holds true for the FB's - - I believe the shutter valve is involved on those cars.

That's what I was going to say drivin... The stock intake has a shutter valve. Not sure which way it actually works, but I do know it prevents backfiring during deceleration on a stock set-up. Anyways, I'd look into that too. I think I have a pic of it.... It's the brass disc top left barrel. I believe it's controlled by vacuum, maybe check the hose that goes to it to see if it's cracked or if the mechanism is in moving order.

Attachment 722901

If it happens to be that AAV then is that harmful to the engine? I'm worried about the engine and components here, if its not killing anything then I'm not gonna worry about it, but if its something like detonation that could cause my apex seals to break then I'm worried.

I've read my FSM manuals and searched for J109 and igniter and only came up with 1 write up that explained how to test and overhaul them. What I'm looking for is more of if it is bad am I going to need to redo the timing or something along those lines.

An afterburn (what most people call a backfire) isn't much risk to the engine. It can be hard on your exhaust system if it gets bad, though. People have blown their mufflers to bits from time to time.

It's basically caused by unburned fuel (from a misfire or a mix error) getting into the exhaust and then igniting.

Allow me to rephrase this. I know the choke isn't stuck. It used to idle really nice at 700. However it idles like crap now at 500 and chokes up to 2k. While the choke is engaged there is alot of jerking coming from the engine like its pissed off. Sorry about my failed realization of this earlier. I just fixed the oil leak yesterday afternoon and with that off my mind and driving it around I was focusing more on the revs and engine activity as opposed to oil pressure since it wasn't pissing oil anymore.

I had the same problem, and my dashpot was stuck. Make sure that works properly and that will help your problem. ( I just WD40'd mine and it's much better now)

Thanks for all the help everyone. I happened to figure it out this past afternoon. It was the "air flow" screw on the side of the carb, I'd forgotten that my uncle was screwing with it back when I was doing my seafoam cleaning. It still backfires a little but I'll work it out with a little tuning tomorrow. At least it calmed the engine down.