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4-AF backfire/poping nosie

1305 Views 6 Replies 6 Participants Last post by  Punisher
4-AF backfire/poping nosie

My 4-AF Corolla has a backfire/pop whenever it is: :thumbdown

Under load (while driving)

around 2000-2500 RPM :barf:

accelerating from standstill

The Carb. has been rebuilt and tuned to specs. I have checked and replaced all vacum lines. The timing was set 5B with the advance line plugged then plugged back in to read 10-16 degrees so timing is set. I have 10,000 miles on the Dis.cap,wires, and plugs and just replaced ignition coil and pick-up coil so ignition is fine. I took the top of the carb off and found black all over the primary (from all the backfire). It rarely pops when reving in park or neutral, but it does rev up to about 2200 RPM and drop back down to 2000, then back up again etc...
I also put a 4'' air cleaner on top, but it still pops with stock air cleaner.

I rebuilt this 4-AF 400 miles ago so I can't see anything wrong interally.

I really want to fix the problem, but I do thing about a weber 32/36 DGEV, the problem with that is $$$ and passing emissions.

aside from the CARB problem there also a static electricity sound coming from around the transmission area. I've checked it in the dark and saw no sparking. Theres got to be a leak somewhere. Could that be related to the Pop?

Thanks for all the info:)
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Usually a pop or backfire is because you're running rich, maybe try leaning out the carb a little?
My suggestion would be to check the egr valve. Now im not sure about your corolla but my little tercel had the same problem it was not all the time while driving like a constant backfire but more like slight popping when slowing down or taking off and if its backfiring up a hill its another symptom of bad egr valve. I would think that a bad valve would set off a cel but anyways check it and if it has a gas temperature sensor check continuity there as well.
I had the same problem with my 4-AF I had a NAPA cap and wire set. I had just installed it and the damn thing started back firing. I took it back and after a few hundred miles it started doing it again. So I ended up getting a carquest cap and wire set and it has been fine since. Don't know if thats your problem, but I can gauruntee you if you have a NAPA cap and wire set it will eventually give you problems. I went through about 6 sets before I finally learned my lesson.
Pop can also happen when you're running too lean. Gas doesnt ignite in the cyl, but in the exhaust pipe or wrong timing. Which I suspect in this case AS:
The timing was set 5B with the advance line plugged then plugged back in to read 10-16 degrees so timing is set.
Should be 5deg BTDC without vacuum advance.
Sorry for a reply 3 weeks later but I finally fix the problem

It was a mix of timing and bad cap and wires

The cap and wires only had 10,000 miles on it guess they sold me a piece of crap

and now timing is 5 BTDC and 10-16 with advance
thanks for the info!:)
pfftt.. this is why I'm an EFI fan now :)
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