Back Firing

[sg1747]

I have a 69 Z with a smog pump and chambered exhaust that back fires when I let off the gas or downshift. Does anyone know what the cause could be?

John

[ssl78]

bad diverter valve or vacumn signal

[Salvatore]

John, What car did you buy? Any pictures? Sam

[1randy]

Possible exhaust leak

[sg1747]

http://i708.photobucket.com/albums/ww86/sg1747/DSCN2120.jpg

[jtower1969]

I'm no expert....but I would vote for lean. Colder temps have more oxegen in the air...you need to put more fuel in the carb to equal this out. I have a baseline for my car and adjust air bleeds up or down 1 1/4 turn before changing jets.

[Xplantdad]

I thought all smog set ups backfired to a point...which is one of the reasons that people took them off... http://www.yenko.net/ubbthreads/images/graemlins/hmmm.gif http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif

[napa68]

Judging by the hood, it's a cross ram. My vote, Too fat!
Excess fuel gets in the exhaust, A.I.R. does its job, wham...backfire! M.O.O. and $0.02 http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif
Tim

[Salvatore]

NIZZZE car John! http://www.yenko.net/ubbthreads/images/graemlins/biggthumpup.gif

[sg1747]

i wish it was a cross ram.wens i will block up all the holes in the exhaust manifold and try that.

john

[Salvatore]

Cross rams don't perform real good on the street except for maybe Kerry Stanleys 69 Z. Hard to drive without a low rear in it.

[69L78]

Sam, There is another exception, you need to take a ride in mine. I was surprised when I bought my car with the X-Ram, it will start as easy as any single 4 bbl car I have ever owned and will sit and idle all day long and not shut off. The engine has 29k org miles and never apart....stock exhaust manifolds and R45 plugs.

[Salvatore]

That sounds good! I guess it is all in the set-up. Hotter plugs do help too. http://www.yenko.net/ubbthreads/images/graemlins/biggthumpup.gif

[69L78]

The 2nd owner who put the X-Ram on in 1981 told me he was using R43 plugs before he went to the hotter R45 and the car would not pull itself up a small hill.

[Chevy454]

Neither of the original chambered exhaust systems we've had had backfiring troubles...the only problem I had with the chambered setup was that it's HORRIBLY restrictive.

[wagonman]

[ QUOTE ]
Judging by the hood, it's a cross ram. My vote, Too fat!
Excess fuel gets in the exhaust, A.I.R. does its job, wham...backfire! M.O.O. and $0.02 http://www.yenko.net/ubbthreads/images/graemlins/dunno.gif
Tim

[/ QUOTE ]

SOME WORDS OF WISDOM HERE!from the previous post.

Your car is backfiring from where?

Intake? or Exhaust?You didnt specify,but i know by experience that your talking about backfiring out your exhaust.When your throttle plate is closed under your extreme high vac conditions as you described.Your engine is backfiring from either experiencing a very rich air/fuel condition or an exhaust leak,bad diverter valve.During this period of deceleration,are you experiencing plumes of black smoke from your tailpipes?(RICH)

Generaly speaking:

"Backfire" is popping through the carb.....usually due to a lean condition caused by improper air/fuel ratio and or timing.


"Afterburn" or backfiring through the exhaust is caused by an excessively rick air/fuel ratio and or timing.

[sg1747]

it back fires from the exhaust it has a 1971 smog set on it.

[Salvatore]

John, You are a drag racer. Process of elimination. Take the belt off and see what happens. Smog systems do make noise and act up. Check all exhaust connections for leaks. Take a plug reading and see where you are at. Check the dwell and timing. Maybe a carb rebuild or valve adjustment. My car has chambered exhaust and mine pops and raps too. Take it to 6,500 rpm's if it goes there smoothly there is NO major damage. May need just a good tune-up or a nice ride in the country!

[Mr.Nickey Nova]

John,
Did you just install the smog system?? If you did then i would say a bad diverter valve too.Friend of mine had the same problem when he installed a rebuilt smog system on his 70 Z/28.

[sg1747]

I am going to try all of the suggestions when I get the car back from the shop. I had to get the trans fixed first so the car is up on the lift. No, the smog system was on it and the previous owner said it was from a 70 Chevelle.