Need Help w/ Backfire Problem

[Red66GT]

This problem has been bothering me for a while, for some reason I just can't seem to get rid of the backfire through my carb when I get on the gas. I can drive the car normally, but as soon as I even get close to flooring it I get backfire through the carb.

Over time I have replaced the distributor (got a new stock one), using the pertronix unit instead of points, got new spark plug wires, spark plugs (with 0.035" gap), tried a different 12 V coil, and recently replaced the timing chain and gears (which helped a little, my old chain was somewhat stretched).

I've retimed the engine correctly, 6 degrees initial, and have the vacuum advance hooked up.

btw, my engine is a 289 A code, any ideas of how I may be able to get rid of the backfire???

 

[whwright]

What type of carb is on the car?

Does it backfire when you "stomp on it," when you slowly depress the pedal, or both? Is it one big "BANG" or lots of little "bangs?"

Backfire often accompanies a lean condition, such as a weak or mis-adjusted accelerator pump. Since you have been through the ignition system pretty well, I'd start looking at the carburetor.

 

[whwright]

After reading your post again, I noticed that you have the ignition set at 6 degrees. That isn't very much. You might try advancing it a little more and see what happens.

Do you have the vacuum advance connected to ported vacuum or manifold vacuum?

 

[68GEETEE]

dwell on your distributor should be set around 29. Try setting this and see how she does.

 

[Red66GT]

Forgot to the mention the carb, I have a new Edelbrock performer series, 600cfm, had a Holley 600cfm, but it still backfired.

It only backfires when I stomp on it, not when the pedal is depressed, also, there are many little bangs, not 1 bang.

It also backfires sooner the greater the load, for example, if I'm going up hill its worse, if the car is sitting still I can stomp on the gas and it rarely shoots fire out the top of the carb.

As far as advancing the timing it seems to want to backfire sooner if its advanced more, and less if its retarded, and if I retard it too much the car has no performance whatsoever.

The vacuum advance is hooked up to my carb (that would be ported vacuum I believe).

I'm going to look into adjusting my accelerator pump, any more suggestions that I can try out?

 

[94CobraPace]

Sounds like a timing back fire. Might want to add a few degrees of timing to see what happens. Then take some away and see if it gets better. Might want to check the balencer to see if is going bad, slipped, and you are getting a incorrect timing reading.

 

[whwright]

It sounds to me like you've more or less ruled out a carb problem or an ignition problem.

How many miles are on the engine? How well does it idle? You might have a burned valve or a weak valve spring.

Did you check all the carb and intake manifold bolts? I've seen some strange and hard to diagnose problems caused by vacuum leaks there.

 

[67GTA-FB429]

When you set your timing, disconnect the vacuum line from the distributor and plug the line (a golf tee works well).

Does it backfire if you "stomp" it sitting in the driveway??

 

[whwright]

Finally! A good use for a golf tee!

 

[SuperDave]

dwell on your distributor should be set around 29. Try setting this and see how she does.

The poster states he has a Pertronix which has no provision for dwell adjustment.

A compression check might reveal a valve or cam problem.

 

[68GEETEE]

The poster states he has a Pertronix which has no provision for dwell adjustment.

A compression check might reveal a valve or cam problem.

Maybe not dwell, but there certainly is a proper gap that has to be set between the magnetic sleeve and the pickup for the unit to function properly.


http://www.fordmuscle.com/archives/2000/02/pertronix/index.shtml

 

[Blown4.6]

Yep, sounds like very lean (too small jets) and/or late timing. My dyno tuner "accidentally" set my initial to 0 degrees (corvette) and used too small primary jets. When I'd hit the gas it would pop back through the carb. He set it back to 10 initial (36 total) and put in the next larger jets.....perfect from that day on.


Good luck. there's not much more embarassing than having people looking at your car at a stoplight with admiration. Then, you hit the gas and it sounds like you're driving some beater.......god that bugged me.