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Author Topic: Backfire through carb?  (Read 1039 times)
discobiscuit
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« on: January 22, 2012, 10:48:22 PM »

In the past few weeks my engine has developed a backfire through the carb.  I have an Edelbrock 1406 and run an Edelbrock 1721 fuel pump so I know the fuel pressure is correct.  I've read where a weak coil can cause a backfire and was hoping someone could give me a few suggestions as to where I shoudl begin looking to resolve this issue.  The coil I have is nothing special, purchased at a local parts store,  and is about 2 years old.  If you think the coil could be an issue could you recommend a good coil to purchase.  I really think items at local parts stores are average at best.  I've recently changed the points, condenser, rotor, dizzzy cap, new plugs and plug wires.  The only thing I have left to do is to check timing.  The dwell is set around 27. 
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Mike S
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« Reply #1 on: January 22, 2012, 11:34:07 PM »

Is the backfire regular or during certain periods of acceleration or decelerating. If I remember right when the dwell slowly drifts from rubbing block wear, the timing retards so I doubt that would be it. Check it though to make sure.

Mike
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BlackoutSteve
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« Reply #2 on: January 23, 2012, 05:28:29 AM »

Usually backfire through carb indicates lean, and most often when the throttle is opened. Enrichment of accelerator pump is required.

Otherwise, it can be ignition lead cross-fire, poor valve seating, cracked dizzy cap..
Any normal timing issues would have to be way off for a spark and an open intake valve to occur at the same time.
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maroman
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« Reply #3 on: January 23, 2012, 08:33:59 PM »

How old is the gasoline? Differant tank fulls doing the same thing?
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Doug  '67 RS/SS 396 auto I know the car since new
Gramps69Z
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« Reply #4 on: January 24, 2012, 09:52:33 AM »

I've recently changed the points, condenser, rotor, dizzzy cap, new plugs and plug wires.  The only thing I have left to do is to check timing.  The dwell is set around 27. 

Was it doing it before the tune up?

Set the dwell at 30 and check the timing.
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Ramjett54
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« Reply #5 on: January 25, 2012, 04:17:09 PM »

If the backfire happens during acceleration is usually a symptom of slow timing or more rarely on street tuned engines it could be a lean condition. Check your timing
If it is more of a continuous popping through the carburetor sound, you probably have valve train problems, broken rocker, bent push rod, broken valve spring, bent valve. Test compression to identify which cylinder.
 
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adjudimo
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« Reply #6 on: January 26, 2012, 05:58:36 PM »

Also, might sound a little elementary; but check and make sure all plug wires are correctly routed in the proper firing order. One wire off or two mismatched can cause backfire through carb.
J.R.
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discobiscuit
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« Reply #7 on: January 29, 2012, 10:03:06 AM »

I installed new points, condenser, rotor, dizzy cap, plugs and plug wires.  Set the dwell to 28 and timing to 10 btdc and still had the backfire through carb once engine was at operating temps.  Very frustrated...I installed an MSD Blaster coil...PROBLEM SOLVED and I'm amazed at the difference.  The Mistress is running like a scalped ape right now. 
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JohnZ
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« Reply #8 on: January 29, 2012, 11:46:41 AM »

Set the dwell to 28

Why 28 when spec is 30*? Reducing dwell reduces saturation time for the coil.
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