Backfiring through the exhaust
#2388499
10/17/17 12:39 AM
10/17/17 12:39 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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I've had the same set-up for years with few changes: Indy SR-headed 440, with Mallory/Hyfire 6AL ignition box (mounted under the hood), MSD coil, Mallory distributor. The only recent change was a couple of months back when I swapped out my 0.031" Holley squirter for a 0.035" squirter to help with a slight off-idle bog (I don't believe that this is the culprit). The car has been having some kind of strange starting issues lately. The battery will seem near-dead after sitting for a few days, and will sometimes die before I can get the engine running. The battery's only 4 years old, and didn't seem to have any issues when I recently put it in my truck. Out of the blue, the engine started backfiring through the exhaust yesterday. At first, I thought that it was loose plug wires, but they checked out. I also pulled all of the plugs and the distributor cap, but everything looks good. I tested the coil's resistances and they were 0.8 ohms across the primary and 7500 ohms across the secondary (higher than the advertised 4500 ohms). I could be wrong, but I'm convinced that it's an ignition problem. Other than the backfires, the engine seems strong while running with the car in park. The backfires aren't consistent. If I bring the engine up to about 2500 rpm I'll get a loud backfire about every 30 seconds. Any idea what I should check next? ...pull the valve cover? ...check compression? Thanks in advance for any help?
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388518
10/17/17 01:03 AM
10/17/17 01:03 AM
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Joined: Aug 2011
Posts: 2,872 Pattison Texas
CSK
master
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master
Joined: Aug 2011
Posts: 2,872
Pattison Texas
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fouled spark plug, that is what I would check 1st.
1968 Charger COLD A/C Hilborn EFI 512ci 9.7 compression, Stealth heads, 4.10 gear A518 ODtrans 4100lb,10.93 full street car trim 2020 T/A 392 Stock 11.79 @ 114.5
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388528
10/17/17 01:22 AM
10/17/17 01:22 AM
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Joined: Oct 2003
Posts: 7,506 Az
Crizila
master
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master
Joined: Oct 2003
Posts: 7,506
Az
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#1 on the hit parade should be to get the battery problem fixed. Start with the battery - charge it and load test it. Then check to see that the charging system is working - 13.8 to 14.2 with the battery fully charged. Next check for an excessive amp draw with the key off. If that all checks out, your back fire problem is not related to the above ( based on what you have already checked). First thing to check is for exhaust leaks at the header flange.
Fastest 300
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388604
10/17/17 09:47 AM
10/17/17 09:47 AM
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Joined: Jan 2006
Posts: 250 Iceland
BadMoonRacer
enthusiast
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enthusiast
Joined: Jan 2006
Posts: 250
Iceland
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After dark, with the car running in the garage; turn all lights off and watch all spark plug wires and the ign ystem. If they are leaking you´ll see it. Been there done that.
Ragnar in Iceland.
66 Charger, 451, .582" lift, 11" conv. 4.10:1. 4118 lbs 11.75@117 mph on regular Firestone Firehawk radials. Icelandic Champion in MC 2007, 2008, 2009 and 2022.
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388606
10/17/17 09:58 AM
10/17/17 09:58 AM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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The plugs looked good, but I can go ahead and swap them out and see if anything changes. I'll also swap my truck battery in and see if that makes a difference. I can't tell if it is backfiring through the same side of the engine each time or not. I'll also check and see if I've got an exhaust leak near the heads. Thanks for the input.
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388610
10/17/17 10:25 AM
10/17/17 10:25 AM
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Joined: Jan 2003
Posts: 13,385 Marion, South Carolina [><]
an8sec70cuda
I Live Here
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I Live Here
Joined: Jan 2003
Posts: 13,385
Marion, South Carolina [><]
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Parts store batteries typically only last 4-5 years. 5 if it's a good one. So get a new battery first and make sure your charging system is working.
If the plugs don't fix it and your wires check good, it's possible you damaged the MSD box w/ the low voltage. MSD boxes don't deal too well w/ low voltage when cranking and trying to run. I've had this happen a couple of times.
CHIP '70 hemicuda, 575" Hemi, 727, Dana 60 '69 road runner, 440-6, 18 spline 4 speed, Dana 60 '71 Demon, 340, low gear 904, 8.75 '73 Chrysler New Yorker, 440, 727, 8.75 '90 Chevy 454SS Silverado, 476" BBC, TH400, 14 bolt '06 GMC 2500HD LBZ Duramax
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2388748
10/17/17 02:32 PM
10/17/17 02:32 PM
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Joined: Nov 2006
Posts: 2,260 IL
furious70
top fuel
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top fuel
Joined: Nov 2006
Posts: 2,260
IL
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I agree low voltage and 6A boxes don't like each other. Also, where is the 6A located? Mine have always been pretty heat sensitive and act up with they get hot or have been heat cycled a bunch.
70 Sport Fury 68 Charger 69 Coronet 72 RR
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2389056
10/17/17 11:38 PM
10/17/17 11:38 PM
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Joined: Jun 2005
Posts: 6,257 gulfport, ms, west mi
rowin4
master
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master
Joined: Jun 2005
Posts: 6,257
gulfport, ms, west mi
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Well I have the same problem. Battery is good, engine starts right up . It has a miss . Pops through the exhaust going down the road . Not a gasket leak , put in new plugs, no change, started in the garage at night with the lights off , reved engine with no spark tracing anywhere. Used those wire testers with the lights in them to check to see if the wires were firing at the plug , all good, Inspected the cap, checked the point gap,and timing . Finally did a compression check. All cylinders were 150 except 2 which were 145. Now what did I miss?
it's ok to butt heads, just don't do it with a butthead
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2389059
10/17/17 11:43 PM
10/17/17 11:43 PM
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Joined: Nov 2005
Posts: 1,943 San Angelo, Texas, U.S.A.
1968RR
OP
top fuel
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OP
top fuel
Joined: Nov 2005
Posts: 1,943
San Angelo, Texas, U.S.A.
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I was swapping out spark plugs, just to make sure that one of the plugs hadn't gone bad, and decided to check the plug wires while I was at it. The multimeter showed that two of the eight had no continuity. ...not 100% sure that that's what is causing the backfires (could still arc across whatever gap exists), but they definitely need to be replaced.
"When I'm in a slump, I comfort myself by saying if I believe in dinosaurs, then somewhere, they must be believing in me. And if they believe in me, then I can believe in me." - Mookie Wilson
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Re: Backfiring through the exhaust
[Re: 1968RR]
#2389114
10/18/17 01:45 AM
10/18/17 01:45 AM
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Joined: Jun 2005
Posts: 6,257 gulfport, ms, west mi
rowin4
master
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master
Joined: Jun 2005
Posts: 6,257
gulfport, ms, west mi
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I did notice the small ground strap was broken from the engine to frame. Do you think that could be the problem?
it's ok to butt heads, just don't do it with a butthead
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