Hi fellas-
I hope someone can help me here, as I've been fighting this problem for a year!
I have a 71 Dart with 383 ->489 Source stroker kit, 10:1 compression, Hughes solid lifter cam, 2" tti headers, holley HP 850 carb, stealth heads, Mallory ignition box and coil, etc. I have a really loud exhaust backfire that occurs ONLY out the passenger exhaust, ONLY at idle, ONLY when the engine is warmed up. It occurs every 5-15 seconds. It's terrible though. It scares people's pets, cops are giving me dirty looks, babies are crying, gearheads laughing at me...not good. The car runs and drives great. Starts easy, excellent power. Sometimes idle is a little rough when hot, but not bad. No driveability problems.
This happened to me once before years ago. I traced it to a bad plug, replaced it, and all was well. Now it's happening again, so I replaced all the plugs, and no better. I replaced all the wires, no better. I have had new plugs that were bad out of the box before, so I tried replacing the 4 plugs on the passenger side again, and no luck. Still backfires. I checked the timing, and it was 10 degrees advanced. I set it to 12 deg BTDC, and the problem persisted. I checked the transfer slot on the carb to make sure it wan't over-exposed and dripping fuel, but it was right on. 4 corner idle was adjusted with no improvement. I took it as lean as I could go, then later tried a little richer. No change with any of it. I then swapped the carb with an edelbrock 800 Thunder AVS I had. The problem was marginally better, but barely. I still had a loud backfire out pass side exhaust at a hot idle. I thought maybe I had a vacuum leak, checked for one, found none, and changed the valley pan gasket anyway because by this time I am grasping at straws. Problem remained. I checked exhaust system for leaks, and it looked good aside from a ticking at the pass side header flange. I replaced the header gasket at the head with a tti copper gasket installed dry. Header leak was much better, but not gone. Backfiring problem no better at all. Now I moved on to compression test looking for a sticky/burnt exhaust valve. I did the test last fall, and don't remember the numbers, but compression was good, all cylinders within a few percent of eachother. Last week I did a leakdown test (still pursuing the possibility of a bad valve). All cylinders were perfectly equal at 6% leakdown. So what next?! I have a few ideas, and I may mention them later, but for now I would love anyone's suggestions on this. Thanks in advance for any help!
Desperate in Buffalo,
-Jay
