Exhaust Backfiring

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AJ/FormS said:
So it's not a backfire;it's an afterfire.
Afterfire is usually an A/F charge in the header catching on fire and burning in the pipe. This type of phonemenon, cannot normally be felt by the driver, as originating in the engine cuz she is not bothered by it. There is no hesitation, or bog, or engine response whatsoever. It just pops a few times or even just once, and then it is business as usual.If this sounds like what yours is doing, you have to figure out how the unburned fuel-charge is accumulating in the header pipes, and how it is being lit off.
Being lit off is usually easier to figure out.Remembering that it is an ignitable mixture in there, that means both fuel and AIR is accumulating there. So either it is a complete A/F charge getting in there from; A) a chamber that did not burn it, or B) it is a fuel charge that did not finish burning in the chamber,before the exhaust valve opened and let it out, or C) one or more, to all the cylinders are dumping a tiny bit of unburned mixture into the pipes , where it accumulates downstream of the collector, finds atmospheric air there, and boom!
So what you may be looking for, since it always happens with a tip-in(The source of the fuel), is late ignition timing(the source of the ignition) AND atmospheric air (the air source)getting into the header, some place close to the exhaust port. It could also be an exhaust valve not fully closing(the ignition source)
So the first thing I would do is probably see how the engine responds to more ignition timing, in an effort to getting all the fuel charge burned,in the chamber( or possibly less, if it is already over-advanced). I would also be looking for how fresh air might be getting into the exhaust system, and finally if those fail. I would do a compression test, and probably a leakdown test too if the compression test looked sketchy.

FWIW
I have seen this happen at lower steady state cruising rpms;with a Vcan pulling timing in,(as it is supposed to), But not dropping out fast enough when the throttle was tipped in.The proof was disconnecting it, and no more afterfire. Then a re-engineering of the timing systems, cured it.
I have also seen it with a burned exhaust valve, (which did idle a bit rough, but hard to tell the difference with a bit of a cam),that ran fine in a throttled steady state. But when the throttle was slammed open, the cylinder pressure blew a portion of the charge into the exhaust port, and my take was the that when the ignition hit, it also lit off that portion, and Pow!
I have also also seen it where the ignition crossfired in the cap, due to an error in rotor-phasing. Also seen crossfiring in cheap,too closely running ignition cables .
These are all afterfires, and NOT backfires, which is a whole nuther story.
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It was the spark plugs. They didn't look bad but were trashed and I did it by gapping them. They were titanium. I knew tgey came pre gaped but didn't know that changing them ruined them. I gapped them at least three times.
 
Snowmobiles do that too. And certain 4-stroke motorcycles. And occasional quads. And once in a blue moon an outboard engine with a coil going down. The point is, it is almost common, in that world.
I mean after the plugs get fouled,maybe even just once, then they are trash. They might look almost new, but the two-bit ignition systems can't deal with it. It used to be a common thing to tell customers booking appointments with misfires, to put all new plugs in it, and roadtest it. I was always backlogged weeks to months, and didn't have time to deal with these kinds of almost trivial issues, and had no desire to lighten their pockets for a plug swap. Invariably the plug change solved the issue. And I won a customer for the major work.

In the automotive world, before EFI,stock coils in stock applications,had pretty good power, and could usually spark anything.As compression and power goes up, sometimes the coils can't keep up. So they invented plugs that were easier to fire, and, occasionally run smaller gaps to make it easier on the coil.Then they added exotic materials to help them last longer.And then we got COP;coil on plug.
So here's my 2cents; go back to a copper plug, or make sure you have a decent coil,(I run the Big Yellow Accell,with .045 gaps)or the problem is likely to re-occur. And find out how air is getting into your pipes and fix it.The popping is from an unburned fuel charge(s) in the pipe,reacting with clean air, being ignited from a following, still-burning charge. It ignites in the pipe and then you get a pop!, as the expanding gas rushes down the pipe. The expanding gas may go in both directions.I suppose with a good amount of overlap, and the right engine rpm, the pulse could even get into the intake.
If one header pipe is sucking air at the head, then it won't scavenge properly,and that means that cylinder will be down on power a bit. If the collector is sucking air, that would probably reduce the strength of the return vacuum pulse, and the engine will be down on power, on that whole side; but probably not near as much as a flange leak. Flange leaks are to be avoided like the plague,almost.
Happy hunting!
 
another example of do the simple inexpensive stuff first-like throw in a new set of spark plugs. and it was not the filter in a different location, other than maybe getting some dirt in the carb. also sounds like if it was spark plugs it was not backfiring but misfiring
 
AJ/FormS said:
Snowmobiles do that too. And certain 4-stroke motorcycles. And occasional quads. And once in a blue moon an outboard engine with a coil going down. The point is, it is almost common, in that world.
I mean after the plugs get fouled,maybe even just once, then they are trash. They might look almost new, but the two-bit ignition systems can't deal with it. It used to be a common thing to tell customers booking appointments with misfires, to put all new plugs in it, and roadtest it. I was always backlogged weeks to months, and didn't have time to deal with these kinds of almost trivial issues, and had no desire to lighten their pockets for a plug swap. Invariably the plug change solved the issue. And I won a customer for the major work.

In the automotive world, before EFI,stock coils in stock applications,had pretty good power, and could usually spark anything.As compression and power goes up, sometimes the coils can't keep up. So they invented plugs that were easier to fire, and, occasionally run smaller gaps to make it easier on the coil.Then they added exotic materials to help them last longer.And then we got COP;coil on plug.
So here's my 2cents; go back to a copper plug, or make sure you have a decent coil,(I run the Big Yellow Accell,with .045 gaps)or the problem is likely to re-occur. And find out how air is getting into your pipes and fix it.The popping is from an unburned fuel charge(s) in the pipe,reacting with clean air, being ignited from a following, still-burning charge. It ignites in the pipe and then you get a pop!, as the expanding gas rushes down the pipe. The expanding gas may go in both directions.I suppose with a good amount of overlap, and the right engine rpm, the pulse could even get into the intake.
If one header pipe is sucking air at the head, then it won't scavenge properly,and that means that cylinder will be down on power a bit. If the collector is sucking air, that would probably reduce the strength of the return vacuum pulse, and the engine will be down on power, on that whole side; but probably not near as much as a flange leak. Flange leaks are to be avoided like the plague,almost.
Happy hunting!
Click to expand...
Yes plugs and the four corner idle not setup right had me making adjustments to the carb that would make it run but not well. Thank you. I also did go over all the exhaust points and re-seal them. It got rid of some small leaks that didn't make much noise. The exhaust on this Barracuda hangs low. Even though I have worked hard to tuck it as high as I can. So once a month I have to go back over it because it is going to scrape on something pulling into a parking lot or even crazy high speed bumps.
 
.090 preload on a 24tpi (.041/turn) adjuster is 2 turns and a hair from 0. On a pedestal magnum, its less. Gapping platinums is a no-no as you have posted. Its nice to fix an old issue with tools instead of parts.
 
the Hughes instructions are use a .090" thickness gauge at the valve tip, remove it and count how many turns to get to zero. then turn it down that many turns, with a 1.5:1 rocker that will preload the lifter .060"
 
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