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.
