I think you have covered all the options pretty well Adam. I am scraping the bottom of the ideas barrel here but maybe something I list will ring a bell with you the next time you look at it?
I think it is going to end up as an electrical problem myself. Bad coil to dizzy lead maybe. It sounds like you are loosing all spark for a moment.
Crossfire in the leads is possible. Run the engine in complete darkness and see if there is any glow on the leads or on the coil face. Make sure those items are clean and crack free.
Other than that I would try an electronic dizzy and a known good coil.
intermittent carb backfire?
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Adam check out the cam lift on the exhaust side make sure all of the valves are opening fully, i had it on a chevy once where it had taken a cam lobe off on the exhaust side it was fine at idle but under load occiasionally it would pop back through the carb this did get worse as time went on.At first I thought it was timing and after many hours chasing wires I took of the rocker covers and discovered one exhaust valve hardly opening. it opened enough to give a good idle but once the rpm was increased a small portion of the spent gasses went down the exhaust port (so no popping at tail pipes) the rest waited until the intake valve opened and left via the carb.
I suppose the same would happen with a collapsed lifter but you would notice a definate ticking from the valve train area. there was no noise when the cam lobe went.
I suppose the same would happen with a collapsed lifter but you would notice a definate ticking from the valve train area. there was no noise when the cam lobe went.
Thanks chaps, much appreciate your help.
Thinking about the physics of it, in order for a flame to pop out of the carb, there has to be a source of ignition for the fuel mixture in the intake, right? So, the most likely ignition source is a spark plug firing when an intake valve is open. Could be because it's firing when it shouldn't due to plug wire cross firing, dizzy cap tracking or malfunctioning MSD type coil. Or, it could be firing when it should, but with an intake valve not fully seating, exposing the intake charge to a spark.
Or... perhaps it could be Guy's theory of a weak exhaust valve, forcing hot exhaust gas back through the intake. Could that be hot enough to ignite the intake mixture? I guess so, but most engine's have a degree of overlap when the intake and exhaust are open at the same time. And EGR systems actively dilute the incoming mixture with hot exhaust gas.
Hmmm... my head hurts
Thinking about the physics of it, in order for a flame to pop out of the carb, there has to be a source of ignition for the fuel mixture in the intake, right? So, the most likely ignition source is a spark plug firing when an intake valve is open. Could be because it's firing when it shouldn't due to plug wire cross firing, dizzy cap tracking or malfunctioning MSD type coil. Or, it could be firing when it should, but with an intake valve not fully seating, exposing the intake charge to a spark.
Or... perhaps it could be Guy's theory of a weak exhaust valve, forcing hot exhaust gas back through the intake. Could that be hot enough to ignite the intake mixture? I guess so, but most engine's have a degree of overlap when the intake and exhaust are open at the same time. And EGR systems actively dilute the incoming mixture with hot exhaust gas.
Hmmm... my head hurts
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Anonymous
cars tend to pop back through the carb if they are too weak., (dont ask why i dunno!) if the throttle pumps arent pumping enough then stamping on the gas will catch it by surprise and it will pop. (dunno why but this seems to happen on loads of american cars!)
can sometime happen if moisture inside dizzy cap but of course u've had that apart
also
idle speed too low
engine too cold
fitting an OEM air cleaner box may help !
$0.02
can sometime happen if moisture inside dizzy cap but of course u've had that apart
also
idle speed too low
engine too cold
fitting an OEM air cleaner box may help !
$0.02