Hi Gary,
1. When the car is in the driveway and you "rev" it up with a quick
release, do you get the same backfire? Or,
2. Does it only backfire when the load on the engine is suddenly
reduced?
3. Is there any "popping" in the exhaust at idle?
4. Does it backfire when changing gears "regardless" of rpm?
5. If distributor timing was the culprit, I suspect that the backfiring
would be coming through the carb not the exhaust. Unless the timing was so
retarded that it was firing when the exhaust valves were open.. However, if
such was the case, you would have no power.
6. If Cam timing was the issue, I can't understand why it would only
occur "once"..this tells me that it is likely one particular cylinder giving
the problem. Cam timing would affect every cylinder.
7. Are you sure that your spark plug leads are fine?
8. I would try a different dist cap.
At the end of the day, it sounds as if you have unburned fumes in your
exhaust that are "exploding" when you take the load off the engine.. This
raises two questions:
1. Where/why are there unburnt fumes and where are they coming from?
2. How are they being ignited?
Unburned fumes can come from an exhaust valve being slightly open during the
compression stroke, (valve clearances too tight?)
Ignition can come from a distributor cap or possibly spark plug leads
arcing? (try running the engine in the dark and see if you can see any blue
arcs)
Have you checked to see if the gaps on the rockers/pushrods are
correct.....I'm wondering if you have one exhaust valve that is too tight
and remaining open on a compression stroke.
Try doing a compression test on each cylinder and see if one stands out as
being low.
Good luck
Paul
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