If this is a North American 76 with the emissions stuff still connected a
faulty gulp valve will cause back-firing (in the exhaust) on the overrun.
Any (probably) internal combustion engine will backfire in the exhaust if
the ignition is in any way intermittent - unburnt fuel will pass through the
engine into the exhaust and be ignited there the next time a cylinder *does*
fire. On a post-63 MGB this is usually accompanied by fluctuations in the
tach reading if the problem is in the ignition LT circuit, but not if they
are in the HT circuit e.g. rotor/cap. Intermittent explosions in the
intake and exhaust can also be caused by sticking valves, or by burnt
valves/seats if regular.
Timing with Pertronix should be the same as from the factory i.e. 10 degrees
at 1500 rpm for a North American, 10 at 1000 for UK, vac advance
disconnected and plugged at the carb in both cases.
PaulH.
----- Original Message -----
> Lucas 25D distributor with a Pertronix ingnition from a previous owned. I
> have checked timing but not sure what it should be set at?
>
> What cause an MGB to backfire?
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