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Q & A: Backfire

I live in Livonia, MI and have a '74 MGB with 84,000 miles. I'm having
trouble finding the cause of a backfire occurring when the throttle is
opened
quickly. It also has a serious lack of power.

So far I have changed the diaphragm on the distributor, the condenser
and
coil, spark plugs, spark plug and coil wires, head gaskets, intake
gaskets,
fuel filter, jets and needles (twin SU carbs), and float valves. The
head has
had a valve job with new guides.

I set the timing at 10 btdc with the vacuum disconnected and checked
the
operation of the distributor and can see the timing advance with higher
rpms,
and retard with a drop of manifold vacuum. There is slightly uneven
deposits
of carbon on the plugs, two of them staying clean and two getting sooty.
Idle
is semi smooth after it warms up and revving up slowly is also smooth
enough.
I'm fairly confident that there is no intake leak.

This engine ran well until now. The vacuum is 17" at idle and the
compression
is 130 psi in each cylinder. Each step has produced no improvement. This
engine also had emission controls that have been removed with the holes
in the
head plugged but the vents around the jets left unplugged but filtered
to
keep dirt out (Plugging them made it run bad).

I know I probably need to do more of an overhaul, but I was trying to
see if
these easier repairs would make it drivable. Should I be looking at
something
else before I get into an overhaul, or could I be overlooking something
on
the things I have already checked? Thank you for your help.

John Harden

John Harden!

I would check the vacuum advance to ensure it is not perforated
(most are). Then, I would set the timing at 15 BTDC at 1500 rpm, vacuum
disconnected. Then, I would make certain that the PCV system... read more