Kai---I'm 95% sure that the problem is spark, or at least spark-timing
related. You said that you have spark at the cylinders which tells me
that the ignition system is switching off and on, as it should, from the
points opening and closing. You feel that it is also timed correctly,
but if you have "spark, fuel and oxygen" it should fire! Unless, of
course, the spark is out of time with the pistons. Let's do a real quick
check, here:
Jog or turn the motor over by hand, until the timing pulley points to
"0" degrees, top dead center. Note where #1 spark plug wire is on the
distributor cap. Take off the distributor cap. The rotor should be
pointing to either #1 or #6 wire terminal. If it's not, somehow you
jumped timing. Proceed from there. Write back if you need further
assistance.
On an engine that was running well, and with no obvious changes starts
to backfire, is almost always caused from an interruption of spark. The
fuel got there without spark to fire it. Fuel loads up in the exhaust
sysem, and ignites when spark comes back on. This does not necessarily
mean that the ignition is out of time. The same thing could happen if
you turned off the ignition (key) then turned it back on, holding the
throttle open all the while. Most likely there's a break in the wire
that takes current to the coil + side. This can be checked by
disconnecting this wire, and using a jumper wire from a fused conection,
make a new contact with the coil + terminal. (I think we've been here
before.)
Do the rotor-pointing bit first, to see what turns up.
Dick T.
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