Geoff---Low speed misfires can be very frustrating to find, as you know.
An 02 sensor's monitor should 'drop a light' when it sees unconsumed air
down the exhaust pipe, showing a lean condition. Unfortunately, this
signal can happen whether a cylinder didn't fire because not enough fuel
was present to make combustion, or no spark was present. The result
would be the same. Lots of air for the split second as it passes the 02
sensor. My first thought here is that you experience a lean misfire at a
certain needle/jet position. This is somewhat unusual if you have a
stock setup. If you are running different carbs, it's possible the
needle match isn't precise to the engine's needs at certain rpms. In
such cases, the monitor should pick this up as a lean condition, as
power would drop, but not necessarily show up as a misfire.
You are describing two conditions here. A low speed misfire, and a power
loss above 3,000rpms. The first one sounds more ignition related, and
the second, fuel. The fact that these can overlap makes diagnosis
challenging.
Dick
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