Tony,

A reluctance to idle without working the throttle and backfiring through the intake are classic symptoms of very lean running. This may be because someone has fiddled with the air/fuel adjustment in the top of the fuel distributor although I would perhaps be more inclined to suggest a vacuum leak. A vacuum leak will show up worst under light load and at low speed, especially at small throttle openings. Remember that any such leaks are not metered by the air flow meter (flap) at the fuel distributor. At high speeds and high loads a vacuum leak will be insignificant compared to the large air flow through the air flow meter and the lack of vacuum under high load conditions will also reduce the leak.

There may be a problem with fuel pressure or a fuel blockage however I would expect this to show up worse under load and at speed and you suggest that it seems to perform well under load and at speed. There could also be issues such as burnt valves or low compression however again your description of how it drives would tend to rule this out. I would suggest having a look for vacuum leaks.

Greg

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