I always thouht the only cause of this could be timing but now I'm a believer that it's always more likely too much fuel that explodes in the tailpipes. My 350 did exactly what yours is doing after carb rebuid. Since the carbs were only supposed to have been cleaned and adjusted (and they seemed to be running lean), I went after the timing and valve adjustment, all to no avail. Later I found out the carb guy had put in bigger jets and didn't tell anyone. Put the old jets back in without touching anything esle and the backfire disappeared.
For what it's worth.
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QUOTE(jimkelly @ Sep 5 2006, 08:58 AM)
as shortly after letting off gas - bang bang bang ?
We just installed a 1.8 with single carb and 050 distributor.
Carb and dist appear to be working fine.
What are the odds that the problem is due to valves needing adjustment as this is what we are gonna try next.