Backfire and quiting

["Michael Thompson" <miket@urgrgcc.edu>]

Hi all,
I just did some valve work and shaved the head in the TR3. It is back on the
road and acting funny. I did not retime it. After a fairly fast run (60mph)
you come to a stop and accelerate away and it backfires through the exhaust
and doesn't want to run. It will restart and continue on. Could someone
remind me what causes backfiring. Timing, water in the gas? dirty carbs?
Thanks,
Mike