I have a 1990 nissan pick up D21 4 cylinder, it would turn over but not start, whats wrong?

I ruled out fuel problems and used a spark tester, found a very weak spark and checked the distributor cap and rotary head(they were corroded) so i changed them. now the spark tester wil flash once or twice then stop(it wont flash again until you shut the battery of and start it up again) any ideas?