Shortly after takeoff, several witnesses heard unusual sounds from the acft. One stated that he heard a sputtering noise & the eng stopped, then it restarted, but was not smooth. The other 2 witnesses heard a backfiring or sputtering noise. Subsequently, the acft entered a steep descent & crashed. Before the plane crashed, the plt had transmitted a mayday call & said the acft was going down. An investigation revealed no evidence of a mechanical problem. Except the left fuel cap o-ring was badly cracked from age & the top of the left wing had signs of fuel streaming (syphoning). Prior to this flt, the acft had been flown from Youngstown, OH to Wilkes-Barre, PA, then on to Danbury, CT. The plt had reported that on that trip, he had seen fuel streaming from the left wing, all the way back, and that he would see a mechanic about fixing the caps. There was no record that the acft had been refueled at Wilkes-Barre or Danbury, prior to this flt. Also, a mechanic had previously advised the owner to replace the o-rings.