If its popping thru the carb My quess it would be timming , Make sure the rotor is pointed at the # 1 plug also I had the same problem years ago My Brother in law and I built a SB motor for drag racing and it got real late by the time We tried to start it ,so We had the timming off enough that it popped thru the carb being that We had been up for 24 hours We keep trying to start it and it keep popping ,We finaly got some sleep and went back to the motor later in the day it turned out when it popped thru the carb it took out the intake gaskit and had a vac. leak so You might want to check that out to after We fixed that it started and ran fine .
Pete
