Check the mechanical and ignition timing... could be very retarded.
I just sent this to another guy a week ago:
Setting timing (both mechanical and ignition) in order:
http://www.cygnusx1.net/Supra/Librar...aspx?S=EM&P=29
http://www.cygnusx1.net/Supra/Librar...aspx?S=EM&P=30
http://www.cygnusx1.net/Supra/Librar...aspx?S=IG&P=19
http://www.cygnusx1.net/Supra/Librar...aspx?S=IG&P=20
When setting timing with the timing light the TSRM shows a SST (Special Service Tool) for jumpering TE1 and E1… just use a piece of wire or a paper clip. You MUST jumper those two connecters when setting the spark advance.