If you have the model number I can supply specific manual and help.
I realize it is Pulse 21 but the model number will narrow it down.
See here - http://www.cozyparts.com/Lennox-Parts/forum/Lennox_service_manual/Furnaces/G21_GSR21_10-01-2001.pdf
I would refer to the gas intake diaphram material and make sure it is clean.
Check the drain is not backed up.
Verify intake and vent pipe are clear.
Those arfe the critical itiems for start up.