Two pipe system. Problem is when burners first ignite it seems that pressure builds up inside burner box, causing flames to go on and off rapidly, sounding like a machine gun. Pressure switch that connects to collector box flutters as this is happening.
Switch is rated at .40WC. Checked and pressure seems to start at .40 and flutters some, then as burners smooth out goes up to .60WC. Gas pressure is 3.4WC. Removed exhaust pipe from top of furnace, pressure switch wouldn't even close. Doesn't seem to be restriction in pipe,and pipe seems to be properly pitched toward furnace.
This one has me stumped. I jumpered switch, burners still ignite poorly. Not as bad. Any help would be appreciated.