[mygirlsdad77]

I am working on a 66000 BTU input, 53000 BTU output carrier furnace. All of the electronics, 120 and 24v components are working just fine. Gas pressure is a steady 3.5 on the manifold side. Vent is not blocked in any way. I have even taken the vent off just to be sure. My problem is, when the unit fires, the flame is off and on in all three burners. For about five seconds, and then clears up and burns just fine. However, sometimes the flame is so sporadic for so long in the very left tube, that the flame sensor doesn't read the flame, and you know what happens when that happens three times in a row. I have checked the heat exchanger, and it show no leaks. I am lead to believe that it is the combustion blower(draft inducer) I am pulling a .5 vacuum with my meter. Is this enough. It is apparently enough to close my pressure switch, wich has no vacuum rating on it, of course, that would be to easy. I can't think of any other reason for problem. I am looking for people who have actually had this problem. Any help will be appreciated.