I have a byrant 90 from about 1992, it ran fine until a few months ago. From then on I have an intermittent ignition problem. From what I can tell the unit has a spark ignition that ignites the pilot light and that ignites the burners. The problem I am having is that the induction fan run about 30 seconds and turns off, but no ignition happens. I have checked the vacuum switch by taking the hose off and sucking on it and it does click. Some times it will not work all day and other times it will work fine for a week. It seems that when it is cold outside below 40 degrees it does not want to work anymore, but when it warms up a little it will work fine. I tried to take a look at the pilot light area but it is buried deep and I can't see it. Does anyone have any idea what it could be or maybe have a link to a repair manual so I can further diagnose it myself, I am a automotive engineer but I have not messed with furnaces before so a manual should be enough for me to diagnose it.
i know this is going to sound stupid, but take out your burners and clean them real good, take some sandpaper to your sensors and clean it as well, you got to start simple and move to the more complicated