[cabmandone]

If it was me.... I'd turn off power, turn off gas and disconnect gas line, label and remove (I wouldn't label but you should if you're not familiar with reading wiring diagrams) wires from gas valve, remove screws that hold manifold in place, remove valve and manifold, remove orifices, hold manifold with valve up, holes down and tap gently to make sure there is no debris in manifold, clean orifices and reinstall, reinstall valve and manifold, reconnect wires, reconnect gas line, turn on gas, open port on valve inlet side and bleed air from gas line, once air is bled and you no longer smell gas from bleeding air fire the furnace and check operation. You might consider removing the burners and cleaning as well. None of the above is difficult or overly technical just take your time.

If you're worried about getting the orifices clean, almost any HVAC company should have the orifices in stock. I probably have some laying around for natural gas from systems I converted to propane. The orifice size will be stamped on the end of the orifice you'll see something like 32 (or whatever size NG orifices are) stamped on the face.

One other thought... When you blew into the heat exchanger you simulated the draft that is supposed to exist in the heat exchanger. If you clean the burner orifices and the problem persists, you could have some blockage in that heat exchanger. When the system lights it's not so much blowing gas into the heat exchanger as much as being drawn into the heat exchanger. That's why when one gets blocked, the flame becomes lazy and yellow and you start tripping a rollout switch.