Gas and Oil Home Heating Furnaces - induced fan pressure problem

:confused: i have an armstrong air , high eff. furn. approx. 10 yrs old. my problem is not enough pressure at the switch for it to come on. right now i have the switch bypassed with a jumper wire and the furnace works fine. if i pull the plastic tube off the fan motor and apply pressure to it manually (via sucking like through a straw) i hear the switch click and the furnace starts.
i removed the fan motor and clean some rust and paint flakes out. when it was re-installed it still doesn't create enough pressure. when i put the tube to my ear i can hear and feel suction but obviously not enough. any ideas
would be greatly appreciated , thanks

Check the hole that the tube plugs into for obstructions.

the hole and the tube are clear. it seems like there's not enough pressure . there is pressure there or suction , just seems to be not enough. the fan motor doesn't make any excessive noise but is it possible it is not up to par and not blowing the way it should .

It's also possible that the fan does make enough suction, but the sensor is no longer sensitive enough to sense it.

Make sure your PVC pipe is cleared outside, and also the intake at the burner box is not plugged up.

Check the exaust and intakes like Jay said. There's a possibility something made its way in there.