Make sure to clean the pilot jet tube next to the main jet tube. These are notorious for clogging because the hole is so tiny. It should pull out with some needle nose. If it wont come out do not squeeze harder, or you will crush the tube. Heat up the area with a small torch to get it hot, no more than a couple seconds of heat from a small propane bottle torch. Assuming you have the carb completely unhooked and you are working with just the carb body and everything else is stripped from the carb including the bowl seal. Heat it up at the base of the tube for a second. Then try and pull the tube out. Works every time for me. Also if you have a fixed main jet express carb you can pull and twist the main jet out if you can't seem to unclog it first. I only advice you do this if you cannot get the jet clean because 9 chances out of 10 the o-ring seal is brittle and with crumble after you have pulled the jet out.
Check your timing and points. Are your points worn? Is your spark plug cap threaded in to the spark plug wire tight? Make sure everything under your seat is connected including the battery. It depends on what year your express is. Later expresses have a square plug that must be connected to the battery to get spark.
Also unhook your kill switch behind your headlight and see if this changes anything. It will the black wire with white stripe coming from your throttle. Loose and faulty kill switches sometimes will cause this.
You can buy a new key switch for about 8 bucks shipped on ebay that works excellent for express'. You just have to cut the plug off and hard wire it to the existing wires.
Let us know what you find out.