I hope I'm not getting too remedial on ya, but here are some things to check.
Put these in the order that works best for you:
If you haven't seen it, review the attached document.
Take a look at these videos:
and look for things in your carb that might be out of place.
The slide rises under engine vacuum, so look especially for things that are preventing vacuum from getting to the top of carb, the upper diaphragm chamber, and for things that would make the vacuum ineffective (again, some sort of defect in the diaphragm itself
). It could be something as obscure as one of the fingers on the white dealy-bop-thingy covering the hole. Drilling the hole to 7/64" cannot be a problem, unless the wrong hole was drilled.
Make sure that you have no intake leaks on the engine side of the carb, such as a loose or mis-installed intake boot or a cut, crack, or hole in the boot.
Make sure that the bowl vent is clear, even going so far as to remove the vent elbow and make sure it is clear.
Do a fuel level check on the bowl by attaching a bit of 1/8" clear hose to the nipple on the bottom. Apply vacuum to the petcock to make sure that it is open. Bring the hose up along side the carb and open the bowl drain. The fuel should rise up to about the gasket level on the bowl.
Do a fuel flow check on the petcock by disconnecting it from the carb, applying vacuum to the petcock to insure the petcock is open, then flip the lever to "On" and then "Reserve". In both positions you should get a strong and steady stream of fuel.
If the vent is clear, the fuel level is right, the petcock flows right, the diaphragm and vacuum passages are good, and there are no intake leaks, you will probably need to disassemble the carb and give all (and only) the metal bits a 24 hour soak in Pine-Sol, then clean every passage and orifice really well.
That's about all I can think of right off the top of my head. If you can report any other symptoms that might provide clues, that might be helpful in diagnosis.