How to capture a perfect Zapdos

Get a Staraptor and level it to around 70, make sure it knows Final Gambit. Give it vitamins or EV train it enough so that its HP stat is exactly 226, and its Speed is 181. These values are exactly one point less than a Zapdos with perfect HP and Speed IVs.

A perfect Timid Zapdos at level 70 should have these stats:

227 HP
136 Attack (not relevant)
145 Defense
201 Sp.Attack
152 Sp.Defense
182 Speed

When you send out the Staraptor, Intimidate will only activate first if Zapdos is slower (and therefore it definitely has an imperfect Speed IV) so you'll know whether to reset or not. If Zapdos's Pressure activates first that means its Speed IV is definitely 30-31. You'll need to use a Speed O-Power before each battle to make sure you outspeed the Zapdos if Pressure does activate first, otherwise it can one-shot your Staraptor with Discharge.

If Pressure activates first then you're ready for the next step, finding out the HP IV. Use Final Gambit and it should either kill the Zapdos or leave it with 1 HP. If Zapdos survives, its HP IV is guaranteed to be 30-31, if it doesn't survive, soft reset and try again!

At this point all you have to do is throw a Master Ball and hope theDefense, Sp.Attack and Speed IVs are good, as well as making sure it has the nature you want. Don't forget to put your Synchronizer at the front!

Happy hunting~