Vacuum Leak
I had the same problem the other day. Mine was a vacuum leak. It only did it once in a while, and I took it all apart to find 3 of 4 vacuum lines had been removed along with the air box and the vapor evap recover system, and then the carbs were capped with loose fitting rubber caps (ala the prev owner

). The topper is that the vacuum switch was capped with electrical tape. So when I hit it, once in a while it'd hesitate, then go like normal... and occasionally I'd get a little popping backfire. Luckilly mine is pretty cut and dry in terms of trouble shooting.
So yah, try ljangell's method if you like. One that works for me is to remove the vacuum lines, plug one end, and hook the other to a bike pump. I then immerse them in water (a 5 gallon bucket is handy, or anything else you can easilly get the tube into and see) and gently pump the tube with air. This way the entire surface of the tube is being tested. You'll probably get immediate resistance so don't have an aneurysm pushing too hard... it won't take too much if there is a leak. If you see bubbles then you've found a problem... so much as it isn't coming from where you plugged the end. Hopefully you'll find the culprit, and if not you can reinstall making sure your ends are secure and airtight, and start 'er up. If it's still doing it, then you can move on to the carbs. Let us know what you find. Good luck.