Bike won't run after top-end work

[Juan1]

Background: I purchased a GS500 that ran poorly and leaked lots of oil.  Since then I've had a machinist replace a couple of burnt exhaust valves, I've replaced all of the seals on the top end, reset the timing, and set the valve clearances to spec.  After all of that, I put my GS back together and she fired right up.

Problem: I tried driving my bike the following weekend, and two blocks away the GS started trying to die at idle.  3 blocks away the bike stopped running and wouldn't restart.  Due to life being busy, I left the bike sitting for 3 months.  Today when I tried starting the GS the bike wouldn't start, and eventually backfired.  When I say the bike backfired, I mean it sounded like a 12 gauge went off, and it set off the neighbor's car alarm. 

Question: What do you guys think the issue could be?  I plan to spend next weekend replacing the old gas, checking the spark plug gaps, checking to see if the timing chain slipped, and rechecking the valve clearances.  Is there anything else I should explore?  This bike has me at wits end.

[weedahoe]

Im willing to bet your timing is off

[adidasguy]

Got gas in the tank?
Got the gas turned on?
Got the vacuum line connected to the petcock?
Check the float levels in the carbs? (Could be not enough gas or you're flooding it.)

[stokes776]

Make sure both ignition coils are connected. I disconnected one accidentally.

I sat there for a few minutes trying to get it to start, and when it finally did, HUGE backfire.  I took it for a ride, but nearly died and totally weird response to throttle.

Forums helped me out, found the spade connector was disconnected.