Having an occasional yet repeating problem.  The engine dies when starting out from a stop.  I just start moving from a dead stop, 3-5 seconds later, blub, blub, blub, dead. Starts up after a little cranking, idles perfect, revs in neutral perfect, holds a higher rpm in neutral perfect, put her in gear start going, dies.  

However, after going through this start, go, die, cycle several times, the engine then runs perfect.  Did a hundred miles on her a couple days ago absolutely no problem.  Today... start, go, die about 7 times then she ran perfect.  Usually happens in the morning right after she warms up and I start out for the day.  But, she has done this to me after riding for an hour.  Always after I stop and then go, blub, blub, blub, dead.  And it doesn't happen every day.  Just occasionally.  

I've torn the carb apart a couple times in the last month, thinking it must be a carb problem.  Everything appears to check out.   Any ideas?

I replaced the stock petcock with a Raptor a couple months ago.

When did it start having the problem?
Did you replace the petcock, hoping to solve this problem?

The part that's really tough to figure out is why is it doing this only sometimes?  

Other times it runs great all day.  Most of the time when it does it, it's when I first start out in the morning, but I warm it up first.  It's also done it to me after an hour of riding.  And after a few attempts and sitting and holding a high rpm (3,000+), it then runs great.  

Fuel's clean, plugs good, carbs cleaned, but sometimes going from a stop it just dies whether I pull in the clutch or not.  Then starts right up after a little cranking.  

Check for an air leak between the carb and the head. A loose clamp may cause a vacuum leak under load that isn't there with a light load. I'd also check the float height.

Make sure the spark plug is clean and gapped properly. Make sure it is tight when reinstalling it. Check your compression release cable for proper adjustment. Sounds like low compression or a weak spark.

Savageman - I tried changing the plug.  No difference.  And this is an intermittent problem.  Doesn't happen every time.  And when she's decided to run, she runs great.  

The compression release... She idles fine, revs fine, let the clutch out and blub, blub, blub, dead.  Doesn't matter if I pull in the clutch, give'r more throttle, nothing keeps her from dying.  Then she starts up after a little cranking.  Gotta crank a little bit more than normal, but starts right up, idles fine, revs fine.  After a few cycles of start-go-dead and she decides to run great again.

One thing that comes to mind is an electrical issue.

If it were just fuel it would be all the time wouldn't it?