Stumped: Bike idles fine when cold, dies when warm.

[Juan1]

My bike has this issue:

When I'm not on the bike it will fire right up and idle great when its cold.  After about 5 minutes, the RPMs slowly drop and in a few more minutes the bike dies.  Giving it throttle helps when the RPMs first start sagging, but doesn't do anything when the bike is almost stalling.

When I'm on the bike the bike only works great when I first start it.  Once the bike has been ridden for a couple of minutes and it goes back to idle, it will stumble, not go above about 1500 RPM and eventually die.

Once the engine cools, the bike will fire right back up again and the problems return as the bike gets warm.

Steps I've taken to remedy this that haven't worked:

Replaced the petcock with that from an '04.
Had Buddha take apart the carbs, clean, rejet, set floats, and replace potentially bad seals.
Replaced a couple of the fuel lines.
Checked for cracked boots, but they all check out fine.

Does anyone have any idea what could be causing my problem?  Maybe the valves are too tight?

[The Buddha]

Crank trigger.
See if you lose 1 cyl ... and what if you got on it and rode it on a highway ... WFO ....
Cool.
Buddha.

[bobthebiker]

Sounds like you need to be using a volt/ohm meter on your ignition system to me. I had a similar issue, except I had a defective coil on one cylinder, and the bike would just feel like a very anemic 250.

[Juan1]

My problem appears to be remedied: 

This is how I resolved the issue:  I replaced the fuel petcock (which I think was bad) and had the carb rebuilt (which I think was unnecessary).  I started my bike and it once again ran rough.  On a whim, as the engine was running very rough and about to die, I switched the petcock to prime.  Suddenly, the bike ran like a dream.  Today I rode for a bit over half an hour, and the bike ran flawlessly.

Conclusion:  The problem part was either the vacuum line that runs from the carb to the petcock (explains why my bike doesn't work when in the "On" position), the petcock, or both.  I believe both were the problem, but those that encounter these symptoms should start be replacing the vacuum line and the line's clamps.