Fuel delivery problem??

Ok, my first question here....

I have a '77 kz650b with mikuni carbs and no accel pump. The bike had been sitting for 2 years. I took the carbs apart, cleaned them out with carb cleaner, put them back on the bike and put fresh gas in the tank with some carb cleaner you mix with gas. The bike started up and ran great. I took it out for a spin today (about 15 miles) and all of a sudden it slowly started losing power and died. After it set for about 5 min, I was able to restart it with full choke on. If I turned the choke off, it would immediatly die. Also, with the choke on, it would run for a few min then slowly lose power and die. There is some rust in the tank but the petcock screens are in good shape.

My questions are...

Is this a fuel delivery problem or something else?
Do I need to rebuild the carbs?
Replace or rebuild the petcock?

I'm at a loss because I'm mainly a car guy and usually deal with fuel injection. I have no problem rebuilding carbs, but I'm worried I'll mess up all the adjustments.

Most likely crud from the gas tank has clogged your pilot circuit. Cleaning is required plus a cleaning of the tank. I would start this effort by removing and cleaning the petcock (the parts inside the tank), taking note of excessive debris in the tank and install an inline fuel filter. Then clean the pilot circuit on the carbs and replace the pilot jets. You have also described the symptoms for a vacuum lock where fuel won't flow because a vacuum has set up in the tank. Try popping the gas cap up next time this happens. If the bike will start/run immediately, then the vent in the cap is obstructed. Take the cap apart and clean everything good.

Thanks for the advice. I tried opening the cap with no luck. So, I took a peice of hose and attached it to the petcock and found that I only had a small dribble of gas coming out. Basically, the carb bowls would fill while the bike was starting and sitting then it would slowly drain the bowls as it ran and not get replenished fast enough so it would slowly run out of gas. Took the petcock off and apart and gave it a good cleaning and had great gas flow. So now it's back off and the tank is being cleaned. Going to clean the carbs for the third time tomorrow and that should fix the problem.

Also, expect my carbs to show up at your place this winter for rebuilding.

If you had that much sediment, your cleaning will likely be temporary. I would consider buying another tank that had minimal rust inside and washing it with phosphoric acid and getting it repainted. I suspect your tank would require repainting after fixing... perhaps POR-15 would work but that is a lot of crud. Buy some inline gas filters in the mean time to collect the sediment.