I picked up an 89 about a month ago. I have restored a bike in the past and am pretty mechanically inclined. I rebuilt the carbs with OEM gaskets and seals and put it all together yesterday. She fired right up and idled nicely. It's not on the road yet, (I have another bike) but I did rip it around the block a few times. Ran fine. Today I noticed I left it on Prime. Again fired right up stayed right on idle. I switched it to run, and it ran fine for 3 or 4 minutes then stumbled and stalled. I couldn't get it to refire. Finally I put it back on prime for a few minutes and it started and ran fine, I switched back to run and it ran for a couple minutes then started to stumble, while it was still running I switched it back to prime and it ran well again.

When it's running, it runs very well. I don't think it's the carbs. It's almost like it's starving on run once the bowls start to run low.

Anyone had this experience?

First thing is to check the condition and routing of the rubber vacuum lines, if there's an air leak - a split in one, it'll cause your symptoms

Thanks I'll do that first, hope its that easy. I'm thinking at idle its opening the valve but not like it should. The bike sat for a few years and the hoses are a little stiff, but pliable. I noticed the vacuum line is a "formed" tube. so was reluctant to use anything else. Clamps are a bit wimpy. If necessary, do you know of any rebuild kits for the petcock?

The vacuum hose doesn't have any leaks. I also started it on prime and removed the vac line from the petcock and it's got plenty of vacuum. Next step open the petcock and see if there are any obstructions?