'91 dies after warm-up

[Shadow_Splat]

Hi, I recently picked up a '91 and have been able to get it to run with some minor fixing. I changed the bottom-end of the carbs with a new gasket set with float needle. What happens is after maybe 3 minutes of warming up with a slight assist from the choke, it idles, around 1.5k with choke off, for another 2-3 minutes then it just dies. I'm new to motorcycles but not new to engines. I'm an auto technician and I have the Clymer manual. The only way I am able to get it to run afterwards is to crank with the petcock set to prime, then I just flip it back to ON. Once it is running, the idle jumps to 4-5k then hovers somewhere around 3-4k. I have the choke all the way off and and the throttle out. I try to fiddle with the choke to see if I can lower it somehow. Doesn't work. Any ideas on what is happening?

[The Buddha]

I think you are a shade rich ... and maybe dont have the petcock working too well either.
Cool.
Buddha.

[ke7syv]

Well if it runs on PRIME but not ON then you most likely have a petcock issue. The petcock has a diaphragm in it to allow the flow of fuel when the motor is creating vacuum. Although, how it dies is a good indicator of the cause. Do you think its a fuel problem or something with the ignition?

[Shadow_Splat]

Well if it runs on PRIME but not ON then you most likely have a petcock issue. The petcock has a diaphragm in it to allow the flow of fuel when the motor is creating vacuum. Although, how it dies is a good indicator of the cause. Do you think its a fuel problem or something with the ignition?

Well it's not that it runs only on Prime but that the only way I can get it started. I flip it back to On when it does end turning over. Is there a test or something for the petcock to see if it is still good?

[Shadow_Splat]

Also would the fuel grade (eg. 91 octane) have anything to do with the idle jumping up so high?

[The Buddha]

After rejet, run 87. Its rich enough to choke a horse ...
Cool.
Buddha.

[DoD#i]

Well if it runs on PRIME but not ON then you most likely have a petcock issue. The petcock has a diaphragm in it to allow the flow of fuel when the motor is creating vacuum. Although, how it dies is a good indicator of the cause. Do you think its a fuel problem or something with the ignition?

Well it's not that it runs only on Prime but that the only way I can get it started. I flip it back to On when it does end turning over. Is there a test or something for the petcock to see if it is still good?

<smack> (that would be a wrench to the side of the head.) Do you know what a carburetor bowl is? Do you know what happens when you fill one, and the stop filling it? The engine runs for a while, then quits. Oh, that happens to you every time you put it on prime, then flip back to on? hmm, I wonder if it might be the octane of the fuel, as opposed to a petcock that shuts off every time you turn it "on". Hint - it's not the octane.

Step one in halfway intelligent problem analysis - put the thing on prime and leave it there. Does it keep running - I bet it will. Perhaps badly, because you probably have a vacuum leak, but at least it won't run out of gas after a few minutes.

Why a vacuum leak - well, its a vacuum petcock, and it's not working - either because the hose isn't connected (leak) the hose is cracked (leak) or the diaphragm is toast (leak). Leave the petcock on prime and plug the vacuum port for the vacuum petcock. Does it work better? Follow the chain of events - it's mechanics, not rocket science.