I have an odd problem: my '83 GS750ED will start easily with choke but will stall the instant the throttle is opened, even a very small amount. It has fuel, and the carbs were professionally cleaned and synced 4-5 months ago. About 3 months ago, knowing I wasn't going to be riding for awhile, I added some Sta-bil to the fuel and rode it for about 5 miles to make sure the float bowls had the 'preserved' gas. Last week I put the battery back in and started it. It started, ran fine with good throttle response. But as it was idling I noticed fuel leaking, apparently coming out the bottom of the airbox. I drained the float bowls and checked the petcock. I had vacuum signal to the petcock and it flowed fuel in 'reserve' and 'on' positions (with vacuum) but not in 'prime'. No fuel was coming back through the vacuum line, and the petcock didn't leak without vacuum applied. I pulled the tank and removed the petcock; no signs of rust or debris on the screen or in the tank. I reinstalled the petcock, put the tank back on the bike and fired it up. No more fuel leak, but now it stalls as soon as the throttle is opened.
I replaced the petcock today. New unit, tested it and it works fine. The stalling problem persists. I replaced the fuel line, the vacuum hose, and checked the vent tube. Again, the bike starts immediately and will run as long as the choke is on. This seems like an extreme lean condition to me, but it doesn't make sense. Everything is stock, there are no false air leaks that I can find. I hope someone has run across this problem before and found a solution, 'cause I'm lost.
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