new problem today - ??? - leaking float needle, hydraulic lock (left carb only) - possible?

I have had exactly what you describe occur due to float needle not seating/admitting raw fuel into the cylinder/locking it up when trying to spin the (Alton) starter. It was the one time I forgot to turn off the fuel! However, in my case I had to pull the spark plug/spin the motor to expel the fuel. I agree that it seems that it shouldn't bleed down on its own but if the crank was able to rotate JUST far enough to crack open a valve, it might have relieved some of the pressure just as you released the starter button. That's the only thing I can think of that would explain the lockup going away by itself if it was hydraulically locked.

In my case, the float had hung up on the gasket.

Bernhard said:

OK here's one, I have a 650 jap 4 that has vacuum petcocks, I forgot to close the tap off and came out the same afternoon to start the bike up on the button one sunny day, didn't notice the smell of gas, just heard a slight crunch when pressed the starter button, this was the sound of a Morse chain breaking. Worse, the same chain also drives the piggyback alternator- so no charge to the battery. Even worse, its a chain that is unique to that one particular model and unobtainable. The air was a bit blue that afternoon at my house.

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What bike?

If I forget to turn off my fuel tap I don't get any problem. I did have a leaking carb one time and that was because the red viton tip of the needle had swollen out of the crimping one side and got off centre, causing the leak. New needle fixed the problem and haven't had any issues since.

concours said:

What bike?

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Yank import 650 dohc Nighthawk.