What you're describing is dieseling, which has been a problem on and off with LBCs forever. As doc50 describes, it's often caused by carbon buildup, which is caused by incorrect carb or timing adjustment, oil burning, and probably other things I'm not thinking of. You need to be sure everything is adjusted right. A fresh tune-up is a good idea if you haven't done that yet. Then, take it for a long drive, where it has plenty of time at full temperature.
If that doesn't help, the simplest thing is the stall-it-with-the-clutch solution.