streetracer - I've seen sport engines where the idle screw may not hold the carb barrel in place. Just an idea. If you hit the brakes and the carb closes completely, that is what is causing the engine to die. When you hit the brakes, the carb should still have a small opening, allowing the engine to get air (and stay running).
If the clutch spring is really loose, it may well be a clutch problem.
mabuchi - I'll make another call tomorrow. Last time I talked to them, they were working on getting you taken care of.
horatio - I don't have even the slightest idea, haha.
birkett - Not a problem.