Motorcycle Repair: Short Whistling, Then Stalls, rd200, gas tank

Question
Hi Gary, I have a Yamaha 1974 RD200 that is producing a short, high-pitched whistle out of the left exhaust pipe.  It start fine, idles fine before going in to gear, but once I try riding the bike, the "whistling" begins and the bike wants to stall out.  I've cleaned the carbs, drained and re-filled the gas tank and replaced the spark plugs which did not help much.
From reading and talking to more experienced riders, I am wondering if it is either dirty or off-timed points and/or a clogged exhaust baffle.  Thanks!

Answer
Colin, Check for a vaccum leak at the intake first then pull the intake boots off carbs and visually look into the carbs.Twist the throttle and see if both slides pull up at the same  time and not one ahead of the other. Then check the pipe for a restriction which may involve removing the pipe and blowing compressed air through it.