In truth, I've never had a problem - I tend to bang the throttle to the stop and hold it there for extremely long lengths of time...30-40 miles or more.
As long as you have a good mix, (40:1 or 43:1), very good maintainence...and a solid machine under you, you should not have a problem.
Two things to mention on that...downhill, keep the throttle open, at least a little, for proper lubrication to the engine...and don't shut down immediately from a full-honk ride over significant distance...let it idle 3-5 minutes and then shut down to avoid temperature difference from affecting seals and whatnot.
Also, check bolts and fittings regularly if you ride like I do, cause riding flat-out causes them to loosen faster.
But having thrown down what has to be well over 50,000 miles on mopeds, and most of that at flat-out...I've never seen a problem with it, really.