It could be a few things, your high speed jet may have a bit of crap stuck in it causing it to not suply enough fuel. It could have some crap stuck in the float needle causing it to fill slowly and at high speed it is using up all the fuel in the bowl. Or simply it may just need a tune.
What does the exhaust smell like at full speed? if it smells lean, it will be your high speed jet, not giving you enough fuel, it could be a blockage or just tuning. I'm not too sure about what the carby looks like on these engines, but it wouldn't hurt to remove the high speed jet and give it a blow out with cabry cleaner.