How old is the engine? just wondering if it's used and had a hard life with a fair bit of blow by. If it's a fairly fresh engine and it's only ever been run for a few minutes at a time, it could be just that the cylinder has never been hot enough to prevent too much wall wetting, where petrol sticks to the 'cold' cylinder walls and gets swept into the crankcase on the power stroke.
Could also be something else, but if it seems to running well otherwise, it may be worth getting it running for a good 10 mins or so on fresh oil and check again for a petrol smell.
Hope it helps and good luck tracking down the cause