Clam,

Since ducted thrust relies heavily on efficency, your prop tip clearances are the problem. Try reinforcing the ducts with .58 oz glass cloth and finishing epoxy resin, Pacer Z-poxy works great. Line the ducts with glass and resin. Squeegee out as near to 50% of the resin as you can get. The weight of the glass/resin will be nothing compared to the thrust you will gain after re-fitting props for a tighter fit. Also, since you are forced to use a smaller diameter prop right now, you are losing thrust there as well. Use the largest diameter prop you can without them hitting the duct walls. Try changing the pitch as well. Hope this helps ya.[8D]

Dan