Try a 14-6 or a 14-8. The heavy plane probably dosn't need a "speed" prop, Unless you already know its exceeding 80 mph with what you've got and it acts like it needs more speed...

Remember that when you change pitch, you should also change dia... The 13-6 is a decent prop for break-in of the .90 class 4-stroke, but its definitly not the best for flying. A 13-4 would be way too light of a prop, and you might over-rev the engine (very bad for 4-strokes... bends pushrods and valves.)

13 lbs is on the heavy end for most sport modelers using a Saito .90. We tend to want the performance of a high power aerobatic aircraft, and for that we ned a high power to weight ratio. You'll NEVER hover that plane with that engine. It won't have the ability to climb vertically forever either. (A .91 4-stroke MIGHT over a 11 lb plane... but not easilly.) You'll find that most people would put a 1.20 on instead of the .90 with that heavy of a plane. (and might as well have the "ballast" be a heavier, higher power engine...)