What is the recommended octane to be used in HP 500 carb motors??

87 octane

So other than waisting money, there is absolutely no reason to use any other octane in stock carb 500's??

Actually the 87 will make the most power and will burn the cleanest and most completely. No reason to run higher octane. Higher octane will cause more black exhaust residue on the transom.

Thats great news.