TOPIC: High elevations?

Family a nd I got a chance to spend the week at upper twin lake in the Eastern Sierras last week. Elevation is about 7200 feet, I live at sea level. Motor ran great, ran rich, and could not get it past 12 MPH. Is this normal? It usually runs about 31 mph WOT. Do i need to buy a set of carbs with jets set for high altitude?
Also, I want to upgrade from the original 2 blade prop. Its a 15 spline 48-31072 13 inch 17 pitch 2 blade, I was hoping for better performance, any of you folks have any suggestions? BTW its a 1965 650.
Heres some pics for you. My almost 4 year old son is a troutslayer, he takes after dad, of course. Great trip, cant wait till next year!!
Where is the San Diego crew? We need a cruise.
Cheers!

Yes that is normal just need the correct jets for it at that altitude.