Are all the fuel lines good? Is the pulse line good? Check for air leaking, most likely at the rubber boots on the carbs. What happens to the CHT/EGT during this time? What happens if you leave it to cool down for about an hour? Is the exhaust not maybe compromised....ie. plugged? Have you changed y...

3 questions here: 1) How long was the shaft?
2) What type of steel was the shaft made from?
3) How was the shaft connected to the gearbox?

Just Testing