He is an interesting question. So I have two of the same tractors. 2000ish Simplicity Legacy 24.5 Briggs & Stratton Diesel. One motor has 700 hours on it and starts and runs perfectly. The other has 300 hours and from a cold start it chugs, puffs some smoke, and even sounds like it is knocking a bit. But then after 20 sec it runs smooth just like the other one. What is going on?