For myself, it was always the way the cowl was held on in the front. Hard to find any other faults because the series 1 is faster than the series 2. Even Andy will tell you that.....
I ran the series 1 since he started building them in 1990 and any newer ones I had were the series 1. Some guys will say that the series 1 was harder to launch but its all in the set-up.