Sometimes, runtime is a problem of the tracks we're running, so no matter what you do... is a throttle time problem (faced it and was unable of doing more than 4:45... ).

If my experience can serve to you. Did from a 5mm venturi a 5.3mm and used an INS box. This helped me to make 5 minutes and very few seconds, lost a tad of power, but at least was able to finish the qualififiers.