With both pairings riding Honda machinery, the Crowe brothers built up a 4.7-second lead on Birchall and Rousseau by Glen Helen on the first lap, stretching it to seven seconds by Ramsey.
Despite a fightback from Mansfield-based Birchall and his new French passenger, the gap increased by a further two seconds by the end of the opening lap.
Molyneux and Roberts slotted into third place a further 35 seconds back following an early battle with the pairings of Lewis Blackstock and Patrick Rosney and Alan Founds and Rhys Gibbons on lap one.
But a time credit awarded to Founds and Gibbons, whose outfit was black-flagged part-way through the lap, saw them slot into fourth behind Molyneux and Roberts early on lap two.
Putting in a dominant second circuit, the Crowe brothers upped the pace as they overhauled Birchall and Rousseau to the lead on the road and 21 seconds ahead by the Bungalow.
That push saw the brothers put in their first 120mph lap on the 37.73-mile (60km) Mountain Course, finishing the circuit in 18 minutes 48.747 seconds.
Molyneux and Roberts, mounted on Kawasaki machinery, followed more than a minute behind as the Manx pair managed to pull away from Founds and Gibbons and Blackstock and Rosney – both on Yamaha outfits.
The uncle and nephew pairing of Founds and Gibbons took fourth place, six seconds back on Molyneux but four ahead of Blackstock.
