TRI CITY — Sustaining momentum has been difficult enough at full strength this season for the South Umpqua football team. On Friday, the Lancers were asked to do it against the second-ranked team in the state and without the services of their starting quarterback. As South Umpqua’s 48-18 loss to Siuslaw in the teams’ Far West League opener seemed to indicate, it was all a little too much to ask. Sophomore quarterback Erik Johnson’s 6-yard touchdown run cut Siuslaw’s lead to 14-12 midway through the second quarter. Two possessions later, Siuslaw linebacker JB Dobson blindsided Johnson with a sack that …