North Plains is a ways away in the opposite direction from Larch Mountain.

If you're certain that you don't mean one of the 3 other MT's on that side of the hills your most direct route is probably as follows.

From North Plains -> West Union all the way up (it turns into Thompson)->skyline->descend the opposite side of Thompson->left on Cornell-> descend to NW. 24th Ave-> turn right-> forward untill 24th turns into a 1way->left turn-> right turn onto NW 23rd-> left on NW Everett->right on 3rd ave->left on burnside->right on MLK-> left immediate after onto Ankeny-> Straight on Ankeny->right on 28th-> left on Stark-> continue on stark-> pass MT Hood Community College, descend, etc... stay on main road untill you come to bridge over the sandy river-> Right on Scenic Hwy-> follow Scenic Hwy into Corbett->Keep Going-> pass Women's Forum-> Right on Larch Mt Road Proceed 10 miles to where the gate is most likely still closed (it was 1.5weeks ago) due to snow-> if gateis open proceed another 14.? miles to top....

That's going to be one really long day on the bike for you, especially if you don't have a ride meeting you