Strait frame machines are sensitive to several unseen items. tire inflation one side to the other, crown on a paved road can cause a little drag on one side, jack up one wheel at a time and check rolling resistance, a dragging brake shoe can drag one wheel effecting steering going down the road, even internal leak in the steering valve. After you have checked all the stuff in the firs couple of posts and can't find anything, time to go further, those machines seem to always wander a little so you probably won't get straight line steering without some steering wheel work by the operator, get every thing adjusted the best you can and any loose linkage fixed, check the items at the start of this post. is the best you can expect