Slightly loosen the nuts clamping the gearbox to the cross tube on the frame underneath the gearbox. To tighten the chain turn the rearmost nut on the gearbox adjuster bolt at the top of the gearbox a little in an anti-clockwise direction. Turn the forward nut on the stud a little in the same direction to draw the gearbox back. Check the adjustment through the inspection cover on the primary chain cover. It is desirable to tighten up the clamping nuts underneath before checking the adjustment. Having arrived at the correct adjustment tighten all nuts fully, particularly both nuts on the gearbox adjuster bolt. From ½" to 3/8" up and down movement on the chain is correct. Always check in several places in case of variation.