Try installing some thinner than standard nuts on the set screws. Tighten the set screws, then reef the nuts down. It may be all you need to keep tight. LOL, I always say try the rest, then move over to best, Kioti.
I should have never sold my last DK 40 I had in stock. That 4 cylinder engine purred like a kitten. I never find another new one for 12 grand.