The crankcase expands as it gets hot, and thus tends to tighten the chain. When cold the chain should have not less than ¾-in. side movement between the sprockets on one run of the chain. Hold the sprockets so that one run is tight when checking the slack.
To make the adjustment, loosen the three nuts holding the magneto to the chain case and prise up the magneto slightly until excess slack is taken up. Tighten the nuts and check chain again before re-fitting the chain cover. See that joint faces are clean and flat and smear with jointing compound. Re-fit cover, using a new joint washer (K240), and tighten the screws evenly and carefully.