This is probably an issue with either the clutch spring, or the sprocket bearing, if there is any drag from either of the components it will drag the engine down to a stop when the brake is applied, check the clutc spring if you can pull the shoes out by hand the spring has streched and will require replacment, turn the sprocket around manually, if there is any resistense ( drag on the crank ) then grease the sprocket bearing.
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This is probably an issue with either the clutch spring, or the sprocket bearing, if there is any drag from either of the components it will drag the engine down to a stop when the brake is applied, check the clutc spring if you can pull the shoes out by hand the spring has streched and will require replacment, turn the sprocket around manually, if there is any resistense ( drag on the crank ) then grease the sprocket bearing.