IF the chain is slack enough to jump there will be a lot of crank movement not thansfered to cam / distributer movement. Turn the crank forward and back to chesk for lost motion. If definate answer isn't found in that check, pull the number one plug and bring the piston to tdc. Now check to see if the balancer has seperated and slipped.
Too often the timming is suspected fault when fuel quality and delivery is the culprit.
