Started Yamaha Seca 750 1982 on 53 degree morning as usual, moved choke to off position, was reving a little to keep running when suddently I was pumping oil all over driveway. Shut off quickly. Oil was pouring like a spicket like blown back through 4 carbs and dripping out box connected to air cleaner. No outward appearances on engine of blown headgasket. Is this one of those inside problems; crankcase chain, tensioner? air cooled, shaft drive Yamaha