Hello, I would like some assistance with a ST90 breaking in procedure. I recently began witha new OS No. 8 plug and new fuel. The engine has been difficult to start and once running will not stay running when the battery is disconnected. I have left the start battery on to gain some run time and managed to get up to speed for a while (half throttle)and it ran a few minutes and stopped. I have the high needle valve at 4.5 turns. One tank thru but have now ruined the plug. What should I do next? Is the engine piston/sleeve tighten when the temp is raised? The fuel is fine. The plug was new. Thanks, Bob