It's all vacuum and timing. The timing is off some where, that makes it run backward, to advanced. The belt might be off 1 tooth or 1 on the dist. I know you don't like to here that but there is somthing wacky going on there. My concrete pump guy has a diesel that will run backward when it gets stalled and cranked backward by the hydraulic system. But it will continue to run that way tell you shut it down. Direct injection of fuel. Yours will only run that long until the fuel it can burn in the exhaust is gone. That tells me that somthing is wrong on the vacuum lines.