I had the same problem with my 5.9, my roto start wasn't fully charged. It spun fast but lacked the torque needed to turn over the engine while it was running so rich. You could try loosening the glow plug 1/4 a turn as well, to ease off the compression. Just tighten it back up once it's running. I also leaned the bottom end 1/2 a turn before it even thought about starting. That engine is so rich at factory settings! You're probably not flooding it with the priming, it's flooding it's self because it's taking so much fuel through the carby. Even now my XL is completely run in, it still won't start at factory settings. Dont start doubting the engine just yet, personally I reckon it's beaut, goes like a shower of the proverbial.