The clattering could very possibly be coming from the roto start itself. It's not uncommon. They're also prone to eat gears. I helped a guy out a couple of weeks ago, and with a roto start the one way bearings kept breaking.
He replaced it with a pull start and suddenly it was a pleasure to get it started. The pull start spins the engine much faster than a roto start.
Before you start tuning the HSN you should hold the car off the ground, once it's up to temp, and start to give it short bursts on the throttle to clear it out. Once it's cleared out, that's when you start to drive it and adjust it for WOT. If it quits quickly, that's when you have to richen it. If it bogs, and you can keep it from stalling by letting off the trigger, it can be leaned.