Does the engine run well when your glow ignitor is still attached? If so, it could be a glow plug issue. Engines still breaking in sometimes 'eat' a plug or two when new. What happens if you start at 1/2 throttle (don't make a habit of it though)?

I highly doubt it's a tuning issue since air density doesn't have such a pronounced affect that your engine won't even run. Are you sure the engine isn't starting backwards?