2005 Triumph 790 Bonneville Won't start when cold.?

The bike is stock with the exception of removed exhaust baffels. The carbs were re-jetted to compesate for modifications to mufflers and was fine until recently and now it won't start if cold. It's garaged in So. Cal., and we've tried all cold start stratagies, including warming carbs, manifolds, barrels and float bowls. I know other Triumph owners have experienced this problem and Triumph denies any trouble, so who knows how to fix this difficulty?