Well, at this point I would almost think something has to be damaged: cracked intake, warped float bowl or throttle body, leaking brake booster, missing/open vacuum port or bad vacuum hose somewhere. Unless that manifold or carb has God's own personal warp in it, it should be sealed.
A/F ratios at, or lower, than stoichiometric (a.k.a. "rich", meaning they have a higher amount of fuel in them) are easier to light than those above stoich ("lean", where fuel concentration is lower than optimal). Any harder-to-light mixture requires more timing advance, which is why most folks would poop themselves if they saw where their timing is with the cruise set at 60MPH (it's often pretty close to, if not over, 50°BTDC with a stone-stock, factory-set ignition system). It's also why you can, and should, crank up a couple of extra degress of advance when using a well-designed cold-air intake--cold air makes for a denser charge regardless of A/F ratio, which means there's more of it to light. Denser mixtures are harder to light as well. The statement about extra fuel cooling the mixture and making it harder to light, while being somewhat technically correct since cooler air is further from its ignition point, is incorrect from the stoichiometric and practical standpoints since it actually makes combustion much easier.
The only thing worse than factory timing is retarding past it. You should be running a lot more advanced than 2.5°BTDC regardless. Factory timing is never optimal and the car should always be tuned beyond it while staying within the limits of use. The factory timing is always set for "some dumbass", meaning the engineers thought of everything reasonable until one of them piped up and said, "Some dumbass is going to try to pull a 35' travel trailer with six overweight passengers, a trunk full of lead, and a piano strapped to the roof up Pike's Peak in the middle of summer." Then they all nodded in agreement and backed off the timing another 5° to account for that guy. If you're not that guy, you need to toss the factory spec out the window and start anew... whether you're looking for better economy or more power.
