[dsraven]

sometimes the brake booster can be a big vacuum leak. you could disconnect the the hose at the engine, plug the hole then try to start it. or pinch the vacuum line off and try to start.
exhaust systems also get plugged or a baffle breaks free inside the muffler and plugs the exhaust. giving the muffler a good bump with a dead blow hammer or your hand may reveal a rattle inside the muffler which indicates a problem. usually the engine will start though, just not rev and then stalls. rodents also like to pack stuff into the pipes if left for awhile and the mice are looking for a new home. I had a motorcycle repair for a guy who couldn't get it to start and when we got working on it it back-fired and a whole whack of popcorn kernels blew out like a shotgun. ran fine after. lol