Vacuum gauge will show a big leak. Little ones need the carb cleaner test. Spray Carb cleaner around the intake gaskets, etc. If the engine idle speed changes, there's your leak. This will make it lean out. Do you have a carb or FI? Other thing to check is timing. Did someone have the distributor out? Maybe it's a tooth off? Break out the timing light only after fixing the initial mechanical setting. Usually if the coil is bad it will break up when at higher RPMs.
