84 Bronco w/300 6: Runs Rough

OK, here's the story. Replaced the old 300 six banger about 2 years ago with a remanufactured one. It ran fine for six or eight months, no problems whatsoever. Then, it produced an intermittent "miss". At idle, it will run fine for a few seconds, then start missing, and sound much like an engine with a burned valve...for a few more seconds...then begin running nomrally again. While driving (it's a 4 speed with granny gear transmission), you can feel it lurch, and the power drop back a little even while holding the accelerator in one position. Then, the same thing happens, a few seconds later, the power returns, and it runs fine. At idle, the engine sounds as if it might be dropping a couple hundred RPM's or so when it starts missing, then will go back to normal idle speed when the miss stops. It's been in the shop twice, had the sensors checked, and nothing was found wrong. I replaced the coil, distributor, plugs, wires, module, cap and rotor, everything I can think of...to no avail. The engine only has about 12,000 miles on it so far, so it should be running much better than this. The last mechanic told me to "just drive it and bring it back when it gets worse". He adjusted the timing a bit, but that didn't seem to change anything.

Any suggestions?

maybe a vacuum leak or clogged fuel filter or air filter

yea sounds like a vacuum leak.... check the line that goes into that air box

UPDATE: Checked all vacuum lines, found no leaks...but did replace a few that looked pretty old. Had carb rebuilt, to no avail...still have the miss. New plugs, wires, and cap installed (AGAIN!), which didn't help. Still have a dead miss on #1 cylinder at idle. Getting fire to the plug. Ironically, the plug removed from #1 cylinder was DRY! Not saturated in gas! Plug appears to be firing at higher RPM's, as engine smooths out. My mechanic simply gave up, saying it *could* be either a burned valve, or a leaking intake gasket. He doesn't seem to think it's a valve issue, since the engine has only 12,000 miles on it, but didn't perform a compression test. Where do I go from here??? All sensors test ok, no vacuum leaks, fresh rebuild on the carb, new plugs and tune-up...nothing changes the skip.

our van does the exact same thing.. its a straight 6 4sp.... it misses sometimes and idles weird... but it still enough to keep it running.... it has a sticky valve so i think your problem is probably valve related

you could do a compresion test yourself. Oreilly's sells one for 20 bucks i think. Its not hard. I had a similar prob with my 351 in my bronco. I finally got pissed and bought a fuel injected 302 for 500 and gonna install it after i paint it.

Spray the intake manifold gasket area with carb cleaner while the engine is running. If the gasket is leaking, the engine will smooth out and the idle will rise a little. Pay close attention when you spray around cylinder #1.

Problem fixed...turned out to be a leaking intake gasket. The old intake wasn't in the best shape, so it was replaced, as was the exhaust manifold. Expensive...but she's purring again.

Thanks for all the help, everyone!