1970 mustang fastback (302) suddenly does not idle and seems to run rough unless I rev it up. Was running fine then something changed.
There are many possibilities. Start with checking with a tach/dwell meter and vacuum gauge. Check dwell & timing and check engine manifold vacuum. Check to see if the problem is within one particular cylinder by watching rpm as you pull one plug wire at a time to look for rpm drop. A good cylinder will drop rpm as you pull plug wire. Note the amount of rpm drop on each cylinder. If you find rpm stays the same or increases, the problem is in that cylinder. Pull spark plugs to look for any signs of problems. Check vacuum hoses for signs of cracks or splits. If you have an auromatic trans, check at vacuum modulator for signs of fluid. With a hand held vacuum pump, check distributor vacuum advance unit. The problem may be as simple as a fouled spark plug. Let me know what your dwell and vacuum reading is. What carburetor is on your vehicle?
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Your dwell should be 28 degrees and timing should be 6 degrees. Do a compression test to make sure there are no mechanical problems. Let me know what you find.
See this link for basic tune-up: http://1970mgr.org/_Tuneup.htm