My truck wont start I have fuel and spark I have
replaced the ecm rotor cap new plugs wires and
put a new starter switch and still won't start
You say starter switch? So what does it do, not turn over or not start? And you say there is spark and fuel? Was the distributor moved?
If you changed out the starter, are you saying it is not turning over? Or will it turn over but it does not appear to get any spark? It could be a computer needing program, lose sensor wire, blown fuse to the fuel pump, or the computer running the spark plugs needs attention. See if you can get the fuses checked, see if there is spark at the plugs, and if it gets fuel. If it is not turning over, check the battery for a full charge. Make sure the wires are connected, on and tight.
Since you say you have fuel (squirting into the carb, I'm assuming) and spark (didn't need to replace the switch if you have spark); '79s did not have a computer so that's not an issue. If the distributor has three or four wires entering at the base in the back - from the firewall then you may have an electronic advance (this electronic control module (E.C.M.) can go bad but it wouldn't kill the engine completely), but they were not put on 305s standard until late '82. In any case try rotating the distributor counter- clockwise between five to ten degrees. This will advance your timing. If it fires up after that then you have a timing issue. If your distributor is the old vacuum type you may have a leaky vacuum hose or fitting or the centrifugal advance mechanism in the distributor may be stuck, have a broken spring or just plane worn out.
(continued) if for some reason you do have an electronic control module, the sensor it uses is called a "knock sensor" mounted in the engine block. Either the sensor has gone bad or the E.C.M. is bad.(E.C.M. is located under the dash behind the center heater vent) The knock sensor can be tested with an ohm meter an I have no idea how to test the E.C.M.
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