Hey guys we have been restoring a 71 Roadrunner with matching numbers 383. The motor was rebuilt (not by me, person I bought it from did). The car had already been painted under the hood when we got it so the first thing we did was get the car running. Which we did and we got it running really good. Well this was probably 4 months ago. Nothing under the hood has been touched since it was running. Now the car is all but finished (just need to finish installing some trim). And it will not start. It will start just for a minute it sounds like its running maybe on 2 or 3 cylinders then just dies after a few seconds (cant keep it running). Its getting plenty of fuel. The coil is good, it puts off a good strong blue spark. However when i remove a spark plug wire and try the old screw driver trick to see if its getting spark.. its a very week spark and very inconsistent. You can turn it over and over for 10 or 15 seconds and it might spark 3 or 4 times or it may not make a spark at all in that time. So I'm almost positive the issue is in the ignition.. I've checked all connections. I think my points seem to be ok (it has dual points) but I'm not 100%. I replaced them back when we got it running...

Really need help on this as its puzzling the crap out of me and I dont know where to go.

 

How about 12 volts directly to coil +, start and only run for a short time to see if it will, then take off the wire and don't leave it connected.

This will bypass all wiring, switches and ballast and give you a direction where to look.

A electrical meter can diagnose just the same, but the wire is quick and easy test.

 

Please realize the two conditions ------which are very different----- and which ties in with the post above ------when cranking

1....Cranking using the key. There is supposed to be a contact in the ignition switch always known as "IGN2" and traditionally brown (check your diagram) this wire is hot ONLY in "crank" from the switch and feeds 12V direct to the coil +

2...Cranking by jumpering the starter relay. This does not engage "1" above, and may throw you astray when testing, as the ignition will still "be going through" the resistor.

Also, if the ignition switch or bulkhead connector has a problem with the IGN2 circuit, jumpering the relay may GIVE you spark where you had none in some cases.

3....Last the "ignition run" (IGN1) circuit goes dead in cranking, meaning, the IGN2 circuit is the only source of power in cranking

(Takes big breath)

So ya..........hook a clip lead direct to the coil + terminal and see if that "improves" the spark

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TESTING the spark.

Weak spark can be LOTS of things after the coil............bad coil wire.........wet / dirty / carbon tracked cap..........bad rotor........and who knows......you may have picked the only bad plug wire to test with.

So START checking spark by using a grounded probe or a parts store spark checker right at the coil. Ground a screwdriver with a clip lead, and get someone to twist the key. Alternatively, make "rig" a gap out of a plug. From the coil tower to ground, with the bypass circuit properly in play OR with 12V "rigged" to the coil, you should have a "hot" blue spark about 38" or longer

This spark should be "rhythmic" as the engine turns. Sporadic spark usually is a distributor pickup related problem.

Check the coil wire with a multimeter for resistance. The "old school" standard used to be that plug wires should be LESS than 1000 ohms per foot. Most of the time they are well below this. "Shake" the wire when testing.