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jckinthebox 07-15-2014 07:15 PM

390 suddenly stops running
I have a 1968 coupe with a 390/C6 that I have been restoring for a couple years and I was a few hours away from road worthy when all of a sudden the 390 that had been gotten to running decent developed a severe chop to the idle and stalled. I tried restarting it while giving it some gas and stomping on the pedal made virtually no difference, it sound like all of a sudden its decided to run on a couple cylinders here and there..

We pulled the valve covers off and all rockers looked fine. Checked compression, all good. Replaced coil checked spark plugs and wires. Im about out of ideas, any help as to what in the world my issue could be would be helpful.


jmk3 07-15-2014 08:52 PM

I had a similar situation with my 68. Would run good for a while then die suddenly. I hooked it up to a battery charger and it stayed running turned out there was not enough voltage at idle and it would stall. The funny part was the battery would start the car fine but not keep it running. Hook up a charger if you have one and see if it helps.

jckinthebox 07-15-2014 09:34 PM

I'm fairly certain that the battery isn't an issue, like I said it's been running fine for a while and then decided to quit on me, I'm thinking more along the lines of a timing chain issue or maybe a distributor. Are either if those p

jckinthebox 07-15-2014 09:35 PM


boogerschnot 07-16-2014 02:17 AM

Could be fuel related. How long did it sit and how old is the gas? Could have sucked something up and the carb is clogged. For it to just stop running all of a sudden that would be my guess. Try to manually choke it to see if that helps or use some gas poured down the carb to get it started first and see if it runs for a few seconds on all cylinders.. If it sounds normal, when you start up by pouring a littl fuel in, then its probably carb related. If it doesnt sound "normal" before it dies, you may have a slipping timing chain.. maybe..

musnicki 07-16-2014 02:53 AM

I agree with Justin on the fuel issue.

Also, does your 390 have a fuel rail installed? I don't know how the old FE motors ran with those fuel rails, if you have one, I would by-pass it. The fuel hits that tube and boils from the heat coming off of your intake. Other than that, its probably some sort of vapor lock, either in the fuel line or heat off of the intake hitting your carb bowls.


jckinthebox 07-16-2014 06:56 AM

Ill definitely give that a try but im hesitant to think that it will be the fix because when the car isn't running and you pump the gas you can clearly see gas being dumped into the carb. Although I suppose if it were clogged it could be one of the jets or something. Ill try it out and see what happens.


Gun Jam 07-16-2014 11:12 AM

spark seems more likely than fuel if the accelerator pumps are dumping fuel into the carb then it should run for a few seconds at least.

Take a volt meter to the battery whats the voltage?

Pull the boot off the coil and place one end of something like a pipe about 1" above the coil where the boot was and the other end across a good ground like the intake or bare metal and crank the motor over do you see spark jump out of the coil to the pipe?

Take your volt meter to the coil wires whats the voltage?

What ignition are you running...points or electronic?


jckinthebox 07-16-2014 11:33 AM

battery has 14+ volts on full charge which is when all these tests are done so that most likely isn't the issue and the coil is not even a day old at this point so I would hope that its fine but I can try to see how far the spark wants to jump. Its a points style ignition and which coil wires are you talking about?

Seeing that, as far as I can tell, i have both fuel and ignition, im just lost.

Im hopefully going to have some time to pull the timing chain cover after work today and check out the alignment there

Gun Jam 07-16-2014 11:58 AM

You cant just keep ignoring suggestions you've gotta try at least the simple ones or find a simple way to disprove them.

Before we spend hrs pulling a timing cover lets cover the basics!!

1) you pulled the valve cover with the valve cover off do you see the valves move when you crank over the motor...obviously yes because you claim compression was fine on all cylinders so right there we know with nearly a 100% confidence that the timing chain isnt could have skipped a tooth but you claim it ran at idle for a few ran crappy but it ran then died never to restart....well that makes a skipped tooth seem less likely and maybe that whole timing cover adventure is misguided at this point...maybe

2) you threw a new coil at it but if the old coil wasn't getting power a new coil wont help

3) NEW is not a verification of "GOOD" new and good should never ever be assumed to mean the same thing...ever

4) you see fuel going into the carb ventures when you move the throttle even if nothing else on the carb was working with the exception of the throttle plates 3 good shots of accel pump squirt and a 10 second wait before crank would get the engine to fire and run for a few does not so let us investigate something more likely than fuel at this time.

5) based on the above lets do something to confirm that we have spark at the coil. This coil is the main coil that plugs into the distributor....I am unaware of the existence of another coil that may come to mind. Do the pipe test and see if the spark will jump out of the coil to the grounded pipe...this is super simple and takes almost no effort....If you see a nice fat spark then go ahead and check out that timing chain. If you see no spark then its something else like a broken wire or bad points

In place of the pipe you can use a wrench or bare wire anything thats metal that can touch a ground like a bolt or unpainted frame, motor piece and the other end hovers 1" above the coil where the boot fit onto.

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