After fitting hei to my 67 bird I have been having trouble with intermittent engine run on. Nothing nasty a little splutter every now and then. I have eliminated all possible causes except one, installing a diode to stop back flow of power from alternator light, I only recently found info on this. Has anyone installed this before? I know how to do it just not 100% sure which wire it should go on. Cheers Jason
Doesn't sound like ignition run on, that would more than likely not be intermittent. Have you put a test light on the feed to the HEI to see if it's still powered after shut down?
I had to install the diode on mine, but that was with an HEI and an MSD box installed as well.
Yes, I have. Mine had the same symptoms. Once in a while it was as if there was a time delay in the ignition switch, I'd turn it off and a few seconds later the engine would shut down. I associated it to a worn ignition switch. Even with the switch in the off position I had a slight voltage reading at the ignition terminal. Then I installed a relay for the HEI powered from a junction block on the firewall and controlled by the ignition switch. This gave the distributor battery voltage on start up and 14.5 volts running. The problem intensified due to the relay needing very little voltage to close and supply the distributor with full system voltage. I installed a diode in the alternator turn on wire that goes to the idiot light and it cured the problem. The engine now shuts down immediately I turn the switch. the problem may have been solved with a new switch but I had a diode in my tool box so I used that. When I get a new switch I'll by-pass the diode and see if the run on starts again.
The dash warning light and/or the resistance wire with gauge should have enough resistance to stop feed-back from the alternator to the ignition but some after market ignition boxes and HEIs will run with even a little feed-back voltage. The diode is installed in the wire that runs from the ignition switch through the dash light and to the alternator. Install the diode with the stripe end towards the alternator.
It could be your problem is the switch, the resistance wire, the voltage regulator [if you have a stock type alternator and voltage regulator] or the dash gen warning light. Check all those fore proper operation.
If you're talking dieseling, then it could be gas octane. The one time I put 87 octane in mine when I ran out of gas, it wouldn't stop running. I had to stall it with the clutch.
is your dist vac connected to ported or manifold vac?
are you running stock heat range plugs?
is your eng high perf?
what is your compression?
if your timing is EXACTLY the same at idle it still might be less or more at higher rpms causing more heat in the cyl so it might create pre ignition .
its remotely possible the increased voltage is heating your plug tips up a bit moire and enough to cause pre ignition.
i would retard timing 3 degrees and see if it stops.
if it pings under acceleration try less timing also and/or higher octane as suggested.
