I put a new demon carb on my car today and was testing the electric choke and in a haste left my key in the on position for about 4 hours. Started the car drove it all around no problems, and I was trying to diagnose another issue I have so I pulled the distributor cap and to my suprise the top of the cap was burned through in the center as was the center of the rotor. and the inside of the cap is all discolored as well. I am using as stock HEI ignition with a cap and rotor from summit. Was this caused strictly from leaving the key in the position for a few hours, or there something else I should be looking at?

The only time that powered was applied to your Dist. was when you were driving around. Now the normal power pulled from an HEI is about 6 amps. What it looks like is an internal short within you HEI. What might have happened is the transformer inside of your HEI shorted internally and that is what happened. Usually they will just quit woking but it isn't unknown for them to spark or even cause fires. I woould disconnect your HEI and smell the transformer. If it smells burnt it probably is. Let me know if you see or find anything more.

There was definitely a fire the rotor is completely burned as is the top of the cap. It's hard to tell if the HEI it self smells as the entire unit stinks like burned plastic. So leaving the key on was just a coincindence?

Heh, the key should be coincidence. However, don't rule this out yet. Have you ispected for any wires that might have ground themselves? Or in a more unlikely event due to your situation you might have a melted hareness. Here's what I mean. If you had a short somewhere that didn't blow a fuse like it should have it would cause any of your hot acc wires to melt shorting it to something else like to the Dist. check your ignition switch all the way to the fuse panel. There might be something there or like you said coincedence. Personnely when it comes to electrical in my car I want to make sure of everything. It most likely your HEI even if it isn't fried now replace it.

I called summit and they sent a UPS call tag to have the cap and rotor returned. I pulled the hot wire leading to the distributor and it is new condition , I also checked the other wires in the harness with no visable problems.

I checked all wires they're all good. I had been running the current distributor with this cap and rotor with no problems for the last couple months. I put a new motor in last week, and with that I changed the spark plugs and alternator.

1. The new plugs are NGK and gapped .045
2. The alternator was externally regulated the new one is internal, I rewired according to the guide on this website.

Could either of these have caused my problem? For now I am going to throw the old cap and rotor on and see what happens I am going to replace the distributor in the near future but not until I figure out what caused this cap and rotor melt. Any help is appreciated.

Greg

If its any help...... I left my key on with the car not running several hours once, and when I came back the HEI module was DEAD!

Not really addressing your question, but make sure your fusible link from the battery is in place. You need some way to ultimately fuse the power to the car. Most do, but not all circuits have add'l fusing and the fusible link is the only way to "topside" all the power (except the cable to the starter) to the car. I've seen the fusible link replaced with a hard line and a fire started because it couldn't pop the link when a short was encountered.

Not really addressing your question, but make sure your fusible link from the battery is in place. You need some way to ultimately fuse the power to the car. Most do, but not all circuits have add'l fusing and the fusible link is the only way to "topside" all the power (except the cable to the starter) to the car. I've seen the fusible link replaced with a hard line and a fire started because it couldn't pop the link when a short was encountered.

Good idea!

I have one more question, should the engine block be grounded to the frame via a ground strap? I would think the motor mounts would provide a good ground, but any advice would be appreciated.

Mine isn't, I have the - batt cable grounded to the alt bracket and the pigtail to the fender - no grounding issues.

I have seen this before in taxi cabs. The center contact on the rotor is not touching the coil terminal in the center of the distributor cap. Either the contact on the rotor was pushed down or the distributor cap was not correctly seated on the distributor. What happens is the contacts ark inside the distributor and burn the cap and rotor. On taxis it was due to high mileage between tune ups wearing away the coil contact until arcing started burning the rotor. The worst that I saw was a no start and the advance weights were clearly visible through the hole in the rotor.
Paul