Shuts down after prolonged idle

[JIMOCO]

Went to the DQ drive thru the other day. Temp was about 72 deg. Since it was a nice evening, the line was long and moved slow. I was in line about 15 minutes when the engine started idling rough. Water temp was holding steady at 180 deg. Oil temp peaked at between 195 and 200. The idle got to the point where I had to lightly depress the gas pedal to smooth it out and hold it at about 1,000 rpms. As I moved up in line, I backed off the gas and immediately the engine was about to stop running. (the road is on a slope which made avoiding rolling into the car behind me interesting) Needed to shut off the car at the order and pick up windows (noise). Both times the engine was difficult to start but it did. Drove home and let it idle for a few minutes on the driveway and all was fine. This is not the first time a long idle resulted in the engine running rough and potentially stalling. Thoughts on the cause?

[svassh]

Had a similar issue with my 66 Vette recently. Turned out to be the voltage regulator. Would stumble and die when it heated up. I finally diagnosed it by multi meter on the battery while running. Over time the voltage would just drop till it died even though battery had enough charge,

[jamminj]

could be vapor lock or sticky floats
I noticed you had a similar problem last December also at high speed
they could be related

[MB750]

I was also gonna suggest vapor lock. What's your engine setup?

[JIMOCO]

Engine is an Engine Factory 347 Stroker with Edelbrock heads, intake, 650 CFM car band mechanical fuel pump (consistent 6.5 psi). I did not notice any fluctuation on the volt gauge when I was having the issue but I was watching the temp gauges more intently. Today, I tested today with a multi meter. I have near 13 volts at the battery with engine off and about 14.8 volts while running. Findings are consistent with my gauge. My thought is things look good there but will try again with the engine hot (raining here today). It is interesting that you mentioned vapor lock and my issue in December. It got cold here and I did not check the carb before the driving season ended. Had rotator cuff surgery and am now getting to full recovery. I need to get back on the issue and see if the two issues, loss of power at high speed and the more recent rough idle/stalling are related.

[MB750]

Vapor lock is easy to check. During the symptoms, turn off the car (because it's noisy), and unscrew the gas cap. If you hear a WHOOSH sound of air being sucked into the tank, there's your problem. Also try starting the car as well. If it starts easy and idles fine, confirmed.

Then head under the car to see why the breather isn't working. Your tank should be vented somehow to prevent a vacuum being pulled on it during fuel use.

[rthomas98]

What type of ignition coil and where is it mounted? If it is mounted horizontally and is not a "high Vibration" coil that might be your issue. The oil inside the coil is not fully covering it and they start acting up if made too hot. If you are running a ignition box this will only make the issue worse as the coil will be even hotter and not covered in oil to cool it.

If it is vertical or a high vibration ignore this advice.

[klawrence]

If the vapor lock investigation turns up blank, maybe plugs are tool cold and fouling after a long idle ?

[JIMOCO]

MSD Blaster 2 coil. The first one was mounted horizontally and that went after 2,000 miles. I mounted the second vertically. Has been working fine but I will consider replacing if the vapor lock is not the issue.

[sread]

I second the "vapor lock" diagnosis or more probably boiling the fuel...especially since the car probably still has winter blend gas in the tank which is very susceptible to that issue. And the gentleman that described checking for that by opening the gas cap and checking for air rushing in is not really correct. That would be a symptom of a plugged tank vent and might contribute to vapor lock but is not really a good way to check for it. Anywhere the fuel system is exposed to heat will start the process and I have even seen the fuel in the carb itself start to boil if exposed to enough underhood heat. I would avoid letting any carbed engine, especially a high performance engine, sit and idle any more than necessary.