Hey all. I am curious about a random popping in the exhaust after the car has been out on a hot day. I have 57 t-bird with a 312 all stock. It has 4160 Holley. The vacuum is pulling about 19.5 inches. I am not sure what the timing is set at. The timing was set using the vacuum gauge. I run high test fuel 91 octane. The car generally has a very smooth idle and I do not get the popping on a cool to warm day. The engine temp was about 196˚ at the thermostat housing. I also notice on hot days my fuel bowl is about half full and there is a little bubbling action. I am not sure if that has anything to do with the popping.

Thanks all

Intermittent ‘popping’ at the exhaust is usually associated with a lean condition.  Hot fuel can cause this but having too large a spark plug gap or a weak spark can also contribute to a random ‘popping’ or miss at the exhaust.  Too much ignition timing at idle can also play into this.

Lorena, Texas (South of Waco)

Thanks Ted for the response.
I guess the easiest thing to try first would be adjust air mixture screws - run it a bit richer. Retard the ignition a bit next. Check plug gap - I think they are set at factory .036. It only does it when it is running in the hotter range. Under normal conditions at about 175 -185˚ idle in park it has the nice even burble out the rear pipes.

I think you are onto it with the hot fuel.   This might be interesting to you  http://www.eps-hane.com/techtips5.html

Thank you for this info. I now remember this happened last year and mysteriously went a few fill ups later. The boiling fuel, the smell of raw gas in the garage after shutting off the car, the hot fumes coming from the carb vent. After a few more fill-ups it went back to normal. Does this explain the rougher idle when it gets hot? When the car is cooler the idle is very smooth.

I would say so.  I had similar issues here in atlanta with a flathead and modern gas.   A higher octane rating will actually lower the evaporation point as well.  It might be worth getting through a tank or two then give it a summer "tuning" on a warm day.

The times when I experienced this was vapor lock or a spark plug wire going bad-ended up in a fouled plug-you might pull your spark plugs and read them before going arms deep into the carb. JMO-easiest to rule out.

Mike Rizzo

1963 F100 "Rudy"

Daniel Island, SC